Studio Technologies 742A User Manual
Audio Mixer
User Guide
Issue 1, July 2012
This User Guide is applicable for serial numbers:
M742A-01151 and later
© 2012 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
50267-0712, Issue 1
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Audio Mixer
Table of Contents
Introduction ................................................................... 5
System Features ........................................................... 6
Configuration Review .................................................... 9
Installation ..................................................................... 10
Post-Installation Calibration .......................................... 17
Operation ...................................................................... 18
Technical Notes ............................................................. 24
Specifications ................................................................ 28
Block Diagram
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Introduction
What This User Guide Covers
This User Guide is designed to assist you
when installing, configuring, and using
the Model 742A Audio Mixer. Additional
background and technical information is
also provided, as well as a product block
diagram included at the end of this guide.
System Overview
The Model 742A Audio Mixer is expressly
designed for use in electronic-news-gathering (ENG) vehicles and small production truck applications. Many other fixed
and mobile audio production, on-air, and
utility applications can also be supported.
Model 742A features include four mic/line
inputs with +48 volt phantom power, four
line inputs, two output buses, LED level
metering, flexible monitoring, and AC and
DC powering. The Model 742A was specifically designed as a dual-channel audio
mixer, rather than as a typical stereo device. This approach makes it excellent for
“dual-path” on-air applications. As a utility
mixer the Model 742A can create two independent monaural signals, excellent for
general signal monitoring, talent cueing,
or intercom support uses.
The Model 742A is targeted for use by
operators with numerous audio sources
that need to be quickly and reliably adjusted and routed to create one or two
independent audio mixes. In these days
of operators having too many tasks to
handle, and too little time to do them, the
Model 742A is a refreshing combination
of performance and simplicity. Using their
experience in mobile broadcast applications, the engineers at Studio Technologies were able to design the Model 742A
to include all the crucial features required
to meet the needs of fast-paced newsgathering operations, while still providing
the operator with an easy-to-use product.
The hallmarks of the Model 742A are
application flexibility, simplicity in use,
audio quality, and long-term reliability.
For powering, the unit can be directly
connected to an AC mains source. A true
“universal input” design that’s intended
Figure 1. Model 742A Front Panel
Figure 2. Model 742A Back Panel
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for worldwide use, the input voltage can
range from 100 to 240 volts, 50/60 Hz. A
source of 10 to 18 volts DC can also be
connected to power the Model 742A. If
both AC and DC sources are connected,
the AC source will power the unit while the
DC source will serve as a backup. In this
way a battery can be connected, ready to
serve in a standby capacity.
A carefully selected feature set ensures
that the Model 742A will be a “team
player” when it is integrated into an audio
system. A number of internal configuration
“jumpers” allow the unit’s performance
to be tailored to the needs of specific
installations. And while there’s flexibility on
the inside, the operator is presented with
an easy-to-use set of front-panel controls
and indicators—the operator typically
never has to access the back panel to
operate the unit. In this way the goal of
delivering successful on-air and production audio, day-after-day, can best be
achieved. The Model 742A’s audio quality
is “pro” throughout. The components were
carefully selected to deliver low-noise, lowdistortion performance.
Long-term reliability was also part of the
Model 742A’s design criteria. To that end
the unit’s enclosure is made of lightweight
yet rugged aluminum with a steel front
panel for overall rigidity which adds to
rack-mounting stability. On the inside,
a conservative circuit design combines
state-of-the-art integrated circuits for sonic
quality with numerous filtering and protection components for long-term reliability.
System Features
Mic/Line Inputs
Four input channels are provided for connection to microphone or line-level signals. The electronically balanced circuitry
is “ruggedized” for reliable operation
under tough operating conditions. The
low-noise, low-distortion, high-headroom
audio performance is what’s expected
of sophisticated “pro audio” equipment.
Features provided for each input channel
include an input sensitivity button, rotary
level control, signal level status LED, and
output bus assignment switch. To support
condenser microphones, the four mic/line
inputs can be individually configured to
supply 48 volt phantom power. Four DIP
switches, accessible on the back panel,
allow individual channel phantom power
on/off control.
For ease of use, each input’s rotary level
control sets the gain of the input circuit as
well as the level being sent to the selected
main output bus (or buses). For convenience the input sensitivity button, as with
the other Model 742A operator controls,
is located on the front panel. For operator
assistance, a dual-color level status LED
provides signal present and peak level
indication.
Associated with each mic/line input channel is a 3-position output bus assignment
switch. This allows each input to be assigned to bus 1, bus 2, or both, quickly
creating two fully independent audio
“feeds.” Bus assignment switches, rather
than the more-typical “pan pots,” were
specifically selected for the Model 742A’s
design. This allows a more positive assignment of an input source to the desired
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output bus (or buses), minimizing the
chance of unwanted “leakage.” Pan pots
are fine in an audio mixer destined for use
in stereo music situations, but are inferior
when used for two-bus applications.
Line Inputs
Four input channels are provided for
connection to line-level signals. Features
provided for each line input channel include a rotary level control, level status
LED, and output bus assignment switch.
The rotary level control is used to set the
amount of input signal that is sent to the
selected main output bus (or buses).
The dual-color level status LED provides
signal present and peak level indication.
A 3-position switch allows the input signal
to be assigned to the desired output bus
(or buses).
compressor on/off switches are provided
on the Model 742A’s front panel. Internal
configuration jumpers determine the compressor circuits’ operating modes. From
the factory the jumpers are set so that the
compressors’ operating threshold is 6 dB
above the nominal +4 dBu output level.
This is an excellent general-purpose setting for broadcast use where voice signals
are the primary audio content.
To meet the needs of other applications,
a technician can change the jumpers to
make the compressor threshold 2 dB
above the nominal +4 dBu. This could
prove useful when using the Model 742A
with level-sensitive RF transmission systems. For other applications a technician
can set the jumpers to a third position
which completely disables the compressor
functions.
Main Output Buses
Signals from the eight input channels
(four mic/line and four line) are routed
and combined to create the two main
output buses. A dual rotary control is used
to independently set the overall level of
the two buses. An electronically balanced
output circuit is associated with each
main output bus. They provide line-level
signals capable of driving balanced or
unbalanced loads of 600 ohms or greater.
Separate studio-quality audio compressor
circuits are provided to control the dynamic range of each main output bus. An
LED indicator is associated with each
compressor, lighting whenever the circuit
is actively controlling signal level. Far from
simple “clippers,” the compressor circuits
utilize sophisticated laser-trimmed voltage-controlled amplifier (VCA) integrated
circuits for quiet, low-distortion operation. To help minimize operator error no
Monitor Section
The Model 742A’s monitor section provides two line-level monitor outputs and
a “stereo” headphone output. Monitor
outputs 1 and 2 are electrically balanced
with their input sources associated with
output buses 1 and 2, respectively. The
headphone output has main output bus 1
as its left-channel source and main output
bus 2 as its right-channel source. Associated with the monitor section is a dual rotary level control that allows independent
setting of the monitor output levels. The
level of the headphone output is always
adjusted using these controls. Internal
configuration jumpers are used to select
the exact signal sources for the line-level
monitor outputs. They can either be “post”
the controls, allowing an adjustable output
level, or set to a fixed –10 or +4 dBu nominal level. This allows the monitor outputs
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to be compatible with a variety of monitor amplifiers, amplified loudspeakers, or
even to serve as an additional set of main
bus line-level outputs.
From the factory the Model 742A’s monitor
outputs are configured to the “post” setting, enabling them to “follow” the setting
of the front-panel level controls. This is
appropriate where monitor loudspeakers
and associated power amplifier channels
are used. A technician can change the
jumper positions so that the monitor outputs are electrically before (“pre”) the level
controls. Two jumper positions are available, corresponding to –10 and +4 dBu
nominal output levels. The –10 position
can be very useful when connecting the
monitor outputs to amplified loudspeakers
that contain user-accessible level controls.
Examples of amplified speakers that contain user level controls include the popular
Fostex® 6301-series. With the jumpers set
to the –10 position, only one level control—the one in the amplified speaker—
would be used to set that speaker’s level.
This can greatly minimize operator confusion and enhance performance.
When the unit is configured for +4 operation the monitor outputs are again “pre”
the level controls, but this time with a nominal level of +4 dBu. This can be used for
various applications, including providing
an additional set of main bus outputs. In
certain cases this may eliminate the need
for an external distribution amplifier to be
included as part of the installed system.
Metering
As previously discussed, individual dualcolor signal present/peak LEDs are associated with the four mic/line input and
the four line input channels. In addition,
two 10-segment LED meters provide an
indication of audio-signal levels present
on the main output buses. These meters
use three LED colors: green, yellow, and
red. The LEDs are calibrated such that the
actual output level, in dBu, is indicated.
Two additional LEDs, one for each output
bus, display the status of the compressor
circuits. They light whenever the compressors are actively controlling the dynamic
range of the audio signals.
Reference Tone
A sine-wave audio tone is provided for
alignment and reference use. From the
factory the tone is configured for 400 Hz.
If required, a technician can revise the
frequency to be 1 kHz. A button on the
Model 742A’s front panel allows the tone
to be connected to the main output buses.
In addition, a dedicated reference tone
output is also provided. This continuous
source of sine-wave signal is accessed by
means of a balanced line-level output on
the back panel. The dedicated reference
tone output is intended to provide a setup
or test signal that is available at all times,
without interfering with normal operation
of the main output buses.
Mounting, Connectors, and
Power
The Model 742A requires one space in a
standard 19-inch rack. Industry-standard
XLR and ¼-inch 3-conductor connectors
are used for all audio interconnections.
To maximize reliability, all connectors
were carefully selected from among
premium-grade models. For example, the
XLR connectors feature metal shells and
are manufactured by Neutrik®. AC mains
power can be connected directly to the
Model 742A by way of a standard 3-pin
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detachable IEC cord set; no external power
supply is used. Having a “universal input,”
the AC mains power source can range from
100 to 240 volts, 50/60 Hz, with no switches to set or straps to change. A source of
10 to 18 volts DC can also be connected
to power the Model 742A. This allows the
unit to operate from a variety of battery
and external DC power supply sources.
The power input circuitry was carefully
designed to allow simultaneous connection
of both AC mains and DC power sources.
In this scenario the AC mains source
will power the Model 742A while only an
extremely small amount of current will be
drawn from the DC source. Upon loss of
AC mains power the DC source will, without interruption, begin to power the unit.
Far from a simple diode “OR” circuit, this
is a true “hot-swap” type design.
identified and changes made as required.
In the case of revising the reference tone
frequency, a calibration trim pot must also
be adjusted.
Compressor Modes
The compressor circuits associated with
the two output buses can be configured
to operate from one of three modes:
• From the factory the compressors are
set to have an operating threshold of
6 dB above the main outputs’ nominal
+4 dBu operating level. In other words,
the compressors will begin to function
whenever the main output levels reach
about +10 dBu, as measured on the
output connectors. This is an excellent
general-purpose setting that is appropriate for most applications.
Configuration
Review
In this section you will be reviewing the
configuration of the Model 742A. Configuration changes will be made if required.
Any changes should be made prior to
installing the unit.
As previously mentioned in this guide,
three Model 742A operating parameters
can be configured to meet the exact needs
of specific applications. These include
compressor mode, monitor output mode,
and reference tone frequency. While simple
to perform, all changes do require the
skill of an experienced and competent
technician. This person must ensure that
AC mains power is disconnected prior to
removing the unit’s cover. Then the locations of the configuration jumpers can be
• Alternately, the compressors’ threshold
can be changed to be 2 dB above the
nominal +4 dBu operating level. This
may be appropriate for applications
where an RF transmission chain places
severe restrictions on the audio signal’s
dynamic range.
• While not recommended for most applications, the compressors can also
be configured to be disabled. With this
configuration, no dynamic range control
will take place. For most on-air applications this is probably not appropriate.
Take advantage of the Model 742A’s
highly effective dynamic range control
to assist in providing clear, intelligible
audio!
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Monitor Output Modes
The line-level monitor outputs can be
configured to operate from one of three
modes:
• From the factory the monitor outputs
are set to follow the position of the front
panel rotary level controls. This mode of
operation is referred to as being “post”
the level controls.
• An alternate configuration is specifically
provided for use in applications where
amplified loudspeakers with built
user level controls are connected.
This “pre” level-control mode provides
fixed nominal output levels of –10 dBu.
This prevents the confusing situation
of having multiple level controls—the
Model 742A’s and the amplified speakers’—both impacting the level of a
speaker. In this case the Model 742A’s
front-panel controls will only impact the
headphone output.
A third configuration provides a “pre”
•
level-control mode with a fixed nominal
output level of +4 dBu. This can be use
ful for applications where an additional
set of main outputs is required. Except
for using ¼-inch 3-conductor jacks
(versus XLRs) the monitor outputs are
identical in quality as compared to the
main outputs. Again in this mode the
Model 742A’s front-panel controls will
only impact the headphone output.
-in
Reference Tone Frequency
From the factory the frequency of the reference tone is configured to be 400 Hz. This
is a nice alternative to the more-typical
1 kHz, a frequency which can soon become very annoying to a listener’s ears. In
most cases 400 Hz will be perfectly acceptable, and actually preferred. But in those
situations where 1 kHz is necessary, two
configuration jumpers are provided to shift
the frequency range so that the oscillator
circuitry can be calibrated for 1 kHz.
Configuration Wrap-Up
-
-
If it is determined that any configuration
changes are required, they should now be
performed. For specific details on how to
make configuration changes, refer to the
Technical Notes section of this guide. The
details provided should make the process
simple and relatively painless.
It’s highly recommended that a Model
742A’s actual configuration be confirmed
prior to installation. Even factory-fresh units
can “mysteriously” end up having their
default configurations changed prior to
actual installation. The same “gremlin” that
borrows and doesn’t return tools, drinks
your favorite soda that you left in the techshop refrigerator, and pulls important patch
cords has been known to sneak inside
equipment and revise settings!
Installation
In this section you will be installing and
interconnecting the Model 742A. The
one-rack-space unit will be installed in an
equipment rack. Audio input and output
connections will be made using the 3-pin
XLR connectors and ¼-inch 3-conductor
jacks. In most cases AC mains power will
be connected by means of a detachable
cord set that is compatible with the Model
742A’s 3-pin IEC 320 C14 inlet connector.
Some applications may warrant connection
of a source of 10 to 18 volts DC which can
be made by way of a 4-pin XLR connector.
The DC source can be used to power the
Model 742A or it can serve as a backup
source for a connected AC mains source.
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