Figure 1 — Model 2 Rear Bracket Mounting Hole Location Diagram
Figure 2 — Model 25A 19-Inch Rack Adapter Diagram
Figure 3 — Model 28A Panel Adapter Diagram
Figure 4 — Model 22 Access Station Diagram
Block Diagrams
Model 2 Central Controller
Model 22 Access Station
Model 32 Talent Amplifier
Model 33 Talent Amplifier
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Issue 8, December 2005 Model 2 User Guide
Page 4 Studio Technologies, Inc.
Introduction
The IFB Plus Series Model 2 Central Controller from Studio Technologies is a highly
integrated, 2-channel IFB (interrupted
foldback or interrupted feedback) system
contained in a single rack space unit. It is
expressly designed to provide talent cueing for ENG, SNG, and mobile production
facilities. The performance and features of
the Model 2 reflect the needs of contemporary applications. Only after extensive
polling of field production and engineering
personnel was the Model 2’s feature group
established. We think you’ll find the Model 2
a great addition to your facility.
What This User Guide Covers
This User Guide applies to Studio Technologies, Inc. IFB Plus Series Model 2
Central Controllers with a serial number of
M2-01361 and later. If you are installing a
Model 2 with a serial number of M2-01360 or
earlier, please contact Studio Technologies
to obtain the appropriate documentation.
This guide is designed to assist you when
installing, configuring, and using the Model
2 Central Controller. It also contains detailed
service information and block diagrams.
Schematic diagrams are available upon
request.
The following items will be covered in this
User Guide:
among many other features. The unit is
powered by 120 or 220/240 V, 50/60 Hz.
Model 22 Access Station
Used by a producer or director to access
the Model 2’s IFB channels. Allows connection of a Studio Technologies Model 11A
gooseneck microphone or a line-level audio
source. Contains two lighted pushbutton
switches which display IFB interrupt status.
Up to four Model 22s can be connected to
and powered by a Model 2 Central Controller. The Model 25A 19-inch Rack Adapter
allows a Model 22 Access Station and
Model 11A gooseneck microphone to be
mounted in a single 19-inch rack space. The
Model 28A Panel Adapter allows a Model 22
and Model 11A gooseneck microphone to
be installed in an opening made in a table,
equipment console, etc.
Model 32 & Model 33 Talent Amplifiers
The Model 32 and Model 33 Talent Amplifiers are self-contained “belt pack” units that
drive talent ear pieces or headsets. Up to
four Model 32 and/or Model 33 Talent Amplifiers can be connected to, and powered by,
a single Model 2 Central Controller.
The Model 32 is intended for use by on-air
personnel, and contains a source switch,
along with an output level control. Either
IFB channel 1 or IFB channel 2 can be sent
to the talent, along with the desired audio
“volume.”
Model 2 Central Controller
An integrated 2-channel IFB unit intended
for applications where space is at a premium, yet high performance is still required.
The unit contains everything required to
implement a full-featured system in a single
rack space. Includes multiple program
inputs, integral telephone interfaces, monitor amplifier, internal interrupt microphone,
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 5
The Model 33 is unique in that a “mix” of IFB
channels 1 and 2 can be created. Two level
controls, along with a source select switch,
allows camera and production personnel to
hear IFB cues from either or both channels.
This allows IFB signals intended for both
technician and talent to be simultaneously
monitored.
Model 2 Central Controller
Contained in a single rack space unit is everything required to implement a 2-channel
IFB system. Features include multiple program inputs, telephone interfaces, voiceoperated (VOX) interrupt, level meters, monitor amplifier, and internal interrupt microphone. Up to four Model 22 Access Stations
can be connected to the Model 2, allowing
producer or director positions to access the
IFB channels. In addition, up to four Model
32 or Model 33 Talent Amplifiers can be connected to a Model 2. The Model 32 and 33
allow personnel access to the IFB channels
using a single standard microphone cable.
The Model 2 packs numerous features
into a single, 19-inch rack space. Standard
connectors are used throughout, including
3-pin XLR-type, ¼-inch 2-conductor,
9-position D-subminiature, and modular
telephone (RJ11-type) jacks. Power is supplied by standard 120 or 220/240 V (factory
configured), 50/60 Hz via a detachable cord.
IFB Channels
The Model 2 contains two independent
IFB channels. Each channel has individual
controls and indicators, including program
source select switches, program level control, 5-segment LED level meter, and LED
status indicators. We’ll describe one channel
in this paragraph. Six switches allow the four
program inputs and incoming audio from
the two telephone interfaces to be selected
as program audio. A level control allows
adjustment of the program audio level
relative to the fixed interrupt audio level.
A 5-segment LED level meter displays the
composite IFB channel audio level, i.e., program and interrupt audio level. The meter
facilitates the rapid setting of the program
level control, as well as providing a general
indication of the channel’s signal level.
Each channel’s composite IFB audio signal
(program and interrupt audio) is sent to four
places: line output, talent amplifier output,
telephone interface 2, and monitor amplifier.
The line outputs provide electronically balanced, line-level signals that interface with
external equipment via two XLR-type connectors. The talent amplifier output provides
channel 1 and 2 audio, along with +22 Vdc
power on one 3-pin XLR-type connector.
Any combination of up to four Model 32 or
33s can be connected to the talent amplifier
output.
Program Inputs
The Model 2 contains four program inputs.
Each can be individually assigned to the
two IFB channels, with the ability to assign
multiple program inputs to an IFB channel.
Program signals enter the unit via four XLRtype connectors on the back panel. The program inputs are electronically balanced with
a nominal input impedance of 24 k ohms.
They feature low noise, low distortion, and
high common mode signal rejection. Each
program input has a trim potentiometer
associated with it. The trim pots, accessible
from the back panel, allow the nominal
+4 dBu input level to be adjusted over a
±8 dB range.
Program audio is muted whenever an interrupt takes place. A sophisticated analog
switch is used to give a noise-free mute with
absolutely no clicks or pops! If desired, a
program “dim,” rather than a full mute can
be implemented by adding two resistors to
the Model 2’s circuit board.
Compressor Circuits
Each IFB channel contains a studioquality compressor circuit. This serves to
even out variations in the interrupt audio signals and smoothly control peak signal levels. They make talent cues more intelligible
Issue 8, December 2005 Model 2 User Guide
Page 6 Studio Technologies, Inc.
and prevent abnormally high signal levels
from reaching a user’s ears. The resulting
audio quality is very, very good.
Telephone Interfaces
The Model 2 contains two telephone interfaces. Both interfaces can be used to bring
audio into the Model 2 from the outside
world. These two audio signals can be independently assigned as program sources for
IFB channels 1 and 2, as well as being used
as an input source for the voice operated
(VOX) interrupt function. Each telephone
interface has a receive level trim potentiometer that is accessible via access holes in the
front panel. The large variations one finds
in telephone line signal levels make “on the
fly” level trimming a useful feature. In addition to receiving audio, telephone interface
2 can be used to originate an IFB feed. A
switch selects if audio will be received from
the outside world, or if audio from IFB channel 1 or 2 will be sent out the interface.
The two telephone interfaces contain a
unique feature which allows two very different types of telephone “lines” to be correctly
interfaced. Each interface can be independently set to operate in either the telephoneline mode or the standard-audio mode. A
telephone line has the profile of being a
2-wire, DC-biased (normally –48 Vdc) circuit
provided by a local telephone company. A
standard audio signal could be provided by,
for example, a fax adapter associated with a
cellular telephone.
When an interface is set to the telephoneline mode and a DC-biased telephone line
is connected, full monitoring and control
is implemented. Each interface contains
a switch that allows the telephone line to
be seized (taken off hook) or hung up.
Loop current is monitored when the interface is off hook. If a disconnect signal (a
momentary break in loop current) is detected the interface will automatically return to
the on-hook state. Each interface contains
an LED indicator that lights whenever loop
current is detected. Interface 2 also implements an auto answer function, automatically taking the telephone line to the off-hook
state when ringing voltage is detected.
In many cases a “telephone line” is actually
provided by a cellular telephone. This cellular telephone may provide an unbiased (no
DC loop current) audio output. The standard
audio mode was designed expressly to
interface with this “cell phone” arrangement.
In this mode, the interface’s loop currentspecific features are disabled, and the interface appears electrically as a transformer
coupled balanced audio input.
When in the standard audio mode, an interface’s front-panel switches are inactive. Also
inactive is channel 2’s auto answer function.
An LED associated with each interface displays when the standard audio mode is selected. Even during operation in this mode,
the loop current LEDs remain active, serving
as a useful diagnostic tool. They will display
if your “standard audio signal” is actually a
telephone line!
Voice Operated (VOX) Interrupt
The Model 2 contains circuitry to allow an
audio signal to serve as both an interrupt
audio source and a control signal. This
eliminates the need for a separate pushto-talk button or contact closure. The VOX
feature allows an audio signal from a remote
source, such as a two-way radio or telephone line, to serve as the interrupt source.
The VOX function was optimized for detection of audio signals in the voice band. As
voice detection is quite tricky to perform,
great care was taken when designing this
function.
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 7
Three audio sources can serve as the VOX
input: receive audio from telephone interface
1, receive audio from telephone interface 2,
or the auxiliary audio input. The auxiliary audio input is a separate line-level audio input
that serves the VOX circuit only. A 3-position
switch selects which source will be used. A
second 3-position switch selects which IFB
channel the VOX interrupt is assigned to, or
if the function is not active. The VOX function
can be assigned to only one IFB channel at
a time. Each IFB channel contains an LED
indicator light to display when a voiceactivated interrupt is taking place.
Internal Interrupt Microphone
Contained behind the Model 2’s front panel
is an internal interrupt microphone. Associated with the microphone are two switches,
allowing the internal microphone to interrupt
IFB channel 1, channel 2, or both.
Monitor Section
The Model 2 contains a simple but excellent monitor section. At the core is a 4 watt
audio amplifier designed to drive an 8 ohm
(or greater) loudspeaker. Associated with
the monitor amplifier is a 3-position source
select switch and a level control. The switch
selects monitoring of IFB channel 1 or 2, as
well as having an off position. A click-free
analog switch mutes the monitor output
whenever the internal microphone or a Model 22 Access Station, if installed and configured, is interrupting either IFB channel.
Model 22 Access Station
The Model 22 Access Station provides the
capability to add up to four additional interrupt locations. Model 22s are intended to be
installed at positions convenient to producers, directors, or other personnel who need
to “cue” talent and related personnel. The
unit consists of a metal chassis containing
two lighted pushbutton switches, unbalanced microphone and balanced line
inputs, and status and control circuitry.
The two high-quality, back lit pushbutton
switches provide access to the two IFB
channels. The lights in the switches display when an interrupt is taking place on its
respective channel. When a channel is idle,
the light is lit dimly. An input select switch
allows connection of a Model 11A gooseneck microphone or a line-level signal. The
electronically balanced line-level input allows interfacing with other communications
equipment, such as an intercom system.
The Model 22 can be configured to mute
the Model 2’s monitor amplifier output. This
function will prevent acoustic feedback from
occurring when a Model 22 is located close
to the Model 2’s monitor speaker.
Model 22 Access Stations are linked to the
Model 2 Central Controller via 9-pin D-type
female connectors. Each access station
contains two connectors, allowing a simple
daisy-chain installation. The nine leads carry
all signals; audio, control, status lamp (tally),
and power. The Model 2 provides all power
required by the access stations. The Model
25A 19-inch Rack Adapter is available to
mount a Model 22 and a Model 11A gooseneck microphone in one space of a 19-inch
rack. The Model 28A Panel Adapter allows
a Model 22 and a Model 11A gooseneck
microphone to be mounted in a panel
opening.
Model 32 & Model 33 Talent
Amplifiers
The Model 32 and Model 33 Talent Amplifiers are self-contained “belt pack” units
that drive talent ear pieces or headsets. A
single 3-conductor microphone-type cable
links the Model 2 with the talent amplifiers.
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Page 8 Studio Technologies, Inc.
Each Model 32 and Model 33 contains both
a male and female XLR-type connector,
allowing simple “loop through” connection
of multiple units. Up to four talent amplifiers
can be connected to, and powered by, a
single Model 2 Central Controller. On each
talent amplifier the audio output signal is
provided on a standard ¼-inch 2-conductor phone jack. An LED on each unit lights
whenever power is present, providing setup
assistance and user confidence. Identical
in size, each is housed in a lightweight, yet
rugged, aluminum housing. A belt clip allows it to be attached to belts, clipboards,
scabbards, pizza boxes, production assistants, etc. An optional mounting adapter is
available, allowing a Model 32 or 33 to be
installed in a permanent location.
The Model 32 is intended for use by on-air
personnel, and contains a source switch,
along with an output level control. Either
IFB channel 1 or IFB channel 2 can be sent
to the talent, along with the desired audio
“volume.”
The Model 33 is unique in that a “mix” of IFB
channels 1 and 2 can be created. Two level
controls, along with a source select switch,
allows camera and production personnel to
hear IFB cues from either or both channels.
This allows IFB signals intended for both
technician and talent to be simultaneously
monitored.
Installation
In this section you will be installing a Model
2 Central Controller in an equipment rack.
In conjunction with the Model 2, up to four
Model 22 Access Stations can be installed.
In addition, wiring can be installed for up to
four Model 32 and 33 Talent Amplifiers.
Internally, the Model 2 has no configuration
switches or trim pots to set. However, there
are several reasons why you may need to
access the “guts” of the unit:
• The factory-selected AC mains voltage
will be either 120 or 220/240 V, depend ing on the shipping destination. It may
need to be revised for your installation.
• From the factory, the Model 2 is set to
mute program audio when an interrupt
takes place. If desired, a program “dim”
function, rather than mute function, can
be implemented.
Refer to the Technical Notes section for
details on revising these conditions.
In addition to the above situations, you may
want to take a look inside and familiarize
yourself with the unit on the rare occasion
that it may need service. (The people here
in the marketing department taught us never
to say a unit may need to be fixed—service
is much nicer!) We are proud of how the
“guts” of the Model 2 look and how it is
constructed so we encourage you to take a
look!
On a more serious side, removing the Model
2’s cover with the AC mains cord connected
exposes you to hazardous voltages.
Warning: Never remove the cover without disconnecting mains power. We
make the assumption that anyone gaining access to the inside of our products
meets the requirements for “qualified service personnel”—including knowledge of
safety precautions.
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 9
Locating the Unit
Physical access and mechanical noise are
the primary factors when choosing a mounting location. You don’t want a cooling fan
blowing directly onto the internal microphone, nor do you want the microphone at
the level of your navel. It is also a good idea
to keep the Model 2 physically separated
from other electronic devices that produce
strong electrical fields. As a device that
contains high gain audio stages, hum and
noise pickup is possible through the chassis and associated cabling. Locating the unit
away from devices such as power amplifiers, power transformers, and lighting controls will help to avoid pickup of unwanted
signals. Precautions were taken to limit the
Model 2’s bandwidth, minimizing the chance
for RF pickup problems.
Mounting
The Model 2 is intended for rack mounting, requiring one 1.75-inch rack space. It
weighs a bit over 10 pounds (4.7 kg), and
operates on either 120 or 220/240 V, 50/60
Hz. The unit is secured to the front mounting rails of an equipment rack using two
mounting screws per side. In addition to the
front mounting “ears,” we encourage you to
secure the Model 2 from the back. Provision
has been made for this to be easily accomplished. On each side of the Model 2’s
chassis are two threaded fasteners intended
to secure user-fabricated mounting bars or
brackets. Use standard 8-32 screws to secure the mounting brackets to the Model 2’s
chassis. To prevent damage to the Model
2’s “guts,” limit the screw length so that the
threads extend into the chassis a maximum
of ¼-inch. Refer to Figure 1, located at the
end of this guide, for a mechanical drawing
detailing the fastener locations. This drawing
will assist you in fabricating the mounting
brackets.
Program Inputs
The Model 2 allows four line-level program
audio sources to be connected. The input
circuitry is designed for a nominal input level
of +4 dBu. A trim potentiometer is associated with each input, giving a ±8 dB adjustment range. The trim pots are not intended
for precise calibration, but strictly to allow
the program input signals to be adjusted to
the same relative level. A little attention to
setting the trim pots will give better operational performance. It will allow an operator,
using the front-panel program switches, to
rapidly change program inputs, while not
giving large changes in level to users listening to the IFB channel. It is anticipated that
the trim pots will be adjusted only upon initial installation or during maintenance, and
are not considered an operator function.
Refer to the Technical Notes section of this
guide for details on setting the trim pots.
Program audio connections are made via
four 3-pin female XLR-type connectors located on the back panel. The program input
circuitry on the Model 2 is direct coupled,
electronically balanced with an input impedance of 24 k ohms. Prepare the mating connectors so that pin 2 is positive (+ or hot),
pin 3 is negative (– or cold), and pin 1 is
shield. With unbalanced signals connect pin
2 to positive, and pins 1 and 3 to shield.
Auxiliary Audio Input
The Model 2 allows an auxiliary line-level
audio source to be connected for use with
the voice operated (VOX) interrupt function.
Identical to the program inputs, the auxiliary input circuitry is designed for a nominal
signal level of +4 dBu. A trim pot is associated with the auxiliary input, giving a ±8 dB
adjustment range. The pot is not intended
for precise calibration, but strictly to allow
Issue 8, December 2005 Model 2 User Guide
Page 10 Studio Technologies, Inc.
the AUX input level to be adjusted for correct VOX operation. It is anticipated that this
pot will be adjusted only upon initial installation or during maintenance, and is not
considered an operator function. Refer to
the Technical Notes section of this guide for
details on setting the trim pot.
The auxiliary audio connection is made via
a 3-pin female XLR-type connector on the
back panel. The auxiliary audio input circuitry is direct coupled, electronically balanced
with an input impedance of 24 k ohms.
Prepare the mating connector so that pin 2
is positive (+ or hot), pin 3 is negative (– or
cold), and pin 1 is shield. With unbalanced
signals connect pin 2 to positive, and pins 1
and 3 to shield.
The VOX interrupt function is quite specialized and may not be used regularly in your
facility. For maximum flexibility, it may be
best to terminate the auxiliary audio input to
a patch point in the audio patch bay. In this
way, a variety of audio sources can quickly
be selected as the VOX interrupt input
source. The VOX feature may sit unused
for months or years, but when you need it
there’s no substitute! The Model 2’s VOX
interrupt works quite well, better than you
may think. (Actually better than we thought,
too, until we tested it!) Try it out and you
may soon find more applications than you
expected.
Talent Amplifier Output
The talent amplifier output is designed to
supply power and audio signals for up to
four Model 32 or 33 Talent Amplifiers. The
talent amplifier output exits the Model 2 via
one 3-pin male XLR-type connector. Pin 1 is
common, pin 2 is +22 Vdc modulated with
channel 1 audio, and pin 3 is channel 2 audio. For convenience, it is expected that the
talent amplifier output will be wired to a main
distribution panel, along with microphone,
camera, and other various connections.
This will allow easy talent amplifier connection using standard flexible microphone
cable. The distribution panel does not need
multiple talent amplifier output connectors
which are “multed” from the Model 2’s talent
amplifier output. Each Model 32 and Model
33 Talent Amplifier has both a female and a
male 3-pin XLR-type connector. This allows
the talent amplifier interconnecting cables
to be “looped” through the talent amplifiers.
This will simplify and, in most cases, reduce
the amount of cabling required to connect
multiple talent amplifiers.
Line Outputs
Associated with each of the two IFB
channels is an electronically balanced linelevel output with a nominal signal level of
+4 dBu. The line outputs are capable of
driving balanced or unbalanced, low or high
impedance loads. The line outputs exit the
Model 2 via two 3-pin male XLR-type connectors. Prepare the mating connectors
so that pin 2 is positive (+ or hot), pin 3 is
negative (– or cold), and pin 1 is shield. To
connect to an unbalanced load connect pin
2 to positive (+ or hot) and pins 1 and 3 to
shield.
Some installations may use the line outputs
to drive wireless IFB transmitters. In other
installations they may be used for special
applications, such as feeding a satellite
uplink. For maximum flexibility it is recommended that the line outputs be wired via
audio patch points, even if you have a dedicated application.
The Model 2’s audio quality and flexibility
make it a useful tool for non-IFB applications. Not to “toot our own horn,” but the
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 11
Model 2’s sonic quality is as good as many
audio consoles, tape recorders, etc. As an
example, the frequency response from program input to line output is better than
±0.5 dB from 20 Hz to 20 kHz. This makes
using one of the IFB channels as a four
input, one output audio source selector for
an uplink application completely valid. Let
the Model 2 become a useful part of your
facility’s “bag of tricks!”
Connecting Telephone Lines
Words of Caution: As with any product,
installing the Model 2 requires a safety-first
approach.
Never install telephone wiring during a lightning storm. Never install telephone jacks in
wet locations unless the jack is specifically
designed for wet locations. Never touch
non-insulated telephone wires or terminals
unless the telephone line has been disconnected at the network interface. Use caution
when installing or modifying telephone lines.
The Model 2 contains two telephone interfaces which can be individually configured
to allow connection to a telephone line or to
a standard audio signal. A telephone line is
designated as such if it has a DC bias current associated with it. This type of signal is
sometimes referred to as a “wet” signal. A
standard audio signal is one that is isolated
from any source of DC voltage. Some fax
adapters associated with cellular telephones
provide this type of “telephone” line. A standard audio signal is sometimes referred to
as a “dry” signal. While this sounds confusing, contemporary applications sometimes
refer to both types of signals as “telco.”
To a large production vehicle that pulls up
to do a two-week golf tournament, a telco
line is one provided by the local telephone
company on an RJ11 jack. To a mobile ENG
vehicle, a telco signal might come from a
cellular telephone which in most cases is a
standard audio signal.
Here is a review of the important features of
the Model 2’s telephone interfaces:
Interface 1
• Telephone line or standard audio mode
select switch on back panel
• Used to receive audio only
• Receive audio used as program and/or
VOX source
• Front panel ±8 dB receive level trim pot
• Switch allows manual off-hook and
hang-up (telephone line mode)
• Auto disconnect upon break in loop
current (telephone line mode)
• Operating mode and loop current status
LEDs
Interface 2
• Telephone line or standard audio mode
select switch on back panel
• Used to receive or send audio
• Receive audio used as program and/or
VOX source
• Sends audio from either IFB channel
• Automatic answer of “ringing” telephone
line (telephone line mode)
• Auto disconnect upon break in loop
current (telephone line mode)
• Front panel ±8 dB receive level trim pot
• Switch allows manual off-hook and
hang-up (telephone line mode)
• Operating mode and loop current status
LEDs
Setting the mode switches and connecting signals is quite easy. Set the telephone
Issue 8, December 2005 Model 2 User Guide
Page 12 Studio Technologies, Inc.
interface mode switches, located on the
back panel, to the desired mode—either
telephone line or standard audio signal.
Use modular telephone cables to mate the
Model 2’s modular jacks (RJ11-type) with
two more RJ11-type jacks. Technically, the
Model 2’s telco interfaces use 6-position
modular jacks with pins 3 and 4 utilized.
If the telephone line mode is selected, the
telco interfaces should, in most cases, terminate on an access panel of some sort. This
will make rapid termination with telephone
company-provided lines. Because of the
presence of high voltage ringing signals, it
is best not to route tele-phone lines through
an audio patch bay.
If the standard audio mode is selected,
a direct connection to the source device
is acceptable, but connecting it via audio
patch points provides better flexibility. In the
standard audio mode the interfaces look like
standard, transformer coupled audio inputs.
Interface 1 has an input impedance of 2200
ohms. Interface 2 has an input impedance
of 2200 ohms in the receive mode, and 700
ohms in the send mode. In standard audio
mode, the interfaces are compatible with
balanced or unbalanced signals. No shield
connection is associated with the telephone
interfaces. Audio signals should be connected to the telco interfaces, via a modular
jack and cable, using shielded cable, with
the shield wire connected to the appropriate point at the end opposite of the Model 2.
The shield wire should remain unterminat-ed
at the Model 2’s end.
The telco interfaces are designed to receive
and, in the case of interface 2, send audio
signals at nominal levels that are correct for
telephone circuits, but which are far lower
than the typical +4 dBu professional audio
standard. The nominal receive signal level is
expected to be –15 dBu, the transmit level
is –6 dBu. Trim pots are associated with
both interfaces, allowing the receive level
to be adjusted over a ±8 dB range. Special
precautions may need to be taken when an
interface is set for standard audio mode and
+4 dBu signals are going to be connected.
These would include the use of an audio attenuator or “pad” for connecting receive audio, and the use of a line amplifier to achieve
send gain from interface 2.
Access Stations
The Model 2 allows the connection of up to
four Model 22 Access Stations. The Model
22 provides an unbalanced microphone
input, a balanced line-level input, two lighted
pushbutton switches, and related circuitry to
access the two IFB channels. Power is provided by the Model 2, so an external power
source is not required. Nine-pin D-subminiature connectors link the Model 22 with the
Model 2 Central Controller and, if desired,
another Model 22. A 5-position screw terminal strip allows connection of a Studio Technologies Model 11A gooseneck microphone
or a line-level signal.
There are three mounting methods appropriate for the Model 22: the Model 25A
19-inch Rack Adapter, the Model 28A Panel
Adapter, or custom. The Model 25A 19-inch
Rack Adapter allows the rapid installation
of a Model 22 and a Model 11A gooseneck
microphone in a single rack space. Refer
to Figure 2 for details on how the Model 22
is physically positioned in the rack adapter.
Plenty of room remains on the right side of
the rack adapter, allowing you to add custom switches, lights, or jacks as your installation may require.
The Model 28A Panel Adapter allows the
installation of a Model 22 and a Model 11A
gooseneck microphone in a “cut out” made
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 13
in a desk, console, or other enclosure. Refer
to Figure 3 for a mechanical drawing of the
panel adapter and the required mounting
hole size.
A custom Model 22 installation is any installation where you devise the mounting
method! Refer to Figure 4 for a mechanical drawing showing the dimensions of the
Model 22. This will assist you in implementing your own mounting method.
shielded cable, with the shield connected at
pin 1 of both plugs.
For best performance, a maximum total
cable length of 500 feet should be used to
link Model 22s to the Model 2. Minimizing
the cable length reduces the total exposure
the unbalanced audio buses have to noise
pickup, etc. Cable runs longer than 500
feet are possible but should be carefully
checked for correct operation.
Contained on both the Model 2 and the
Model 22 are standard 9-pin D-type female
connectors. A “straight through” cabling
scheme links the units together, with pin
1 connected to pin 1, pin 2 to pin 2, etc.
Shielded cable is required as the two audio
buses linking the Model 22s to the Model
2 are unbalanced. The shield should be
connected to pin 1 on the 9-pin plugs. For
reference, the following chart displays the
signals associated with the access station
connector:
For ease of installation, each Model 22 contains two 9-pin D-type female connectors
wired in parallel. This allows signals to be
easily “looped through” on their way to the
next Model 22. For short cable runs, standard 9-pin video monitor extension cables,
commonly used with personal computers,
are an inexpensive and readily available way
to link a Model 2 with a Model 22. Ensure
that any cable assemblies you purchase use
The Model 22 Access Station contains
several configuration switches that must be
set. The input select switch allows the unit to
be set for a mic or a line-level input. In most
cases the Model 11 gooseneck microphone,
available from Studio Technologies, will be
used with a Model 22. Setting the switch
to the mic position activates the mic input
terminals. The mic input is configured only
for use with unbalanced electret-type microphones. The + terminal on the mic input
provides current limited 5 Vdc to power the
electret microphone. Don’t use a dynamic
microphone as damage may occur. The
– terminal is connected to the microphone’s
low signal lead. The connection marked
SHLD is intended for the shield wire of the
microphone cable; it doesn’t carry signal.
The Model 11A Gooseneck Microphone is
an unbalanced electret type, requiring an
external source of DC power. The microphone utilizes a 3-conductor ¼-inch plug
for interconnection. The tip lead brings DC
power to the microphone, as well as having microphone audio superimposed on it.
The ring lead is microphone common. The
sleeve lead is a shield connection.
Included with the Model 11A is a mounting
hardware kit. Included with the kit is a
3-conductor ¼-inch jack used to mate with
the microphone’s plug. At the factory a
3-conductor wiring harness assembly is
Issue 8, December 2005 Model 2 User Guide
Page 14 Studio Technologies, Inc.
attached to the jack. The wiring harness has
a red-colored wire, a black-colored wire, and
a shield wire. One end of the red wire is attached to the tip connection on the jack.
The other end should be connected to the
+ terminal on the Model 22’s microphone
input. One end of the black-colored wire
is attached to the ring lead of the jack.
The other end should be connected to the
– terminal on the Model 22’s microphone
input. One end of the shield connection is
attached to the sleeve of the jack. The other
end should be connected to the SHLD terminal on the Model 22’s microphone input.
Setting the input select switch to the line
position activates the line input, and the
associated input trim potentiometer. The
line input circuit is electronically balanced,
capacitor coupled, with an input impedance of 24 k ohms. Balanced or unbalanced
signals can be connected. The input trim
pot allows signals with a nominal level of
–15 to +10 dBu to be correctly interfaced.
When connecting balanced signals the
audio should connect to the + and – terminals. The shield wire can be connected to
the SHLD terminal associated with the mic
input. Unbalanced signals require a bit more
attention for correct operation. The signal
high lead should be connected to the +
terminal. The signal low/shield wire should
connect to both the – terminal and the SHLD
terminal. Failure to make this connection
can result is large amounts of hum being
induced into the interrupt signal.
If you have connected a signal to the line
input terminals, set the trim pot only after
installing the entire IFB system. Refer to the
Technical Notes section of this guide for
details on setting the trim pot.
The Model 22 contains a monitor mute function, which is intended to prevent acoustical
feedback if the access station is located
close to the monitor loudspeaker associated
with the Model 2 Central Controller. When
the monitor mute switch is set to the ON position, the monitor output is muted whenever
an interrupt occurs from that access station.
Monitor Output
The Model 2 contains a 4 watt monitor
amplifier which is intended to drive a small,
efficient loudspeaker. A switch on the front
panel selects whether IFB channel 1 or 2 will
be monitored. The monitor output is accessible from the back panel using a ¼-inch
2-connector jack. The output can drive 8
ohm or greater loads, and is quite “hi-fi.”
Ensure that both conductors of the speaker
wiring are “floating” (isolated) from ground.
This will prevent ground loops that could
cause oscillation or other strange audio
gyrations. We recommend using a good
quality loudspeaker. If you do so we think
you’ll find the monitor output sounds quite
nice. In our lab, we tested the monitor amp
by connecting it to a JBL three-way speaker
cabinet, with a compact disc acting as our
program source. The amp, within reason,
really “kicked some tail!”
Connecting AC Mains Power
The Model 2 is factory configured to
operate from AC mains power of either 120
or 220/240 V, 50/60 Hz. In most cases,
units shipped to North America are factory
selected for 120 V operation and are supplied with an internal 0.500 A, 5 x 20 mm
fast-acting fuse. Most units shipped outside North America are factory selected for
220/240 V operation and are supplied with a
0.250 A, 5 x 20 mm fast-acting fuse. Before
connecting the Model 2 to mains power,
determine the actual mains voltage and confirm that the Model 2 has been configured
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 15
correctly, as an incorrect setting could seriously damage the unit. Should it be necessary to change the unit’s operating voltage
refer to the Technical Notes section of this
guide. Because the Model 2 contains no
power switch it will start operating as soon
as mains power is connected.
The Model 2 uses an IEC standard connector to mate with the AC mains cord. Normally the cord supplied has a North American
standard plug at one end and an IEC connector at the other. In non-North American
applications the appropriate plug must be
attached. The wire colors in the mains cord,
in most cases, will conform to the internationally recognized CEE color code and
should be wired accordingly:
Connection Wire Color
Neutral (N) Light Blue
Live (L) Brown
Earth/Ground (E) Green/Yellow
Operation
Looking at the Model 2’s front panel from
the left to the right, you should note the
functional groups: internal interrupt microphone, channel 1-related items, channel
2-related items, voice activated (VOX) interrupt, telephone interface 1, telephone interface 2, monitor amplifier, and power LED.
We’ll discuss these groups in the following
sections.
Internal Interrupt Microphone
The Model 2 contains an internal microphone which can be used to access either,
or both, of the IFB channels. Two momentary action pushbutton switches are located
on the left end of the front panel and are
labeled IFB 1 and 2. Pressing either switch
mutes (or optionally dims) program audio,
mutes the monitor speaker output, and connects the microphone to the selected IFB
channel(s). The IFB status LED associated
with each chan-nel will light whenever its
corresponding interrupt button is pressed.
Notice that sound enters the microphone via
the small openings in the front panel above
and slightly to the right of the switches.
The Model 2’s Front-Panel
Controls and Indicators
If you value equipment by the number of
switches and lights per rack space, the
Model 2 is really a good deal—there is more
stuff crammed onto the front panel than we
thought possible! Seriously, the designers
had the difficult problem of getting maximum functionality into a single rack space.
We had heated arguments about the feature
list, what to include, what to delete. In the
end, we feel that all important features were
included. Once you understand all the functions, we think you’ll find the Model 2 quite
powerful, yet easy and intuitive to use.
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Program Select, Level Adjustment, and
Indicators
Two identical sets of controls and indicators
serve IFB channels 1 and 2. Each channel
contains six program select switches, two
status LEDs, a program level control, and a
5-segment LED level meter. The six switches
are used to select which of the four program
inputs, and the receive audio from the two
telephone interfaces, will serve as the program audio source(s). The switches were
chosen to allow more than one source to be
selected at a time. The ability to simultaneously depress and lock multiple buttons is
not a defect, but is a feature which can be
useful in special circumstances.
The red LED, labeled IFB, is lit any time program audio is being interrupted. There are
three ways an interrupt can take place: by
the internal microphone being activated, by
a Model 22 Access Station being used, or
via a control signal from the voice operated
(VOX) interrupt function. The yellow LED,
labeled VOX, is lit any time interrupt is
caused by the VOX function.
The program level control allows the program audio signal to be adjusted relative to
the interrupt audio level. The interrupt level
is internally fixed and serves as the reference. The gain structure was configured
so that the level control set for 50 percent
of rotation (12 o’clock) will give a program
level approximately equal to the interrupt
level. This statement is made under the assumption that a +4 dBu program signal is
selected and its associated input trim pot is
correctly set.
The 5-segment LED level meter displays the
internal level of the composite (program and
interrupt) IFB signal. The three green LEDs
are lit with signals in the normal operating
range. The yellow LED lights with a signal
slightly higher than average. The red LED
lights when signal levels are in the “headroom” area. The ballistics of the meter are
a cross between that of a VU meter and a
peak (PPM) meter. (We affectionately refer to
it as a “PU” meter!) The meter should prove
useful during installation and maintenance,
as well as during normal operation.
A typical interrupt signal will light the green
LEDs, with peaks lighting the yellow LED.
The internal compressor circuits will keep
most interrupt signals from lighting the red
LED. When an interrupt is not taking place,
the level meter will reflect the level of the
program audio source. Setting the program
level control to occasionally light the yellow
LED will give a program level approximately
equal to the interrupt level.
Voice Operated (VOX) Interrupt
The Model 2 contains a specialized function
that allows an audio signal to automatically
interrupt the program signal. By contrast,
using the Model 2’s internal microphone
requires an explicit action by the operator,
i.e., pressing a button to cause an interrupt.
In some cases, this explicit action is simply
not possible. An example would be a director giving cues via a two-way radio or a
telephone line. The VOX circuitry creates an
interrupt control signal by detecting energy
in the voice band. This control signal acts
on the selected IFB channel, interrupting
program audio and routing VOX audio in its
place. The VOX interrupt can be assigned
to either IFB channel 1 or 2, but not to both
simultaneously.
The VOX interrupt function is only sophisticated from an internal-circuitry stand point.
Operation is quite simple, with only two
switches to set. The input switch allows one
of three audio sources to be connected to
the VOX input: audio from the auxiliary audio
input, receive audio from telephone interface
1, or receive audio from telephone interface 2. The output switch is used to select
whether the VOX interrupt function is off (not
used), or is assigned to interrupt IFB channel 1 or 2. VOX operation can commence
as soon as the output is assigned to one of
the channels. VOX interrupt activity can be
noted by observing the yellow VOX LED associated with the assigned IFB channel.
Telephone Interface 1
Two status LEDs, one switch, and one trim
potentiometer are associated with telephone
interface 1. The yellow LED, labeled STD, is
lit whenever the interface is set for the standard audio mode by the switch on the back
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 17
panel. The red LED, labeled LC for loop
current, lights any time DC current is flowing through the interface. Normally in the
standard audio mode, loop current would
not be flowing through the interface and
the LC LED will not be lit. In most cases the
loop current LED lighting would indicate that
the mode switch should be changed to the
telephone line position.
The interface control switch, active in the
telephone line mode, allows the interface to
be taken off hook or hung up. Momentarily
pressing the switch to the up position, labeled MAN OH for manual off hook, places
the interface in the off-hook state. If loop
current is detected, the interface will stay in
the off-hook state and the loop current LED
will light. If loop current is not detected, the
interface will return to the off-hook (idle)
state after a few seconds. Momentarily
pressing the switch to the down position,
labeled HANG UP, immediately disconnects
the interface from the telephone line. In
the standard audio mode, the interface
control switch is not active because the
standard audio mode places the interface
permanently in what is effectively the offhook condition.
Telephone line audio signals can vary greatly in level on a call-by-call basis. To counter
this problem, a level trim potentiometer is
provided for the operator, allowing adjustment over a ±8 dB range. A quiet signal
can be boosted, and a “hot” signal can be
attenuated. The trim pot is accessible via a
small hole in the front panel, directly to the
right of the interface control switch.
Telephone Interface 2
Two status LEDs, two switches, and one trim
potentiometer are associated with telephone
interface 2. The yellow LED, labeled STD,
is lit whenever the interface is set for the
standard audio mode by the switch on
the back panel. The red LED, labeled LC
for loop current, lights any time DC current is flowing through the interface. In the
standard audio mode, loop current will not
usually be flowing through the interface. If it
is, the red LED will light. In most cases the
loop current LED lighting would indicate that
the mode switch should be changed to the
telephone line position.
The interface control switch, active in the
telephone line mode, allows the interface to
be taken off hook or hung up. Momentarily
pressing the switch to the up position, labeled MAN OH for manual off hook, places
the interface in the off-hook state. If loop current is detected, the interface will stay in the
off-hook state and the loop current LED will
light. If loop current is not detected, the interface will return to the on-hook (idle) state
after a few seconds. Momentarily pressing
the switch to down position, labeled HANG
UP, disconnects the interface from the telephone line. An apparent problem occurs
if the switch is used to place the interface
in the off-hook state and then immediately
used to hang up the interface. A pause of
approximately three seconds is required before the manual hang-up function becomes
active after the interface is manually taken
off hook. This is due to the charge time of
a debounce capacitor required in the auto
answer circuit. This capacitor is not associated with interface 1 and the pause is not
required. In the standard audio mode, the
interface control switch is not active because the standard audio mode places the
interface permanently in what is effectively
the off-hook condition.
A second switch controls the audio routing
through interface 2. In the center position,
audio is received from the telephone line.
In the up position, labeled SEND CH1,
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Page 18 Studio Technologies, Inc.
composite IFB channel 1 audio is sent out
the interface. In the down position, labeled
SEND CH2, composite IFB channel 2 audio
is sent out the interface. The overall send
level is not adjustable. Just like the talent
amplifier and line outputs, the interrupt level
is fixed, and the program level is adjusted
in reference to it. The send level has been
internally configured to give the highest signal level possible, without overloading the
telephone company equipment.
On interface 2 a level trim pot is provided,
allowing the receive audio to be adjusted
over a ±8 dB range. A quiet signal can be
boosted, and a “hot” signal can be attenuated. The trim pot is accessible via a small
hole in the front panel, directly to the right
of the routing control switch. The trim pot is
active only in the receive mode.
Monitor Output
A level control and source select switch is
associated with the monitor section. The
level control adjusts the output level sent to
the external monitor speaker. IFB channel 1
is monitored when the source select switch
is in the up position. In the down position,
channel 2 is monitored. The middle position
is labeled OFF and the monitor amplifier is,
as you might guess, off!
Model 2 and, if present, Model 32 and/or
Model 33 Talent Amplifiers should be lit. If
one or more Model 22 Access Stations are
installed, their pushbutton switches should
be dimly lit.
Ear pieces should be connected to the
talent amplifiers. Both the Model 32 and
Model 33 Talent Amplifiers utilize a ¼-inch
2-conductor phone jack for their audio
output. On the Model 32, either IFB channel can be selected as the audio source. On
the Model 33, either or both IFB channels
can be selected as the audio source(s). The
level control(s) should be adjusted for the
desired level during an interrupt from the
Model 2’s internal microphone. Note that the
even when the level control(s) are set to the
fully counterclockwise position, the audio
output will not be fully “off.” This ensures
personnel will never (hopefully!) miss an
important cue because a level control was
accidentally turned “off.”
Once the talent amplifier’s output level has
been set, a Model 2 program source can be
selected and the program level control on
the Model 2 adjusted as required. Remember that the nominal interrupt level is internally set in the Model 2, and acts as
the system reference.
Power Indicator Light
A red LED indicator light located on the right
In the following paragraphs we’ll describe
several simple procedures you can use to
try out the Model 2’s features.
side of the front panel is lit any time mains
power is applied to the Model 2. Since the
Model 2 does not contain a power switch,
the power LED should be lit at all times that
power is applied to your equipment racks.
Connect a high quality audio source, e.g.,
compact disc player, to a program input.
Select it as the program audio source for
one of the IFB channels. Set the program
level control to get the yellow meter LED
It’s Time to Use the System!
Operation can commence after the Model 2
and related equipment have been installed
and connected. The power LEDs on the
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 19
lighting on signal peaks. Test the monitor
amp—it should let you “rock out” pretty
well. If not, put in a maintenance request
for a better speaker!
Bring an audio source into the auxiliary audio input and test the VOX interrupt feature.
Set the VOX input switch to the middle position, connecting the auxiliary audio input to
the VOX circuitry. Use the VOX output switch
to assign the VOX output to one of the IFB
channels. On the channel you selected for
VOX interrupt watch the VOX and IFB status
LEDs light when signal is detected. Use the
monitor amplifier to monitor VOX interrupt
activity.
Try receiving audio via telephone interface 1.
Use the receive audio as a program source.
Use the receive level trim pot to vary the
receive audio level. Use the receive audio
as a source for the VOX interrupt function.
Telephone interface 2 has more features; try
them all. If set for the telephone line mode,
let the interface automatically answer an
incoming telephone call. Use the interface
to receive, as well as send audio. Use the
receive audio as a program source. Use the
receive level trim pot to vary the receive audio level. As with interface 1, use the receive
audio as a source for the VOX interrupt.
Test the Model 22 Access Stations. Is the
interrupt audio loud and clear? Observe the
status lamps inside the pushbutton switches. Do the appropriate lamps light fully
bright when an interrupt takes place?
If an access station is configured to mute
the Model 2’s monitor output, ensure that
the monitor speaker does mute during an
interrupt.
In all cases, you should hear clear, click-free
audio. We intended the Model 2 to sound
great—if not, call us for technical help. A
completed installation should be reliable,
easy to use, and perform to high sonic standards. Questions and comments from the
field are welcomed and encouraged!
Technical Notes
Schematic Diagrams
Schematic diagrams are available to all purchasers of IFB Plus systems. Contact Studio
Technologies’ support, via phone or email,
to obtain the electronic files. The schematics show the graphical representation of all
the electronic components, along with their
electrical value and connections. Traditionally, a problem with schematics has been the
decimal point marking. It either disappears
due to bad printing, or dust, dirt or other
imperfections end up looking like decimal
points. For clarity, Studio Technologies has
adopted a more “European” component
marking scheme. Upon first review it may
seem quite confusing, but it can eliminate
problems. For resistors, the designation “K”
for kilo (1000) has been moved to the decimal point position. A 4700 ohm resistor is
shown as 4K7, rather than 4.7K. An example
for a one percent value, a 49900 ohm resistor would be shown as 49K9. For capacitors,
the letter “r” is simply substituted for the
decimal point marking. A 0.47uf capacitor is
shown as r47uf. Once you are accustomed
to this system you may well prefer it.
Definition of Level
Studio Technologies has opted to use the
dBu designation as it seems to be quite
rational. Using dBm was fine when all audio
line outputs were terminated with 600 ohm
loads. In this way it was easy to say that 0
dBm is 1 milliwatt dissipated in the known
load (i.e., 0 dBm across 600 ohms will measure 0.775 V). In current situations an output
is rarely terminated in 600 ohms; generally
5 k ohms or higher. The dBu designation is
better because it refers to dB referenced to
0.775 V, with no reference to load impedance. This takes into account the current
Issue 8, December 2005 Model 2 User Guide
Page 20 Studio Technologies, Inc.
audio scene where most equipment has a
low output source impedance, and a high
input impedance.
Revising Mains Voltage
The Model 2 can operate from mains power
of either nominal 120 or 220/240 V. Internal
“straps” select the operating voltage. Follow
this section if a change of operating voltage
is required.
The following procedures must be performed by a qualified technician. Operating
the Model 2 with the cover removed exposes the technician to points in the power
input section with hazardous voltages.
1) Ensure that the mains power cord is
removed from the Model 2’s power connector on the back panel.
2) Remove the top cover via the four 6-32
button-head cap screws.
3) Locate the power supply jumper straps.
They are directly adjacent to the power
transformers on the right side of the circuit
board. The circuit board legend shows the
designated locations for the two operating
voltages. From the factory, 0 ohm “resistors” are used for the straps. These resistors
are really just encapsulated jumper wires.
For 120 V operation, two “0 ohm” jumper
straps are installed for each of the three
power transformers (total of six straps). For
220/240 V operation one strap is associated
with each transformer.
4) Review the present configuration. If a
change is required, use a soldering iron and
appropriate tools to revise the straps. With
care, the changes can be made without
removing the circuit board from the chassis.
Removing the circuit board from the chassis is a painful and time-consuming experience. The voltage selection straps can be
removed and installed from the component
(top) side of the circuit board. Ensure that
the ends of the straps do not extend below
the circuit board so as to touch, or even
come near the bottom of the chassis. Failure
to heed this warning can result in safety, reliability, and operational problems. Confirm
that the required straps have been installed
for all three transformers.
5) If the mains voltage was changed,
replace the fuse with the correct value:
0.500 A for 120 V, 0.250 A for 220/240 V.
The fuse is a 5 x 20 mm fast-acting type.
6) Replace the top cover and secure it
using the screws.
7) Before reconnecting mains power, use
an ohm meter to ensure that none of the
straps leads have shorted to metal chassis
below the circuit board.
8) Reconnect the mains power and check
the Model 2 for correct operation.
Dim/Mute Function
From the factory, program audio is set
to mute upon interrupt. If level “dimming”
rather than full muting is desired, a simple
modification can be performed by a qualified technician. Schematic diagrams
should be obtained prior to performing
any revisions.
The following procedures must be
performed by a qualified technician. Operating the Model 2 with the cover removed
exposes the technician to points in the power input section with hazardous voltages.
1) Ensure that the mains power cord is removed from the Model 2’s power connector
on the back panel.
2) Remove the top cover via the four 6-32
button-head cap screws.
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 21
3) Referring to page 2 of the Model 2’s
schematic diagram, find the resistor identification numbers for the dim resistors. From
the factory, resistors are not inserted into
these locations. If resistors are installed,
someone has beaten you to the punch and
already performed this modification!
4) Refer to the chart located on page 2
of the schematic. The chart gives resistor values corresponding to several “dim”
values. The level in dB refers to the program
level drop that will occur during an interrupt.
After selecting and acquiring the desired
resistors, prepare them for insertion into the
circuit board. With care, the resistors can be
added without removing the circuit board
from the chassis. Bend the leads so that
they are ½-inches apart, allowing them to
easily drop into the resistor locations. Trim
the leads so that once inserted and soldered
into the board, the ends of the leads will not
touch the chassis below the board. Again,
ensure that the ends of the straps do not extend below the circuit board so as to touch,
or even come near the bottom of the chassis. Failure to heed this warning can result
in operational problems.
5) Replace the top cover and secure it
using the screws.
6) Reconnect the mains power and check
the Model 2 for correct operation.
Disabling the Auto Answer Feature
In the telephone line mode, telephone
interface 2’s circuitry will automatically go
off-hook upon detecting ringing voltage
on the telephone line. This feature can be
very useful, allowing another site to access
the Model 2 without operator intervention.
There may be cases where this feature is
not desired, and the auto answer function
can be disabled. Referring to page 5 of the
Model 2’s schematic diagram, observe the
ring detection circuit in the lower left section
of the page. If needed contact the factory
to obtain the schematic files. A 5K6 resistor (5600 ohm, 1 watt) is in series with the
optocoupler. Remove this resistor to disable
auto answer. Remember to be careful inside
the cabinet. Disconnect mains power prior
to taking off the cover!
Voice Operated (VOX) Interrupt Release
Time
The VOX interrupt circuit was designed to
have a fast detect time and a relatively long
release time. The fast detect time ensures
that the beginning of a word will not be lost.
The slower release time keeps the circuit
from “modulating” during normal English
language speech patterns. This release time
was scientifically determined—we gathered
a bunch of people in our lab, “listened” to
a number of different release times, then
chose the favorite. Actually, we think the
selected time works quite well. However,
you may have a different opinion. (Do
beans belong in chili? Is Elvis really dead?)
The release time is set using one capacitor. Referring to the right side of page 7 of
the Model 2’s schematic, locate the r33uf
(0.33uf) capacitor connected to the output
of the synchronous half-wave rectifier. Reducing the capacitance to r22 (0.22uf) or r1
(0.1uf) will shorten the release time. Increasing the capacitance to r47 (0.47uf) or 1uf,
will increase the release time. Use safety
precautions if you will be performing this, or
any other, modification. If needed, contact
the factory to obtain the schematic files.
Line Output Nominal Level
The nominal level of the line outputs is
+4 dBu. This should be well-suited for most
professional applications. However, the line
output stage is such that if you require a different nominal output level, you can achieve
Issue 8, December 2005 Model 2 User Guide
Page 22 Studio Technologies, Inc.
it by simply changing one resistor per
channel. Refer to page 9 of the Model 2’s
schematic to observe the two identical line
output stages. If needed contact the factory
to obtain the schematic files. Audio enters
the line driver stage at the internal nominal
operating level of –10 dBu. Each channel’s
composite IFB signal enters a section of
operational amplifier which is configured to
add 8 dB of gain, bringing the nominal level
to –2 dBu. This signal connects to a differential line driver integrated circuit. By the very
nature of a balanced line driver a 6 dB level
boost is achieved. The nominal output level
is now +4 dBu.
You might enjoy a quick explanation of why
6 dB is added by a differential output stage.
We’ll use the example of an input signal to
the differential line driver chip which, upon
taking a “snapshot” view, is swinging positive to a level of +2 V. Observing the balanced output at that very same instant, the
“high” output is swinging +2 V, while the
“low” output is swinging –2 V. The unbalanced input has an excursion of 2 V, but
the balanced output had an excursion of 4
V (+2 V to –2 V). Since voltage gain in dB
is equal to 20 log Vout/Vin, calculating our
example you get 20 log 4/2 = 6 dB! Another
side note, this 6 dB of boost is exactly why
the differential line receiver, used the Model
2’s program input circuits, contains a fixed
6 dB attenuation. The receiver IC gets rid of
the 6 dB gain created by a differential driver,
saving headroom and correctly matching
equipment levels.
Anyway, to modify the nominal output level
requires you to change the gain of an operational amplifier by changing the value
of its feedback resistor. Refer to the chart
on page 9 of the schematic diagrams for
several output levels and the corresponding
resistor values. If you need an output level
that requires less than unity gain from the
op-amp you can do it, but be careful. If you
choose a feedback resistor less than 10 k
ohms a potential stability problem arises. To
eliminate the chance of a problem, connect
a 100pf capacitor in parallel with the feedback resistor to reduce the open loop gain.
As with all changes to internal Model 2
circuitry, taking a safety first approach is
required. Disconnect mains power before
removing the Model 2’s cover.
Setting the Program Input Level Trim Pots
There is no hard and fast rule on how to
best adjust the trim pots associated with the
program inputs. As mentioned in the installation section, the pots are provided simply
to allow the relative levels of the program
input signals to be equalized. This will allow
minimal level changes when an operator is
switching between the four program inputs.
A simple method of using the trim pots to
“calibrate” the program input levels is to
start by connecting an AC VTVM to the line
output of IFB channel 1. Set the front-panel
program level control associated with IFB
channel 1 to the middle position (12 o’clock
or 50 percent of rotation). Set the four program level trim pots to 50 percent of their
rotation. Set the four program sources to
deliver their normal operating signal level to
the Model 2’s program inputs. These signals
could be in the form of alignment tones, or
actual program material. Use the program
select switches to select, one at a time, the
four program inputs. The line output should
give an output level of +4 dBu nominal, with
+8 to +12 dBu probable on peaks. The
actual output level is not important, and a
lower level is much better than a “hotter”
average level. After observing the relative
strengths of the four sources, use one or
more of the trim pots, if required, to get an
equal average level from the four sources.
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 23
If you have a source that is especially low
in level, e.g., –10 dBu, all is not lost. While
getting a –10 to +4 level booster box is a
good idea, the level trim pots can come to
the rescue. On the inputs that have signal
sources of nominally +4 dBu, set their trim
pots fully counterclockwise, reducing their
input sensitivity by approximately 8 dB.
On the inputs that have the –10 sources
connected, set their input trim pots to fully
clockwise, adding 8 dB of input sensitivity. You have now equalized the 14 dB (+4
to –10) level difference using the trim pots!
Using the program input switches, you now
should find relatively equal audio levels. Use
the AC VTVM, connected to IFB channel 1’s
line output, to “tweak” the trim pots.
If you don’t have an AC VTVM handy, or if
you’re not concerned about exact calibration, use one of the LED level meters on the
front panel to set the trim pots. Actually, if
your ears are pretty good use them, and
no meters, to set the trim pots to get equal
levels—that’s what we would do if we were
in the field! Confirm your subjective adjustment with the LED meter and you should
be set.
Setting the Line-level Trim Pot on the
Model 22 Access Station
The Model 22’s line input circuitry allows
balanced and unbalanced signals with a
nominal level of –15 to +10 dBu to be correctly interfaced. The system is designed
to have the Model 22 send interrupt signals
at a signal level of –10 dBu to the Model
2 Central Controller. A level adjustment
trim pot on the Model 22 allows the various signal levels to be adjusted to give the
desired signal strength. Set the trim pot only
after installing the entire IFB system. Start
by connecting an AC VTVM across the two
test points, labeled COM and AUDIO, on the
Model 22’s circuit board. Then, while speaking into the microphone, headset, or other
device that produces the line-level signal,
adjust the trim pot to give an average level
of approximately –10 dBu on the meter.
Peak signals should fall in the –5 to 0 dBu
range. This setting should result in a clean,
clear interrupt signal, without excess compressing by the Model 2’s circuitry.
Model 32 and 33 Output Level
By design, the output level on the Model 32
and Model 33 Talent Amplifiers cannot be
set to fully “off.” While the output level can
be substantially attenuated, it never can be
set for full attenuation. This was to ensure
that talent personnel could never accidentally be fully “isolated” from their program
or IFB source. There may be special cases
where full attenuation is desired. This might
be especially true with the Model 33 Talent Amplifier, where a mix of the two IFB
channels can be achieved. Referring to the
Model 32 or Model 33 schematic diagram,
observe the 200 ohm resistor that is connected between circuit common and the
counterclockwise connection of each potentiometer. This resistor prevents full attenuation. By replacing this 200 ohm resistor
with a “0 ohm” resistor or jumper strap, full
attenuation can be achieved.
Circuit
Descriptions
The following paragraphs describe the
circuitry of the Model 2 Central Controller,
Model 22 Access Station, and Model 32 and
Model 33 Talent Amplifiers.
This information will help you understand
how the units work and, if required, help
identify where a failure may be located.
Issue 8, December 2005 Model 2 User Guide
Page 24 Studio Technologies, Inc.
Please refer to the block diagrams, located
at the end of this guide, and the schematic
diagrams, located in the separate schematic
packet, while reading this material.
Model 2 Central Controller
Power Supplies
The Model 2 contains three independent
power supply circuits. It was felt that reliability would be enhanced by splitting the
circuitry loads, and then optimizing a power
supply for each. Using this scheme, the
Model 2 will run reliably, even with wide
swings in ambient temperature, varying
operating duty cycles, and mains voltage
fluctuations. The result is a unit that should
prove quite hard to kill! The power supplies
use separate step down transformers, the
primary side of each containing two 120 V
windings. This allows them to be configured for nominal mains voltages of 120 or
220/240 V. For 120 V operation the primaries are connected in parallel; for 220/240 V
operation they are connected in series. The
configuration is performed using jumper
straps on the circuit board. For safety, a fuse
is in series with the incoming mains power.
Because the Model 2 is intended for
continuous operation, a power switch
is not included. This serves several purposes: eliminating the chance of a power
switch being accidently turned off, maintaining physical isolation between the nasty
50/60 Hz fields and the sensitive analog
circuitry, and eliminating the physical
space required by a switch.
the power entry connector. The transformer’s secondary is fed to a full wave diode
bridge. The output of the bridge is filtered
with electrolytic capacitors, producing nominal ±19 Vdc.
Two integrated circuit regulators produce
±12 Vdc from the unregulated ±19 Vdc.
Capacitors on the outputs of the regulators
provide stability. Interesting technical note
department: notice that a diode is connected from the output pin of each regulator
to circuit common. These serve to keep the
±12 Vdc rails at, worst case, one diode drop
(0.7 V) away from its ground pin, i.e., the
+12 V regulator will, worst case only, go to
–0.7 Vdc. This is important when supplying
bipolar loads, such as operational amplifiers. Without these diodes the regulators can
“latch-up” upon mains power being applied
or removed. When mains power is initially
applied one of the supplies can “come up”
(get to its operating voltage) sooner than the
other. This voltage is fed back through the
loads (e.g., the op-amps) to the output pin
of the other regulator that is still coming up
to full voltage. Upon seeing this unexpected
opposite polarity voltage on its output pin,
the regulator may get very unhappy, possibly latching into a nonoperating state,
drawing lots of current and burning up! The
protection diodes keep this condition from
happening.
The Model 2 contains an LED indicator light
that displays the presence of mains power
being provided to the unit. The LED is powered by the –12 Vdc power supply bus.
The first power supply generates filtered and
regulated ±12 Vdc. Its transformer has dual
12 V secondaries which are connected in
series. The series connection point provides
circuit common, as well as being strapped
The second power supply generates unregulated nominal +40 Vdc. Its transformer
has dual 12 V secondaries which are connected in series. The 24 Vac, which under
low to moderate loads is quite a bit higher, is
to the metal chassis and the ground pin of
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 25
fed to a full wave diode bridge and a electrolytic filter capacitor which produces nominal
+40 Vdc. The negative pin of the bridge is
connected as the circuit common, as well
as being strapped to the metal chassis and
the ground pin of the power entry connector. The unregulated +40 Vdc is fed to an
integrated circuit regulator which produces
+26 Vdc. This voltage is used by the talent
amplifier output circuit.
The third power supply generates unregulated nominal +25 Vdc and regulated
+18 Vdc. Its transformer has dual 17 V
secondaries which are connected in parallel.
The 17 Vac is fed to a full wave diode bridge
and a electrolytic filter capacitor which produces nominal +25 Vdc. The negative pin
of the bridge is connected as the circuit
common, as well as being strapped to the
metal chassis and the ground pin of the
power entry connector. The unregulated
+25 Vdc is used by the monitor amplifier
and lamp output circuits. The lamp outputs
are used to drive indicator lights in Model 22
Access Stations.
The +25 Vdc is also fed to an integrated circuit regulator which produces +18 Vdc for
use by the access stations. A capacitor on
the output of the regulator provides stability.
The +18 V regulator acts as much as a current limiter as it does a regulated power supply. The regulator limits the amount of short
current that can be drawn by the access
stations. In this way a short-ed access station or related cabling will not bring “down”
or damage the Model 2.
For service purposes, test points are included on all the power supply buses. In
addition, circuit common test points are
included—two on the left side of the board
and one on the right side.
Program Inputs
The Model 2 contains four identical program input circuits. The program inputs
are designed to receive balanced signals
with a nominal level of +4 dBu. A trim potentiometer associated with each program
input allows ±8 dB of input variation. Now
is a good time to note that the Model 2’s
internal operating level is –10 dBu. This was
selected to optimize audio performance—
maximizing the signal-to-noise ratio and allowing ample headroom. The exact purpose
of the program input circuits is to receive
the program audio, separate out hum and
noise, unbalance it, and attenuate it to the
–10 dBu internal operating level. Each circuit
contains a differential input integrated circuit, followed by a variable gain reduction
stage. The program input signals are direct
coupled to the differential (balanced) line
receiver integrated circuits. These devices
have excellent common mode rejection,
low noise, and high slew rate. They contain
two internal 12 k ohm series input resistors,
individually laser trimmed for accuracy. By
design, they provide 6 dB of level reduction.
The nominal –2 dBu signal is capacitive
coupled to an operational amplifier configured as a variable attenuator. The coupling
capacitors are nonpolar type, allowing for
DC voltages of unknown polarity to be
received on the program inputs. The output
of the op-amps have nominal signal levels
of –10 dBu and are direct coupled to the
program select switches. Small capacitors in
the feedback loop of the op-amps ensures
stability at the less than unity gain implementation.
Program Select, Program Muting, and
Program Level Control
Each IFB channel contains a group of six
program select switches. These switches
allow one or more of the program sources
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Page 26 Studio Technologies, Inc.
to be selected from among the four program
inputs and the receive audio from the two
telephone interfaces. During the design
phase, the switches were carefully specified
to allow more than one program source to
be selected at one time. This feature can
prove invaluable in special applications. The
output connections of the switch sections
are connected to operational amplifiers configured as unity gain summing amplifiers.
The output of the summing amplifiers are
connected to sections of SSM-2404 analog
switch, providing program audio muting
during interrupt. These switches perform a
click free off-to-on and on-to-off transition by
internally generating a ramp control signal.
This ensures a very “clean” transition from
program audio to interrupt audio and vice
versa. Optionally, by adding resistors in
parallel with the analog switches, program
“dimming” rather than muting can be implemented.
The output of each analog switch is connected to a potentiometer. The potentiometers exhibit a log (audio) taper, setting the
level of program audio relative to the fixed
nominal –10 dBu internal level of the interrupt audio.
Interrupt Audio Routing and Summing
Two identical circuits control interrupt audio
routing. The composite interrupt audio signals are derived from two operational amplifiers configured as summing amplifiers. Four
audio sources can enter the summing junctions: internal microphone audio, access
station audio, voice activated (VOX) interrupt
audio, and program audio. All audio sources
are controlled by sections of SSM-2404
analog switch. These switches ensure clean,
click-free audio transitions. The outputs of
the interrupt summing op-amps are connected to the compressor circuits.
Compressor Circuits
Interrupt audio is processed using two compressor circuits, one for each channel. Lasertrimed voltage-controlled amplifier integrated
circuits are used to create the compressor
circuits. They provide—along with some operational amplifier integrated circuits, discrete
resistors, capacitors and diodes—everything
required to implement two channels of dynamic range control. The circuit implements
true studio-quality compressors, ensuring
that the talent will get even, intelligible interrupt audio. Even screaming directors will
have a difficult time fooling these circuits!
Internal Microphone
The Model 2 contains an internal electret
microphone, along with front-panel switches
and control circuitry. A microphone preamplifier is created using one section of low noise
operational amplifier. Power is provided to
operate the FET preamp in the mic. The microphone audio signal is capacitive coupled
to the op-amp’s non-inverting input. To provide optimal voice transmission, the capacitor
was selected to roll-off low frequency input
signals; the 3 dB down point is approximately
105 Hz. Two fixed resistors set the gain of the
preamp. The output of the op-amp is capacitive coupled to two sections of SSM-2404
analog switch, part of the interrupt audio
routing circuit.
Voice Operated (VOX) Interrupt
To the Model 2’s designers, the VOX circuitry
is the most interesting as it combines analog
and digital functions to perform a function
that, historically, has not worked very well in
most equipment. Including a VOX circuit was
contemplated with some trepidation. Don’t
get us wrong, it’s not that most VOX circuits
are unusually complicated, it’s just that they
don’t often work very well!
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 27
Audio enters the VOX circuit via a
3-position source select switch. The switch
selects between audio from the two telephone interfaces, and the auxiliary audio
input. The auxiliary audio input circuit is
identical to those used by the program
inputs. The output of the source switch connects to one section of SSM-2404 analog
switch and the input of a bandpass filter.
The bandpass filter allows signals in the 400
to 1400 Hz band to pass, while restricting all
others. This frequency range was selected
to best isolate voice signals, which is the desired VOX interrupt audio source. The 24 dB
per octave slope of the filter is quite effective
in removing signals that could cause false
voice detection. It was felt that providing an
excellent bandpass filter would be the key
to accurate voice detection. While this filter
is more complicated than most VOX circuits
might use, we think it provides the desired
outcome. The output of the filter is sent to
one section of integrated circuit operational
amplifier configured as a synchronous halfwave rectifier. The output of the rectifier is a
DC representation of the energy in the voice
band of the input signal.
A capacitor connected to the output of the
rectifier sets the VOX release time. The
larger the capacitor value, the longer an
absence of voice-band energy is required
for the DC output to fall below the VOX
threshold. The DC voltage from the rectifier
is connected to one section of integrated
circuit comparator. A reference voltage, created by two resistors, sets the comparator’s
switching threshold. A resistor from the output of the comparator to the reference input
provides positive feedback, i.e., hysteresis.
This adds stability to the circuit, preventing
minor variations in the DC input voltage from
causing rapid switching. Since the output of
the comparator is an open-collector type, a
pull up resistor is required. The output of the
comparator is a “voice present” logic signal.
It goes to the logic low state whenever a
voice signal is detected. This logic signal is
connected to one section of the VOX output
select switch. The switch connects the VOX
logic signal to the circuitry associated with
the interrupt logic, program muting, and
status LEDs.
The “voice present” logic signal is also
connected to an inverter gate. The inverted
signal controls the VOX audio analog switch,
mentioned previously. The analog switch
goes to the on state whenever an input
audio signal is detected. The output of the
analog switch is routed by the VOX output
select switch to the interrupt summing bus
for either channel 1 or channel 2.
Telephone Interfaces
The Model 2 contains two telephone interface circuits. Although somewhat similar,
each has unique characteristics. Interface 1
is designed to receive audio from the outside world. Interface 2 can receive audio,
as well as send IFB channel 1 or 2 audio.
In addition, interface 2 contains circuitry to
automatically answer a ringing telephone
line. Each interface contains a mode switch,
allowing the interface to be set to the telephone line or standard audio mode. The
telephone line mode optimizes the interface
for connection to a standard loop start telephone line. This mode is sometimes referred
to as “wet,” a telephone company term
for DC current flowing through a line. The
standard audio mode optimizes the interface for connection to a circuit that does not
have DC current flowing in it. This mode is
sometimes referred to as “dry,” a telephone
company term for a line with no DC current
flowing through it.
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In the telephone line mode, a relay controls if the telco signal is connected to the
interface. In the standard audio mode, the
relay is continually energized. Both interfaces in the telephone line mode provide
a telephone line termination impedance of
approximately 700 ohms. In the standard
audio mode, interface 1 provides a termination impedance of approximately 2200
ohms. This allows an audio source which
uses an operational amplifier output to correctly interface with the Model 2. Interface
2 in the standard audio mode presents a
source impedance of approximately 2200
ohms when sending channel 1 or 2 audio,
and an impedance of approximately 700
ohms when receiving audio.
Now we’ll discuss interface 1 specifics. The
tip and ring of the telephone line enters the
interface via a protection circuit. With the
Model 2, transients are definitely not welcome. A sealed, bifurcated contact relay
controls the connection of the telephone line
to a 600 ohm to 600 ohm coupling transformer. In series with the center taps of the
transformer is a loop current detector. This
circuit is quite interesting, providing both desensitization and over-current protection for
the optocoupler that actually detects current
flow. The output of the optocoupler is conditioned by two resistors and a capacitor to
provide “debounce” of the loop current status signal. An LED, labeled LC, indicates the
presence of loop current. The secondary of
the transformer is connected to one section
of operational amplifier which is configured
as a variable gain buffer. A trim potentiometer allows the gain to be adjusted from 4 to
20 dB. This processes the telephone line audio to get it to the Model 2’s nominal internal
level of –10 dBu.
The interface mode switch controls two
functions: relay status and termination
impedance. In the telephone line mode,
the interface control switch located on the
front panel is active, allowing the operator to
control the relay. In the telephone line mode,
a 1000 ohm resistor is placed across the
secondary of the transformer. This, along
with the op-amp’s 2200 ohm input resistor,
provides the correct termination impedance
for a standard telephone line. In the standard audio mode, the 1000 ohm resistor is
not connected to the circuit, and the 2200
ohm input impedance of the op-amp’s input
resistor provides the input impedance. In
the standard audio mode, the switch is disabled, the relay is held energized, and the
LED labeled STD is lit.
A simple logic circuit implements the telephone interface control function. Two NAND
gates form a flip-flop, the two states being
on-hook and off-hook. In the telephone line
mode, pressing the interface control switch
to the manual off-hook position sets the
flip-flop to the off-hook state, and causing
the relay to energize via one gate of a power
driver integrated circuit. If loop current is
detected the circuit will stay in the off-hook
state. If current is not detected the circuit
will time out and return to the on-hook state.
Pressing the interface switch to the manual
hang-up position forces the circuit to the
on-hook state. In the standard audio mode
the logic circuitry is effectively disabled, the
relay is continually energized, and the LED
labeled STD is lit.
Now we’ll discuss interface 2 specifics. The
tip and ring of the telephone line enters the
interface via a protection circuit and then to
an optical coupler-based ring voltage detection circuit. It provides a logic signal to
indicate when the line is “ringing.” A sealed,
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 29
bifurcated contact relay controls the connection of the telephone line to a 600 ohm
to 600 ohm coupling transformer. In series
with the center taps of the transformer is a
loop current detector. An LED, labeled LC,
indicates the presence of loop current. The
secondary of the transformer is connected
to the input of an operational amplifier which
is configured as a variable gain buffer. A trim
potentiometer allows the gain to be adjusted
from 4 to 20 dB.
Four sections of analog switch set the audio
flow through the interface. Control signals
for the analog switches are derived from
the front-panel audio routing switch, along
with some simple logic gates. One section
of analog switch controls the output of the
receive operational amplifier. It is active only
when the interface is in the receive mode.
This analog switch prevents channel 1 or 2
audio from “feeding back” to other Model
2 circuits when the interface is in the send
mode. The other three sections of analog
switch control the flow of send audio, as well
as adjusting the interface termination impedance. The secondary of the transformer, via
a 1000 ohm resistor, connects to one of the
three remaining sections of analog switch.
In the standard audio mode this switch is always in the on state, keeping the 1000 ohm
resistor effectively connected across the
transformer’s secondary. This resistor, along
with the 2200 ohm resistor on the input of
the receive op-amp, provides a 700 ohm termination impedance. In the standard audio
mode this analog switch is active when the
routing switch is set to send channel 1 or
channel 2 audio. The two remaining analog
switches control if channel 1 or channel 2
audio is connected to the send driver operational amplifier.
The interface mode switch, located on the
back panel, controls two functions: relay status and termination impedance. In the telephone line mode the interface control switch
is active, allowing the operator to control the
relay. In the standard audio mode the switch
is disabled, the relay is held energized, and
the LED labeled STD is lit.
A simple logic circuit implements the telephone interface control function. Two NAND
gates form a flip-flop, the two states being
on-hook and off-hook. In the telephone line
mode, pressing the interface control switch
to the manual off-hook position sets the flipflop to the off-hook state, causing the relay
to energize via one gate of a power driver
integrated circuit. The output of the ring
voltage detector can also set the flip-flop.
If ringing voltage is detected for a sufficient
time period the resistor/capacitor circuit on
the input of the flip-flop is brought to the
logic low state, and the flip-flop is set. If loop
current is detected the circuit will stay in
the off-hook state. If current is not detected
the circuit will time out and return to the
on-hook state. Pressing the interface switch
to the manual hang-up position forces the
circuit to the on-hook state. Notice that there
is what appears to be a “bug” in this circuit
when in the telephone line mode. If the interface control switch is moved to the manual
off-hook position, and then moved immediately to the hang-up position, the phone line
will not “hang up.” This is because approximately three seconds are required for the
ringing detection capacitor to recharge after
being discharged by the manual off-hook
switch action. If a pause of three seconds
occurs, or the interface control switch is
held in the hang-up position for about three
seconds, the telephone line will hang up.
Normal operation will not be affected by this
condition.
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Access Station Interface
The Model 2 contains interface circuitry to
allow up to four Model 22 Access Stations to
be connected. Four types of signals are sent
to and received from the access station: interrupt audio, lamp voltage, control signals,
and +18 Vdc power.
Two identical circuits connect access station
interrupt audio into the Model 2. Each circuit
capacitive couples audio into an operational
amplifier configured as an inverting unity
gain buffer. The output of each op-amp
is connected to one section of SSM-2404
analog switch, part of the interrupt audio
routing circuit. This analog switch produces
a click-free joining of access station audio to
the IFB channels.
Adjustable voltage regulator integrated
circuits are used to provide power for the
pushbutton switch’s status (a.k.a. tally)
lamps located on the access stations. These
regulators were selected because of their
ability to be controlled by a logic signal,
along with their inherent over-current protection.
Three logic signals are ready to be controlled by the Model 22 Access Stations:
channel 1 interrupt control, channel 2 interrupt control, and monitor mute control.
These CMOS-type logic signals are active
low. Pull up resistors maintain a logic high,
as well as providing sufficient current for the
access stations to function correctly. Series resistors and shunt capacitors provide
overvoltage protection when interfacing the
nasty “outside world” signals with the Model
2’s logic gates. Static “zaps” of 8000 volts
shouldn’t damage the Model 2—but please
don’t try to prove us wrong!
Filtered and regulated +18 Vdc provides
power for the access stations. The +18 Vdc
is created by a 3-terminal, integrated circuit
regulator which provides over-current protection. This feature is especially important
as access station cable shorts may be quite
common during installation and testing.
Talent Amplifier Output
Power, IFB channel 1 audio, and IFB channel 2 audio are provided by the talent amplifier output. A low-noise, low-distortion
modulator circuit superimposes channel 1
audio onto DC power. The circuit uses
+26 Vdc from the power supply section
to create a +22 Vdc with channel 1 audio
output. The nominal audio level is –10 dBu,
with over 20 dB of headroom available. The
circuit has over-current protection so that
a shorted cable will not damage the output
circuitry. Another section of operational
amplifier is used to drive channel 2 audio to
the outside world. A resistor and capacitor in
series with the output protects the op-amp
from a short circuit, as well as an accidental
shorting of the power/channel 1 audio connection to the channel 2 connection.
Line Outputs
Each IFB channel has a balanced line-level
output associated with it. The two circuits
are identical. Composite IFB audio at the
internal operating level of –10 dBu is boosted 8 dB by one section of operational amplifier. The output of the op-amp is connected
to a differential line driver integrated circuit.
This integrated circuit provides an electronically balanced, low impedance output
signal. It is capable of driving high signal
levels into 600 ohms or greater. Because of
the nature of a differential driver, an effective
6 dB boost in signal level is achieved, giving a nominal output level of +4 dBu. For
enhanced reliability the output pins of the
integrated circuit are capacitor coupled to
the output connectors. Shorting one side
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 31
of the balanced output to circuit common
(pin 1 of the line output connector) will not
damage the “chip.” Unbalanced operation in
this manner is perfectly acceptable, as long
as the short is made directly at the output
connector.
Monitor Amplifier
An integrated circuit power amplifier provides 4 watts RMS of relatively low distortion audio to drive a monitor loudspeaker. A
switch selects the IFB channel to be monitored. The output of the switch connects
one section of SSM-2404 analog switch
which provides the click-free monitor output
muting function. The output of the analog
switch connects to a log taper potentiometer
via a low-pass filter created using a capacitor and resistor. The wiper of the pot connects to the power amp via a DC blocking
capacitor. Three resistors and a capacitor
provide a half supply bias voltage. The open
loop bandwidth of the power amp is limited
via a capacitor in the feedback loop. The
output of the amp is capacitive coupled to
the output jack. A resistor/capacitor combination connected across the amp’s output
provides protection from oscillation.
Miscellaneous Logic Circuitry
Simple but effective logic circuitry controls
the Model 2’s functions. All gates, with the
exception of the relay drivers, use CMOS
technology for low power consumption
and high reliability. Page 10 of the Model
2’s schematic contains the bulk of the logic
circuitry, and is fairly self-explanatory.
Meter Circuits
Two identical meter circuits monitor the
audio level of the composite IFB channels.
Audio signal enters a meter circuit via an
operational amplifier configured as a halfwave synchronous rectifier. The resulting DC
output is smoothed via a resistor/capacitor
low pass filter. This DC signal is connected
to a specialized meter driver integrated
circuit. It will directly control up to ten LEDs,
lighting them in 3 dB steps. Five of the steps
were selected for Model 2 use. The meter
integrated circuit is set to operate in its bar
graph mode, with the five LEDs connected
in a series arrangement. This arrangement
minimizes current consumption; the same
current is consumed whether one LED, or
all five are lit. The LED current is set to approximately 13 mA by two fixed resistors.
Model 22 Access Station
General Description
The Model 22’s circuitry is contained on
two printed circuit boards. The bulk of it
lies on the electronics board; the two
pushbuttons lie on, you guessed it, the
pushbutton switch board. The boards are
interconnected via a 12-position pin and
socket arrangement. The electronics board
contains power supply, interface, and audio circuitry. The Model 22 interconnects
with the Model 2 Central Controller and, if
present, other Model 22s via 9-pin D-subminiature female connectors. Contained on
the Model 22’s circuits board are two such
connectors, wired in parallel. This allows
“daisy-chaining” of the wiring through one
unit to get to others, without requiring the
use of wiring “splitters.”
Power Filtering, Voltage Dividers, and
Power Supply
Filtered and regulated +18 Vdc is provided
by the Model 2 Central Controller. A diode in
series with the incoming power protects the
circuitry from improper wiring. The +18 V is
used directly by the two 4-section comparator integrated circuits, as well as to create
two reference voltages; +6 V and +12 V.
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These references are created by resistor
divider circuits. The +18 V also passes
through a simple resistor/capacitor low
pass filter. This filtered +18 V, referred to as
+V on the Model 22’s schematic diagram,
is used by the audio circuitry, as well as to
create another reference voltage. This reference, aptly called VREF, is approximately
half of +V and acts as an analog center
voltage point—an artificial “ground.” The
+18 Vdc is also connected to a low-power,
3-terminal regulator. The +5 Vdc filtered and
regulated output voltage is used to provide
power for a Studio Technologies Model 11A
gooseneck electret condenser microphone.
balanced or unbalanced line-level signals.
Unlike the direct coupled configuration used
in the Model 2’s program inputs, capacitors
are used to isolate the audio signal from the
Model 22’s circuitry. The output of the ’2143
connects to a voltage divider circuit consisting of two fixed resistors and a trim potentiometer. This “pad” reduces the line-level
signal to match that of the electret microphone, which is approximately –25 dBu.
The trim pot provides the input level trim
function, allowing a wide range of signal
levels to function correctly. The unbalanced
and attenuated line input signal is boosted
by the preamplifier/output driver op-amps.
Audio Circuitry
The Model 22 allows connection of two different types of audio sources; an electret
microphone, or a line-level signal. A switch
selects which input circuitry is active. In the
mic position, the circuitry is configured to
accept an unbalanced audio source with a
nominal level of –25 dBu. Connected via a
current limiting resistor, +5 Vdc is provided
to operate the FET preamplifier in the electret microphone’s cartridge. Microphone
audio is capacitor coupled to two sections
of low-power operational amplifier which
serve as preamplifier/output driver stages.
The two sections of op-amp are identically
configured as non-inverting amplifiers with
approximately 15 dB of gain, bringing the
signal level to nominal –10 dBu. A note
to you audio purists: the selection of the
LF353 was mandated strictly to address the
power consumption issue, not for its less
than optimal noise figure. The outputs of the
op-amps are connected to two sections of
analog switch.
In the line position the balanced line input
circuitry is active. A differential line receiver
integrated circuit is used to interface
As mentioned two paragraphs previously,
the outputs of the op-amps connect to
sections of analog switch. The outputs of
the analog switches connect to the audio
buses via capacitor/resistor combinations.
The capacitors act as level shifters, moving
the signal from a VREF center to a system
common center point. The series resistor in
the output ensures that in the event of a fault
condition the op-amp will never see less
than a 2000 ohm load. It also protects the
analog switch from transients or shorts to
+18 Vdc in the interconnecting cables. The
analog switches are held in the off condition unless the access station is initiating an
interrupt. At that time audio, via the analog
switches, is placed on the appropriate audio
bus(es).
Logic Circuitry
All logic functions are performed using sections of integrated circuit comparator, along
with two “spare” analog switches. Credit
Mitch, the consulting engineer, for continuing to prove that comparators are about
the handiest “building block” available! The
comparators perform three major functions.
The first function prevents an access station
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 33
from initiating an interrupt on a channel that
is already in the interrupt state. This allows
only one access station at a time to interrupt a channel. This function is operationally
helpful, as well as being required by the
audio bus arrangement. If, for example, two
access stations connect to the bus at once,
the audio level of the two signals would
each drop by 6 dB. This is due to the fact
that the outputs would load each other, creating a voltage divider. The operating state
of the interrupt channels is determined by
monitoring the status lamp voltages. A lamp
voltage is nominally 10 V when an interrupt
channel is idle, 14.5 V when active. The 12 V
reference is used by the comparators as the
lamp voltage “slicing” level. Another section
of comparator is used to “mask” the lamp
signal when the access station has initiated
the interrupt. This prevents an oscillation
condition—can you figure out what it is?
The second function the comparators perform is to activate the interrupt (talk) buses.
When the access station’s interrupt logic
lines go to the interrupt active state the
comparator outputs sink current, pulling the
interrupt buses to the low state. The buses
are normally high, and get pulled low when
interrupt is required.
The third comparator function, if configured upon installation using a slide switch,
causes the Model 2’s monitor output to
mute whenever the access station is initiating an interrupt. This is an important feature,
preventing feedback if the access station
is located in close proximity to the monitor
loudspeaker. Two sections of comparator
monitor the internal interrupt active logic
signals. The 6 V reference is used as the
slicing level. The outputs of the comparators
are connected in parallel, allowing current
sinking whenever one or both of the channels are active. The comparators have open
collector outputs, thus allowing the direct
parallel connection—pretty handy, huh!)
The paralleled comparator outputs are connected, via the configuration switch, to the
monitor mute bus. The bus is normally high,
and gets pulled low by the comparators
when monitor muting is required.
Two analog switches perform a dual role,
provide voltage level shifting, and implementing a delay function. The switches
convert the access station’s interrupt active
logic signal, which is referenced to system
common, to a signal referenced to VREF.
The analog switches which connect audio to
the buses need VREF as their reference. A
combination of a diode, resistor, and capacitor form a delay-on-turn off function. This
allows the SSM-2404 analog switches on
the Model 2 to quietly disconnect the access
station audio buses from the interrupt channel prior to the access station releasing its
connection to the bus. This prevents a nice
loud click from reaching the interrupt output.
Only a delay on turn off is implemented. A
fast off-to-on turn on is required, placing the
access station audio on the bus prior to the
analog switch on the Model 2 quietly ramping to the on state.
Model 32 Talent Amplifier
General Description
The Model 32 Talent Amplifier is a selfcontained module which allows headphones
or ear pieces to be driven with IFB channel
1 or 2 audio from a Model 2 Central Controller. The unit contains circuitry to extract DC
power from the three conductors that carry
power and two channels of audio. Along
with a power amplifier integrated circuit, the
unit contains a channel select switch, power
LED, and level control. The Model 32 contains two 3-pin XLR-type connectors, one
male and one female. The female connector
Issue 8, December 2005 Model 2 User Guide
Page 34 Studio Technologies, Inc.
is intended to interconnect with the Model
2’s talent amplifier output. The male connector is intended to facilitate connection
of additional Model 32 or Model 33s Talent
Amplifiers using standard microphone-type
cables.
The Model 32’s circuitry is contained on two
printed circuit boards, which are interconnected via a 5-conductor flexible jumper
cable. All active circuitry lies on the electronics board; the connectors lie on the connector board—no surprises here!
Power Supply
The Model 2 Central Controller provides
+22 Vdc modulated with channel 1 audio
on pin 2 of the XLR connector. Channel 2
audio is provided on pin 3, with pin 1 supplying common. From the Model 2, the
nominal audio levels are –10 dBu. A
3-terminal adjustable integrated circuit
voltage regulator is configured to provide
+10 V, nominal, from the incoming +18 V
that is modulated with channel 1 audio. The
important characteristic about this circuit is
its constant input impedance characteristic.
The input impedance is fixed at a moderately high value, approximately 500 ohms, and
does not vary appreciably with load. This is
important so that the channel 1 audio signal
is not significantly attenuated, nor distorted
by normal fluctuations in the power draw. An
LED indicator light provides indication that
+10 V power is present.
Power Amplifier
An industry-standard (for good reason)
LM386N-1 power amplifier integrated circuit
is used to drive the audio output. Signal
enters the amplifier circuit via a switch that
selects the input source, either IFB channel 1 or 2. The signals are coupled to the
switch via coupling capacitors, providing
DC blocking, and preventing audio “clicks”
when switching between channels. The
output of the switch is connected, via a low
pass filter, to a log taper audio potentiometer. This pot allows user adjustment of the
output level. A resistor in series with the
potentiometer prevents the output level from
being completely shut off, a useful broadcast feature. A low pass filter on the output
of the amplifier helps to provide stability. The
amplifier’s output is capacitor coupled, via a
series resistor, to the output connector. The
capacitor changes the audio output signal
from being biased at approximately +5 V, to
being biased at signal common. The series
resistor limits the output current in the event
of a shorted output connecting cable.
Model 33 Talent Amplifier
General Description
The Model 33 Talent Amplifier is a selfcontained module which allows headphones
or ear pieces to be driven with IFB channel
1 or 2 audio from a Model 2 Central Controller. The unit contains circuitry to extract DC
power from the three conductors that carry
power and two channels of audio. Along
with a power amplifier integrated circuit,
the unit contains a channel select switch,
power LED, and level control. The Model 33
contains two 3-pin XLR-type connector, one
male and one female. The female connector
is intended to interconnect with the Model
2’s talent amplifier output. The male connector is intended to facilitate connection
of additional Model 33 or Model 32s using
standard microphone-type cables.
The Model 33’s circuitry is contained on two
printed circuit boards, which are interconnected via a 5-conductor flexible jumper
cable. All active circuitry lies on the electronics board; the connectors lie on the connector board—no surprises here!
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 35
Power Supply
The Model 2 Central Controller provides
+18 Vdc modulated with channel 1 audio
on pin 2 of the XLR connector. Channel 2
audio is provided on pin 3, with pin 1
supplying common. From the Model 2,
the nominal audio levels are –10 dBu. A
3-terminal adjustable integrated circuit voltage regulator is configured to provide +10
V, nominal, from the incoming +18 V that
is modulated with channel 1 audio. The
important characteristic about this circuit is
its constant input impedance characteristic.
The input impedance is fixed at a moderately high value, approximately 500 ohms, and
does not vary appreciably with load. This is
important so that the channel 1 audio signal
is not significantly attenuated, nor distorted
by normal fluctuations in the power draw. An
LED indicator light provides indication that
+10 V power is present. A zener diode, in
series with the LED, is used to generate a
5.6 V reference for the circuitry.
Mixer/Output Amplifier
IFB channel 1 and 2 audio is capacitorcoupled to two audio-taper potentiometers.
Resistors in series with the pots ensures
that the audio level can never be fully turned
“off.” The output signals from the pots
connect to an NE55320-type operational
amplifier that is configured as a combining
amplifier. The faithful ’5532 is used both to
mix the audio sources, as well as to drive
the audio output. Signal enters the op-amp
via a switch that selects which of he two input sources are active. Note that the switch
actually selects one source by muting the
other! The op-amp’s output is capacitor
coupled, via a series resistor, to the output
connector. The capacitor serves as a DC
blocking function, changing the output so
as to be referenced to signal common. The
series resistor helps the op-amp maintain
stability, as well as providing protection in
the event of a shorted output connecting
cable.
Troubleshooting
Troubleshooting the Model 2 and related
components is to be performed only by a
qualified technician. Operating the Model
2 with the cover removed exposes you to
hazardous voltages.
Block diagrams are located at the end of
this guide. Schematic diagrams are available upon request. These documents provide assistance during maintenance and
repair. Understanding the schematics and
block diagrams will lead to much better
understanding of the individual circuits and
how they work together (or, in case of trouble, how they don’t work together!)
Model 2 Central Controller
The Model 2 consists of many relatively
simple circuits. Detailed troubleshooting
procedures are difficult to describe because
there are few specific circuits that are prone
to failure. The following paragraphs may
give a few helpful hints if problems do arise.
In all cases, you are welcome to contact
Studio Technologies for technical support.
Wiring Errors
The Model 2 provides +18 Vdc for use by
the Model 22 Access Stations. Up to four
access stations can be connected, drawing
a total maximum of 100 mA nominal from
the +18 Vdc supply. The +18 Vdc is produced by a 3-terminal regulator. Should the
+18 Vdc get shorted to common up to one
amp of current may flow through the regulator. By design, the +18 Vdc regulator does
not use a heat sink. This helps the device
heat faster under excessive current flow,
Issue 8, December 2005 Model 2 User Guide
Page 36 Studio Technologies, Inc.
allowing the internal overpower protection
to shut down the regulator. During the short
circuit condition the power transformer may
vibrate under the strain of excessive current.
Be aware that the monitor output power
amplifier shares this transformer. Should
the +18 Vdc be “dead shorted” the monitor
performance will also suffer.
The talent amplifier output uses currentlimited temperature-sensing components to
create the +22 Vdc modulated with channel
1 audio. Like the +18 Vdc supply for the access station, a shorted talent amplifier output will result in excessive currents, followed
by thermal shutdown. Removing the short
and allowing time for the device to cool will
restore operation.
Power Supplies
Should problems occur with the Model 2,
the power supplies are the first thing to
examine. The unit contains three independent supplies, each with transformer, bridge
rectifier, filtering, etc. Use the test points to
ensure that the correct voltages are present.
Be aware that the test points for the unregulated supply points can vary greatly from
their nominal values.
There are two very common reasons why
a power “rail” can go down: defective electrolytic capacitor and a shorted power bus.
With time, over-temperature, etc., an electrolytic capacitor can fail. The caps used in
the Model 2 are heavily overrated for normal
use, but still could fail. Use an oscilloscope
to examine the unregulated voltage points.
Check for excessive “ripple.” The 3-terminal
regulators are quite reliable and are not a
weak link. A shorted integrated circuit can
draw enough current to cause a regulator to
go into the thermal shutdown mode. Eliminate the short, wait a minute or two, and the
supply should come up again.
Audio Circuitry
The Model 2 uses straightforward audio circuitry throughout. Normal “follow the audio
flow” troubleshooting techniques should
be used. A digital multimeter, oscilloscope,
signal generator, and audio VTVM should be
all that is required.
Model 22, Model 32, &
Model 33
The Model 22 Access Station, the Model
32 Talent Amplifier, and the Model 33 Talent
Amplifier all have fairly simple circuitry. A
careful review of the block diagrams, located at the end of this guide, and schematic
diagrams, available upon request, should
allow rapid problem solving. All share the
condition that all signals, including power,
audio, and logic, begin and/or end with the
Model 2 Central Controller.
The first thing to determine is that the wiring
linking the units together is correct. Ensure
that the Model 22 is getting +18 Vdc for
operation. The Model 32 and Model 33 use
+18 Vdc modulated with channel 1 audio.
Use an oscilloscope to check for both the
DC voltage, and the superimposed audio
signal.
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 37
Specifications
Model 2 Central Controller
Back Mounting Locations:
Provision made for attaching customer-fabricated
back brackets. Use four (two per side) 8-32 machine
screws to attach brackets to the sides of the Model 2
chassis.
Mains Power Requirements: 120 or 220/240 V,
±10%, internally configured, 50/60 Hz, 20 watts
maximum
Mains Connector Type: 3-conductor IEC-type
Mains Fusing: 1
Type: 5 x 20 mm, fast-acting
Rating: 0.5 A for 120 V mains power, 0.25 A for
220/240 V
IFB Channels: 2
Channel Features: 6-position program select switch,
program level control, status LEDs, 5-segment LED
level meter
Audio Switching: solid state, ramping “clickless”
integrated circuit analog switches
General System Audio Specifications:
(Measurements taken from program input to line
output)
Distortion (THD+N): 0.03%, measured at 1 kHz
Frequency Response: better than ±0.5 dB,
20 Hz-20 kHz
S/N Ratio: 67 dB
Program Inputs: 4
Connector: 3-pin XLR-type, female, pin 2 high
Type: electronically balanced, direct coupled
Input Impedance: 24 k ohms
Input Compatibility: can be connected to balanced
or unbalanced signals
Common Mode Rejection: 90 dB @ DC and 60 Hz,
85 dB @ 20 kHz, 60 dB @ 400 kHz (typical)
Nominal Input Level: +4 dBu
Input Trim Control: allows adjustment of input
sensitivity over a ±8 dB range, nominal
Auxiliary Audio Input:
Application: used with voice operated (VOX)
interrupt
Connector: 3-pin XLR-type, female, pin 2 high
Type : electronically balanced, direct coupled
Input Impedance: 24 k ohms
Common Mode Rejection: 90 dB @ DC and 60 Hz,
85 dB @ 20 kHz, 60 dB @ 400 kHz (typical)
Nominal Input Level: +4 dBu
Input Trim Control: allows adjustment of input
sensitivity over a ±8 dB range, nominal
Talent Amplifier Output:
Application: intended to provide power and audio
signals for any combination of up to four Model 32
or Model 33 Talent Amplifiers
Telephone Line Requirements (Telephone Line
Mode): 2-wire, loop start, 10 mA loop current
minimum
Ringing Detection (Telephone Line Mode): 40
to 150 V RMS, 18-68 Hz, ringer equivalence 0.93
Number of Rings to Answer (Telephone Line
Mode): 1, or a fraction of one ring
Interconnection: mates with standard USOC RJ11
jack using standard 6-position modular cable
Disconnect (Telephone Line Mode): manual using
front-panel switch or automatically after detection of
250 mSec, nominal, break in loop current
Interface Control (Telephone Line Mode): switch
on front panel allows manual off hook and manual
hang-up functions
Audio Routing Control: switch on front panel allows
interface to receive audio, send channel 1 audio, or
send channel 2 audio
Receive Audio Level: –15 dBu, nominal
Receive Audio Adjustment Range: ±8 dB
Send Audio Level: –6 dBu, nominal
Indicator Lights: 2, one yellow LED indicates opera-
tion in standard audio mode, one red LED indicates
loop current flowing through interface
Access Station Interface:
Connector Type: 9-pin D-type, female
Application: provides power, audio, control inputs,
and status lamps outputs for up to four Model 22
Access Stations
Power: +18 Vdc, filtered and regulated, current
limited
Audio Inputs: one input per IFB channel,
unbalanced, capacitor coupled, 100 k ohm input
impedance, –10 dBu nominal audio level
Control Signals: 3, one per IFB channel, one for
monitor mute function, all lines active low
Status Lamp Outputs: one per IFB channel, 9.6 Vdc
channel inactive, 14.5 Vdc channel active, nominal,
current limited
Monitor Output:
Application: designed to drive loads of 8 ohms or
greater
Connector Type: ¼-inch 2-conductor phone jack
Output Power:
4 W RMS into 8 ohms, measured
@ 1 kHz, 1% THD+Noise
Internal Microphone:
Type : electret condenser
Frequency Response: 3 dB low frequency rolloff
@ 105 Hz
Interrupt Audio Compressor Circuits:
Number of Circuits: 2, one per IFB channel
Type : dual slope
Ratio in Compression Area: 4 to 1
Ratio in Limiting Area: infinity
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 39
Dimensions (Overall):
19.00 inches wide (48.3 cm)
1.72 inches high (4.4 cm)
11.4 inches deep (29.0 cm)
Mounting: one space in a standard 19-inch rack
Connector: two terminals on screw terminal strip
Dimensions (Overall):
6.4 inches wide (16.3 cm)
1.6 inches high (4.1 cm)
5.2 inches deep (13.2 cm)
Weight: 10.4 pounds (4.7 kg)
Model 22 Access Station
Power Requirements: +18 Vdc, 25 mA maximum,
provided by Model 2 Central Controller
Interconnection:
Contains two 9-pin D-subminiature connectors
(female). One connector intended to link Model 22 to
Model 2 Central Controller. The second connector,
wired in parallel with the first, is designed to allow
“loop through” installation for connection to additional Model 22(s).
0.5 W Equivalent to EAO Switch Corp. 11-903-2,
Wamco 0L1100BPE, Lamptronics AS25-8
Audio Inputs: electret microphone or line level,
switch selectable
Mounting:
Rack mounted using Model 25A 19-inch Rack
Adapter. Panel mounted using Model 28A Panel
Adapter. Custom mounting easily accomplished.
Weight: 0.8 pounds (0.4 kg)
Model 25A 19-Inch Rack
Adapter
Application: intended for use with Model 22 Access
Station and Model 11A gooseneck microphone
Dimensions (with Model 22 attached):
19.00 inches wide (48.3 cm)
1.72 inches high (4.4 cm)
5.2 inches deep (13.2 cm)
Mounting: one space in a standard 19-inch rack
Weight (with Model 22 attached): 1.6 pounds
(0.7 kg)
Microphone Input:
Compatibility: designed only for use with 2-wire
gooseneck electret microphone (2 wires + shield).
Intended input level –25 dBu nominal. Model 11A
gooseneck microphone available from Studio
Technologies.
Microphone Power: +5 Vdc, current limited.
Applied to microphone “high” lead.
Connector: three terminals on screw terminal strip
Line Input:
Type : electronically balanced, capacitor coupled,
compatible with balanced or unbalanced audio
signals
Input Impedance: 24 k ohms
Common Mode Rejection: 100 dB @ DC and
60 Hz, 70 dB @ 20 kHz, 62 dB @ 40 kHz (typical)
Input Level: –15 to +10 dBu, input level adjustable
via trim potentiometer
Issue 8, December 2005 Model 2 User Guide
Page 40 Studio Technologies, Inc.
Model 28A Panel Adapter
Application: intended for use with Model 22 Access
Station and Model 11A gooseneck microphone
Dimensions (with Model 22 attached):
8.0 inches wide (20.3 cm)
2.75 inches high (7.0 cm)
5.2 inches deep (13.2 cm)
Mounting:
Designed to be mounted in a panel opening.
Recommended opening size 6.5 inches wide
(16.5 cm), 1.7 inches high (4.3 cm), 6.5 inches
minimum depth (16.5 cm)
Weight (with Model 22 attached): 1.3 pounds
(0.6 kg)
Model 32 Talent Amplifier
Model 33 Talent Amplifier
Primary Application: intended for use with Model 2
Central Controller
Indicator Light: red LED indicates operation of
internal power supply
Power/Audio Input:
Connector: 3-terminal XLR-type female
Signals: pin 1 common, pin 2 +18 Vdc modulated
with channel 1 audio, pin 3 channel 2 audio
Audio Levels: –10 dBu nominal
Power Requirements: 12 to 35 Vdc, 20 mA normal
operation, 110 mA worst case (shorted headphone
output)
Loop Through Connector:
Type: 3-terminal XLR-type male, connected in
parallel with input connector
Purpose: Intended to be used to connect multiple
talent amplifiers in a “loop through” arrangement
Audio Amplifier:
Output Connector: ¼-inch 2-conductor (monaural)
phone jack
Load: intended for connection to headphones or ear
pieces with impedance of 8 ohms or greater
Source: IFB channel 1 or channel 2, switch
selectable by user
Output Level: user adjustable via log taper
potentiometer
Maximum Output Power: 170 mW into 8 ohms,
@ 1% THD+Noise, 400 Hz
Distortion (THD+N): less than 0.15%, measured
at 400 Hz, 100 mW output power into 8 ohms
Frequency Response: optimized for voice response
and high frequency rejection. Low frequency rolled
off, 3 dB down at 120 Hz. High frequency rolled off,
1.7 dB down at 20 kHz
Dimensions (Overall):
3.6 inches wide (9.1 cm)
4.8 inches high (12.2 cm)
1.9 inches deep (4.8 cm)
Mounting:
Intended for portable applications. Contains integral
belt clip. Optional Model 36 Mounting Adapter allows
Model 32 to be permanently mounted.
Primary Application: intended for use with Model 2
Central Controller
Indicator Light: red LED indicates operation of
internal power supply
Power/Audio Input:
Connector: 3-terminal XLR-type female
Signals: pin 1 common, pin 2 +18 Vdc modulated
with channel 1 audio, pin 3 channel 2 audio
Audio Levels: –10 dBu nominal
Power Requirements: 12 to 35 Vdc, 21 mA normal
operation, 34 mA worst case (shorted headphone
output)
Loop Through Connector:
Type : 3-terminal XLR-type male, connected in parallel
with input connector
Purpose: Intended to be used to connect multiple
talent amplifiers in a “loop through” arrangement
Audio Amplifier:
Output Connector: ¼-inch 2-conductor (monaural)
phone jack
Load: intended for connection to headphones or ear
pieces with impedance of 150 ohms or greater
Source: IFB channel 1 and/or channel 2, switch
selectable by user
Output Level: user adjustable via two log taper
potentiometers
Maximum Output Power: 35 mW into 150 ohms,
@ 1% THD+Noise, 400 Hz
Distortion (THD+N): less than 0.15%, measured at
400 Hz, 32 mW output power into 150 ohms
Frequency Response: optimized for voice response
and high frequency rejection
Dimensions (Overall):
3.6 inches wide (9.1 cm)
4.8 inches high (12.2 cm)
1.9 inches deep (4.8 cm)
Mounting:
Intended for portable applications. Contains integral
belt clip. Optional Model 36 Mounting Adapter allows
Model 33 to be permanently mounted.
Weight: 0.6 pounds (0.3 kg)
Weight: 0.6 pounds (0.3 kg)
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 41
Model 36 Mounting Adapter
Application: intended for use with Model 32 and
Model 33 Talent Amplifiers
Dimensions (overall, with Model 32 or 33
attached):
5.4 inches wide (13.7 cm)
4.8 inches high (12.2 cm)
2.0 inches deep (5.1 cm)
Specifications and information contained in this User
Guide subject to change without notice.
Issue 8, December 2005 Model 2 User Guide
Page 42 Studio Technologies, Inc.
Figure 1. IFB Plus Series Model 2 Central Controller Rear Bracket Mounting Hole Location
(for fabricating customer-provided brackets)
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 43
Figure 2. IFB Plus Series Model 25A 19-Inch Rack Adapter
(used with Model 22 Access Station and Model 11A Gooseneck Microphone)
Issue 8, December 2005 Model 2 User Guide
Page 44 Studio Technologies, Inc.
Figure 3. IFB Plus Series Model 28A Panel Adapter
(used with Model 22 Access Station and Model 11A Gooseneck Microphone)
Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 45
Figure 4. IFB Plus Series Model 22 Access Station Overall Dimensions
Issue 8, December 2005 Model 2 User Guide
Page 46 Studio Technologies, Inc.
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