Studio Technologies 750 User Manual
Model 750
Model 750 Audio Mixer
User Guide
Issue 3, May 1997
This User Guide is applicable for serial numbers 201 and later.
© 1997 by Studio Technologies, Inc., all rights reserved
5520 West Touhy Avenue
Skokie, Illinois 60077 U.S.A.
Telephone (847) 676-9177
Fax (847) 982-0747
www.studio-tech.com
50165-597, Issue 3
Model 750
Table of Contents
Foreword
Introduction .................................................................. 7
System Features.......................................................... 8
Installation .................................................................... 10
Configuration ............................................................... 16
Operation ..................................................................... 23
Technical Notes ........................................................... 36
Specifications............................................................... 38
Appendix A-1 ............................................................... 40
Appendix A-2 ............................................................... 41
Block Diagrams
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 3
Model 750
Foreword
With all the pressures in life to get things done faster, work more hours, finish one job so
you can get on to the next, its rare to get a chance to try and do something really right.
Thats what Studio Technologies attempted to do with the Model 750 Audio Mixer. The
goal was simple, to make a great audio mixer for ENG, SNG, and mobile broadcast applications. By great we meant not just meeting specifications or price issues, but to address
the real needs of personnel out in the field. And not only to meet their immediate needs,
but to provide resources that they can adopt and use in the future. While getting something perfect is impossible, I can say that I'm very pleased with how the product turned
out, and hope that you share my enthusiasm. I think that you'll find the Model 750 a pleasure to use, providing the resources required for efficient, easy-to-use, day-after-day
operation.
A special thanks to Tony Raven, Chief Engineer at Shook Electronics USA. It was Tony
who suggested that Studio Technologies create an audio mixer for mobile broadcast
applications. He supported his idea by spending many hours with me on the telephone,
reviewing lists of features, making comments, and keeping us focused on meeting the
needs of field personnel. Thanks again Tony!
Many other broadcast professionals contributed their time, expertise, and good ideas to
help make the Model 750 what it is. Without their help the unit would definitely not be as
finely tuned as it is. Thanks to you all!
Many people at Studio Technologies contributed toward making the Model 750 a reality.
Fred Roeck did the mechanical design and prepared the production documentation.
Carrie Loving designed the graphics and provided engineering CAD support. Al Lux designed the printed circuit boards. Mitch Budniak, Ace Consulting Engineer, designed the
power supply and did several overall reality reviews. Jim Cunningham, with assistance
from Gary Hebert at THAT Corporation, designed the limiter/compressor circuit. Joe
Urbanczyk designed the computerized test routines for the Audio Precision System One
and worked on agency approvals. Larry Leviton wrote the software that drives the itty-bitty
micro-controller in the Setup section800 bytes in C! Minas Serbekian and his crew built
the prototype units, ensuring that the final design was "manufacturable."
Please contact me with your questions, comments, and suggestions. I can be reached by
voice at (847) 676-9177, fax at (847) 982-0747, or via the Internet @ www.studio-tech.com.
Sincerely,
Gordon Kapes
President
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 5
Model 750
Model 750
Introduction
The unique requirements of electronic
news-gathering (ENG) trucks, satellite
news-gathering (SNG) trucks, and specialized remote broadcast applications
demand more than just a general-purpose
audio mixer. Thats why Studio Technologies created the Model 750, a special
piece of equipment for a special application. Only by taking the time to speak with
industry professionalsdesigners of ENG
and SNG vehicles, along with numerous
field technical personnelwere the necessary specifications established. Using that
information as a road map to guide the
process, additional features, operating
ergonomics, and overall look-and-feel
issues were addressed. The result is a
high-performance, easy-to-use audio
mixer expressly designed to go on-theroad.
What This User Guide Covers
This User Guide is designed to assist you
when installing, configuring, and using the
Model 750 Audio Mixer. A limited amount
of background technical information is
also provided. Should you require detailed
technical information please refer to the
Model 750 Service Guide. The Service
Guide contains detailed service information, including schematic diagrams. The
Service Guide is not shipped with each
Model 750, but is available from the factory upon request.
Figure 1. View of Model 750 Front Panel
Figure 2. View of Model 750 Back Panel
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 7
Model 750
System Features
Mic/Line Inputs
Four transformer-coupled input channels
are provided for connection to microphone or line-level signals. The circuitry is
ruggedized for reliability under tough
operating conditions. A full set of features
is provided for each input channel, including selectable input sensitivity, +48V
phantom power, low-cut filter, phase
reverse, level control, and pan pot. For
operator assistance, a bi-color LED provides signal present and peak level indication. Two illuminated push-button switches
control the routing of the input signal to
the main and auxiliary (AUX) audio buses.
For convenience the four mic/line input
sensitivity buttons, like all the Model 750s
operator controls, are located on the front
panel.
Stereo Line Inputs
Two stereo line-level input channels are
provided, and are intended for connection
with audio signals associated with video
playback. The channels support real
world broadcast applications, where left
and right signals may, or may not, be an
actual stereo pair. Routing buttons allow
each left and right input to be assigned to
the left output bus, to the right output bus,
to both output buses, or muted. A level
control, along with a bi-color signal
present/peak LED, supports each left
and right input. Two illuminated buttons
control the routing of the signals to the
main and AUX audio buses.
Main Output
Signals from the mic/line input channels,
stereo line input channels, and the setup
section (discussed later) combine to
create the main audio output. The main
output bus is stereo, but can be used to
create two independent monaural audio
mixes. Two studio-quality, stereo limiter/
compressor circuits are provided to control the dynamic range of the main audio
bus. Far from a simple clipper, each
limiter/compressor circuit utilizes a sophisticated laser-trimmed voltage-controlled
amplifier (VCA) integrated circuit for quiet,
low-distortion operation. For installation
flexibility, three sets of stereo output circuitry provide access to the main stereo
bus: two are electronically balanced, linelevel; the third is transformer-balanced,
with its output level selectable for mic or
line.
AUX Output
The Model 750 provides a second stereo
audio bus that can be configured to operate in either of two modes: as an auxiliary
(AUX) output or as a monitor interrupt
function. In the AUX output mode special
applications are easily supported, such as
creating a mix-minus feed or serving as
a second stereo output for on-air use.
Signals from the mic/line input channels,
stereo line input channels, and the setup
section can be assigned to the AUX bus.
Access to the AUX bus is provided by
means of an electronically-balanced, linelevel output.
In most cases, however, the AUX interrupt
mode will be selected, allowing the normal
monitor source to be interrupted and the
AUX signal monitored in its place. Model
750 operators will find this function extremely useful, enabling one or more of
the input channels, or the setup section,
to be monitored without interfering with
signals assigned to the main output bus.
Two DIP switches, accessible from the
Issue 3, May 1997 Model 750 User Guide
Page 8 Studio Technologies, Inc.
Model 750
front panel, allow the interrupt function to
be configured; one switch selects whether
the monitor speaker output will be interrupted by the AUX signal, the other
whether the headphone output will be
interrupted.
Configurable Outputs
Four monaural, balanced, line-level outputs can serve in a wide range of installation-specific applications. DIP switches,
located on the back panel, are used to
select from the five available signal
sources: main output left, main output
right, AUX output left, AUX output right,
and mic/line input channel direct. The DIP
switches can be set to provide additional
stereo outputs, create monaural outputs
from the main or AUX outputs, or to provide direct outputs from the mic/line input
channels.
and an externally-provided mix-minus
signal. A mono function allows the monitored audio source to be checked for
phase cancellation problems and acceptable sum-to-monaural performance.
The AUX interrupt mode allows monitoring
of selected input channels and the setup
section without interfering with normal
operation of the main audio bus. For
operator assistance, the meters can be
configured to monitor the AUX bus whenever the AUX interrupt mode is active.
Mechanical VU-type meters, along with
peak level LED indicators, provide a userfriendly indication of audio-signal levels.
Fanatics for addressing the details, Studio
Technologies has even eliminated the
issue of burned-out meter-scale illumination by implementing a solid-state, LEDbased lighting scheme.
Monitor Section
Reminiscent of much larger audio consoles, the Model 750s monitor section
gives the operator an extensive set of
resources. Separate stereo outputs and
level controls are provided for connection
to a monitor amplifier and headphones. In
addition to monitoring the main and AUX
audio buses, provision has been made for
monitoring two external audio sources.
These stereo inputs are intended to be
connected to off-air, microwave, or satellite receivers. External Monitor Input 1 is
compatible with +4 signals, while External Monitor Input 2 can be set, using a
front-panel DIP switch, for 10 or +4
compatibility.
Multiple sources can be simultaneously
selected for monitoring. This can prove
useful by allowing, for example, an operator to simultaneously monitor a local mix
Setup Section
Unique to the Model 750 are the features
provided by the setup section, the highlight being the ability of the operator to
record and playback three voice identification segments. The Left+Right segment
allows a voice message of up to 12 seconds in length to be recorded, with playback going to the left and right output
channels. The Left Only and Right Only
segments allow recording of up to 4 seconds each, with playback going only to
their respective channels.
The voice segments can prove useful,
allowing the facilities that receive the
vehicles signal (generally provided by
microwave or satellite links) to quickly
learn more about the origin of the transmission. Information such as vehicle
identity, physical location, the names of
technical and on-air personnel, mobile
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 9
Model 750
telephone numbers, and left and right
channel identifiers can easily be sent.
The voice audio source is selectable, with
the choices being either the internal microphone or an externally-connected signal.
The setup section also contains a digitallycontrolled sine-wave generator which,
under software control, produces a
0-level single-tone reference and multistep tone sequences. The single-tone
output is selectable for either 400Hz or
1kHzthe two industry-standard frequencies. The tone sequences allow frequency
response testing of a transmission chain,
with the choice of either a basic or an
advanced sequence. In the basic setting
a 1kHz 0-level reference is generated,
followed by individual 10kHz, 1kHz, and
100Hz tones. In the advanced setting a
1kHz reference is generated, followed by
14 individual tones, ranging from 15kHz to
50Hz. At the end of each sequence a left/
right channel identifier is generated. It
consists of a short burst of 700Hz sent
only to the left channel, followed by two
short bursts of 700Hz to the right channel.
While the voice and tone setup signals can
be assigned to the main and AUX output
buses, a direct line-level stereo output is
also provided. This allows the generation
of setup and test signals without interfering with normal operation of the main and
AUX audio buses.
Talk Back
The Model 750 includes a talk back
function. Two push-button switches allow
the selected voice source to be routed to
the setup direct output and/or the main
and AUX outputs.
Mounting, Connectors, and Mains
While the Model 750 is loaded with features, it only requires two spaces in a
standard 19-inch rack. XLR-type and
¼-inch 3-conductor audio connectors are
used for audio interconnection. Mains
power is factory configured for 100, 120,
or 220/240V, 50/60Hz.
Installation
In this section you will be installing the
Model 750 Audio Mixer in an equipment
rack. Input and output connections will
be made using the Model 750s multitude
of jacks. AC mains power will then be
connected.
During normal operation, no access to the
back panel of the Model 750 is required.
However, there are five trim potentiometers that may need to be adjusted prior to
the Model 750 going into service, depending on the requirements of the specific
installation. Since part of the calibration
process involves observing the frontpanel-mounted VU meters, you may want
to perform the calibration process on-thebench, prior to mounting the Model 750
in an equipment rack. In addition, there
are a number if DIP switches that may
need to be set. Refer to the Configuration
section of this guide for details.
Realistically, as long as you can get the
assistance of another person for about
5 minutes, go ahead and install the Model
750, wire it up, and then worry about the
calibration. You will need a highly-skilled
and articulate helper, capable of describing the position of the VU meters indicators with words such as: too high,
down a little, almost, up a tad,
and OKyouve got it!
Issue 3, May 1997 Model 750 User Guide
Page 10 Studio Technologies, Inc.
Model 750
Locating the Unit
Physical access and mechanical noise
are the primary factors when choosing a
mounting location. By its very nature as an
audio mixer, an operator will need easy
access to all the controls, as well as good
sight lines to the meters. In most cases the
internal microphone, located on the upper
right side of the front panel, will be used,
so be careful that you dont have a cooling
fan blowing directly onto it.
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
that radiate strong AC fields is an excellent
idea. The left side of the Model 750, which
contains the microphone preamplifiers, is
the area most sensitive to noise pickup.
Also, it is possible that the Model 750s
power transformer may radiate energy into
adjacent equipment. It is located at the
right, bottom rear corner of the unit, and
may need to be physically isolated from
sensitive equipment, especially videotape
recorders.
Balanced Connections
(Input & Output)
Ring ()
Tip ( + )
Sleeve
(Shield)
(Switchcraft No. 297, Neutrik NP3C, or equivalent)
Figure 3. Recommended phone plugs
Dont be concerned about our use of
phone jacks as part of a professional
product. The jacks used feature goldplated contacts for high reliability. As
much as we wanted to, there was simply
no room in only two rack spaces to use all
XLR-type connectors.
Caution: For reliable audio interconnection, the phone plugs you use must
comply with industry-standard RS-453.
Switchcraft No. 297 or Neutrik NP3C
are highly recommended.
Mounting
The Model 750 is intended for rack mounting, requiring two standard rack spaces
(3.5 inches). It weights approximately 14
pounds (6.4kg), and operates on either
100, 120, or 200/240V, 50/60Hz. The unit is
secured to the front mounting rails of an
equipment rack using two mounting
screws per side.
Mic/Line Inputs
The Model 750 contains four input channels that are compatible with microphone
or line-level sources. Each of the four
Mic/Line inputs use a single 3-pin female
XLR-type connector. Prepare the mating
connectors (males) so that pin 2 is signal
high (+ or hot), pin 3 is low ( or cold),
and pin 1 is shield.
It is expected that the mic/line inputs will
Audio Inputs and Outputs
The Model 750s audio input and output
connections are made using 3-pin XLRtype and ¼-inch 3-conductor phone jacks.
be wired to input/output connector panels
located on the side or rear of the vehicle.
Its also nice to have one or two of the
inputs multed to connectors inside the
vehicle, allowing the quick connection of
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 11
Model 750
microphone or line-level signals. To minimize noise and other problems, it is recommended that the mic/line signals arent
wired by way of points on the patch bay.
Stereo Line Inputs
The Model 750 contains two stereo linelevel input channels. They are intended
for connection to sources located within
the vehicle, specifically the audio output
channels from videotape machines. The
direct-coupled, electronically-balanced
circuitry has extremely low noise, high
common-mode rejection, but no special
protection from overvoltage or static.
(Dont get us wrongthe stereo line input
circuitry is better than whats found on
most audio mixers available today
theyre just not as tough as the mic/line
inputs!) The stereo line input circuitry is
intended for connection to balanced, linelevel signals with a nominal operating level
of +4dBu.
Each stereo line input channel uses two
3-pin female XLR-type connectors. Prepare the mating connectors (males) so
that pin 2 is signal high (+ or hot), pin 3 is
low ( or cold), and pin 1 is shield. With an
unbalanced source connect pin 2 to high
(+ or hot), and pins 1 and 3 to shield. If
connecting to an unbalanced source in
this manner results in hum or noise, connect pin 2 to high (+ or hot) and pin 3 to
shield; leave pin 1 unterminated. For
flexibility of use it is recommended that the
stereo line input connections be routed
through points on the patch bay.
Main Outputs
The Model 750 contains three sets of
stereo outputs associated with the main
audio bus. Main Outputs 1 and 2 are
electronically balanced, have a nominal
level of +4dBu, and are capable of driving
balanced or unbalanced loads of 600
ohms or greater. Main Output 3 is transformer balanced, with its output level
selectable, using a front-panel button, for
microphone or line level. It is capable of
driving full level into balanced or unbalanced loads of 2k ohms or greater.
Main Output 1 and 2
Main Output 1 and 2 are intended for
connection to devices located within the
vehicle. It is recommended that either
Main Output 1 or 2 be used as the on-air
source. Each output utilizes two ¼-inch
3-conductor phone jacks for interconnection. Prepare the mating connectors
(plugs) so that tip is signal high (+ or hot),
ring is low ( or cold), and sleeve is shield.
Whether these outputs are wired via the
patch bay will depend on the specific
installation.
Note that while the electronically-balanced
output circuitry is capable of driving loads
of 600 ohms or greater, the output level
will drop slightly as the load impedance
approaches 600 ohms. A 0.5dB difference
in output level can be expected as the
load impedance changes from 10k ohms
to 600 ohms.
While balanced operation is preferred,
unbalanced operation is not a problem
for the electronically-balanced output
circuitry. To connect to an unbalanced
load connect the tip to high (+ or hot),
and both the ring and sleeve to shield. For
optimal unbalanced operation, it is important to connect both the ring and sleeve
together on the plug that mates with the
Model 750.
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Page 12 Studio Technologies, Inc.
Model 750
Main Output 3
Main Output 3 has been optimized for
connection to devices located external
to the vehicle. This ruggedized, transformer-coupled stereo output is selectable, using a front-panel button, for
microphone or line level. It is recommended that Main Output 3 be wired, via
points on the patch bay, to the vehicles
input/output connector panel. Two 3-pin
male XLR-type connectors are used for
interconnection. Prepare the mating connectors (females) so that pin 2 is high
(+ or hot), pin 3 is low ( or cold), and pin
1 is shield.
Main Output 3 was specifically designed
to provide a fully-isolated output with good
sonic performance. It was NOT designed
to drive full level into 600 ohm loads! While
a 600 ohm load can be connected without
damage, a 2dB level drop should be
expected. Used with 2k ohm or greater
loads, Main Output 3 will provide its full
specified performance.
AUX Output
A stereo output is provided to give access
to the Model 750s AUX audio bus. It is
intended that the AUX audio output will be
connected to devices within the vehicle.
The electronically-balanced, line-level
outputs have a nominal level of +4dBu,
and are capable of driving balanced or
unbalanced loads of 600 ohms or greater.
Two ¼-inch 3-conductor phone jacks
used for interconnection. Prepare the
mating connectors (plugs) so that tip
is signal high (+ or hot), ring is low ( or
cold), and sleeve is shield. To connect to
an unbalanced load connect the tip to
high (+ or hot), and both the ring and
sleeve to shield.
Even if there are no immediate plans
to connect the AUX audio outputs to
external devices, it is recommended that
they be terminated on patch points. In this
manner they will be accessible for those
one-in-a-million situations that seem to
arise far too frequently!
Configurable Outputs
Each of the four configurable outputs is
electronically balanced, has a nominal
level of +4dBu, and is capable of driving
balanced or unbalanced loads of 600
ohms or greater. The outputs are intended
to be used only within the vehicle. DIP
switches are associated with each
configurable output. Details on using
these switches can be found in the Configuration section of this guide.
Each of the configurable outputs is accessible via a ¼-inch 3-conductor phone jack.
Prepare the mating connectors (plugs) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. To connect to an unbalanced load connect the
tip to high (+ or hot), and both the ring
and sleeve to shield.
It is highly recommended that the configurable outputs be routed via the patch
bay. Or, if there are no immediate needs
for these outputs, be certain to terminate
them on the patch bay for future use.
Sometime, somewhere, they will get used.
Setup Direct Output
Direct access to the setup sections voice
and tone signals is provided by means of
a stereo line-level output. As one of the
unique features of the Model 750, it should
definitely be utilized! This output is intended to be connected to the audio input
associated with the video/audio switchers
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 13
Model 750
bars and tone position. For maximum
flexibility, it is recommended that these
signals be connected by way of the patch
bay. In this way they can be accessed for
test and calibration purposes.
The electronically-balanced, nominal
+4dBu outputs are accessible via two
¼-inch 3-conductor phone jacks. The
circuitry is capable of driving balanced or
unbalanced loads of 600 ohms or greater.
Prepare the mating connectors (plugs) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. To connect to an unbalanced load connect the tip
to high (+ or hot), and both the ring and
sleeve to shield.
Two trim potentiometers are used to set
the output level of the setup direct output.
Refer to the Configuration section of this
guide for details.
External Monitor Inputs
Two external audio signals can be connected, allowing them to be monitored
independently of the internal audio buses.
It is intended that the external monitor
inputs will be connected to outputs associated with off-air, microwave, or satellite
receivers. Both inputs are stereo, electronically balanced, and interface using
¼-inch 3-conductor phone jacks. External
Monitor Input 1 is compatible with signals
that have a nominal level of +4dBu. External Monitor Input 2 is compatible with
10dBu or +4dBu signals. A DIP switch,
located on the front panel of the Model
750, is used to set the input sensitivity.
Refer to the Configuration section of this
guide for configuration details.
As an installation aid, a connection made
only to the jack labeled L/MONO routes
the signal to both the left and right input
circuitry. This allows a monaural signal to
be monitored in a 2-channel mono format.
If you wish to have a mono input connected only to the left input, simply insert
an unterminated plug into the associated
right input jack. This will break the normal connections that link the left and right
input circuits.
Prepare the mating connectors (plugs)
so that tip is signal high (+ or hot), ring is
low ( or cold), and sleeve is shield. With
an unbalanced source connect the tip to
high (+ or hot), and both the ring and
sleeve to shield. If connecting to an unbalanced source in this manner results in
hum or noise, connect tip to high (+ or
hot) and ring to shield; leave the sleeve
unterminated.
Monitor Output
The monitor output is designed for connection to an audio power amplifier associated with a pair of monitor loudspeakers.
In mobile applications it may prove common for loudspeakers with internal amplifiers, such as those from Fostex, to be
connected to the Model 750s speaker
output. The outputs are electronicallybalanced, line-level, and can drive balanced or unbalanced loads of 600 ohms
or greater.
The monitor output utilizes two ¼-inch
3-conductor phone jacks for interconnection. Prepare the mating connectors
(plugs) so that tip is signal high (+ or hot),
ring is low ( or cold), and sleeve is shield.
To connect to an unbalanced load connect the tip to high (+ or hot), and both
the ring and sleeve to shield. Whether
these outputs are wired via the patch bay
will depend on the specific installation.
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Page 14 Studio Technologies, Inc.
Model 750
Record/Talk Back External
Audio Input
The Model 750 contains an internal microphone which is used by the setup section
for voice recording, as well as by the talk
back functions. In special cases it may be
desirable for an external source of audio
to be used in its place. A ¼-inch 3-conductor phone jack allows the connection
of a line-level signal, with a nominal level
range of 20 to +4dBu.
Prepare the mating connector (plug) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. With an
unbalanced source connect the tip to high
(+ or hot), and both the ring and sleeve
to shield. If connecting to an unbalanced
source in this manner results in hum or
noise, connect tip to high (+ or hot)
and ring to shield; leave the sleeve
unterminated.
It is intended that a microphone associated with an intercom system headset will
serve as the external audio source. Intercom stations often provide an amplified,
line-level version of the mic signal. It is
often referred to as a hot microphone
output.
A trim potentiometer is associated with the
external audio input, allowing adjustment
of the input sensitivity. Refer to the Configuration section of this guide for details.
Information is also provided on how to set
the front panel DIP switch to select the
external audio input.
Connecting AC Mains Power
Figure 4. Detail of back panel showing voltage
configuration chart
The Model 750 is factory configured to
operate from AC mains power of either
100, 120, or 220/240V, 50/60Hz. In most
cases, units shipped to North America
are factory configured for 120V operation
and are supplied with an internal 0.5A,
5 x 20mm fast-acting fuse. Units configured for 100V also utilize a 0.5A fuse. Units
configured for 220/240V operation are
supplied with a 0.25A, 5 x 20mm fastacting fuse.
Before connecting the Model 750 to mains
power, determine the actual mains voltage
and confirm that the Model 750 has been
configured correctly. As could be expected, an incorrect mains configuration
could seriously damage the unit.
Should it be necessary to change the
units operating voltage refer to the Service Guide, a separate document available
upon request from Studio Technologies.
Model 750 User Guide Issue 3, May 1997
Studio Technologies, Inc. Page 15
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