Studio Technologies 740 User Manual
Model 740
Model 740 Audio Mixer
User Guide
Issue 3, May 2004
This User Guide is applicable for serial numbers:
M740-00151 and later
© 2004 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
50026-0504, Issue 3
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Model 740
Table of Contents
Introduction ................................................................... 5
System Features ........................................................... 6
Configuration Review .................................................... 8
Installation ..................................................................... 9
Post-Installation Calibration .......................................... 14
Operation ...................................................................... 15
Technical Notes ............................................................. 21
Specifications ................................................................ 24
Block Diagram
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Model 740
Introduction
What This User Guide Covers
This User Guide is designed to assist you
when installing, configuring, and using
the Model 740 Audio Mixer. Additional
background technical information is also
provided. A product block diagram is
included at the end of this guide.
System Overview
The Model 740 Audio Mixer is expressly
designed for use in compact electronicnews-gathering (ENG) vehicles. Many other
audio production applications can also be
supported. Model 740 features include six
mic/line inputs, a monaural output bus,
LED metering, and a flexible monitoring
section. It’s important to note that the feature set was carefully selected to provide
exactly what’s needed —and no more!
Keeping it simple was the design mantra.
The Model 740 is targeted for use by
operators that have a multitude of tasks
to perform, and too little time to do them.
Using its experience in mobile broadcast
applications, Studio Technologies was
able to design the Model 740 to include
all the crucial features required to meet
the needs of fast-paced news-gathering
operations, while still providing the operator with an extremely easy-to-use product.
The hallmarks of the Model 740 are application flexibility, simplicity in use, audio
quality, and long-term reliability. A number
of internal configuration jumpers allow the
unit’s performance to be tailored to the
needs of specific installations. The carefully selected feature set ensures that the
Model 740 will be a “team player” when
it’s integrated into an audio system. 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 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 740’s audio
quality is “pro” throughout. The components were carefully selected to deliver
low-noise, low-distortion performance.
Model 740 Front Panel
Model 740 Back Panel
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Model 740
Reliability over the long haul was a Model
740 design criteria. To that end the unit’s
enclosure is made of steel, combining
strength with effective RF rejection. On
the inside, all components are mounted
on an FR4 (fiberglass-based) circuit
board assembly.
System Features
Mic/Line Inputs
Six input channels are provided for
connection to microphone or line-level
signals. The electronically balanced
circuitry is “ruggedized” for reliability
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, and level status LED. For
ease of use, the rotary level control sets
both the gain of the input circuit and the
level being sent to the main audio bus.
For convenience the six input sensitivity
buttons, like all the Model 740’s operator
controls, are located on the front panel.
For operator assistance, the bicolor level
status LED provides signal present and
peak level indication.
Main Output
Signals from the six mic/line input channels combine to create a monaural signal
called the main output bus. A rotary control sets the overall level of the main bus.
An electronically balanced output circuit
is associated with the main output bus.
It provides a line-level signal capable of
driving balanced or unbalanced loads
of 600 ohms and greater. A studio-quality
audio compressor circuit is provided to
control the dynamic range of the main audio bus. An LED indicator lights whenever
the compressor circuit is actively controlling the signal. Far from a simple “clipper,”
the compressor circuit utilizes a sophisticated laser-trimmed voltage-controlled
amplifier (VCA) integrated circuit for quiet,
low-distortion operation. To help minimize
operator error, no compressor on/off switch
is provided on the Model 740’s front panel.
An internal 3-position configuration jumper
determines the compressor circuit’s operating mode. From the factory the jumper
is set so that the compressor’s 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 installations,
a technician can change the jumper to
make the threshold 2 dB above the nominal +4 dBu. This could prove useful when
using the Model 740 with level-sensitive RF
transmission systems. For other applications a technician can set the jumper to
a third position which completely disables
the compressor function.
Monitor Section
The Model 740’s monitor section provides
a line-level monitor output, along with two
headphone outputs. Associated with the
monitor section is a rotary level control
and a 3-position source select switch. The
monitor output is compatible with a variety
of monitor amplifiers and amplified loudspeakers. For flexibility, two headphone
output jacks are provided, one on the
front panel and one on the back. Each
headphone jack is supported by an independent output circuit. A 3-position toggle
switch is used to select the audio source
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to be monitored. This allows monitoring
of the main audio bus, along with two
external monaural audio sources. These
external monitor inputs are intended to
be connected to devices such as off-air,
microwave, or satellite receivers. Alternately, other mobile broadcast support equipment, such as a PC-based editing system,
can be connected.
Unique to the Model 740 is the ability to
configure the monitor output to meet the
needs of the “real world.” This is accomplished by using an internal 3-position
configuration jumper to select the monitor
output mode. From the factory the monitor
output is configured to “follow” the setting
of the front-panel level control. In addition,
the monitor output mutes whenever a pair
of headphones is plugged into the frontpanel jack. A technician can change the
jumper position so that the monitor output
still follows the setting of the front-panel
level control, but is not impacted by use
of the front-panel headphone jack.
A third jumper position allows a technician
to configure the monitor output such that
it is electrically before (“pre”) the level
control. As with the previous mode, the
output isn’t impacted by use of the headphone output. This mode is very useful
when an amplified loudspeaker that contains a user-accessible level control is
connected. Examples of amplified speakers that contain user level controls include
the popular Fostex® 6301-series. With the
jumper in the pre-level-control position,
only one level control—the one on the
amplified speaker—would be used to
set the speaker level. This can greatly
minimize operator confusion while still
utilizing the resources of the Model 740’s
monitor section.
Metering
A 10-segment LED meter provides an
indication of audio-signal level. An internal
selection jumper determines the audio
source for the meter. From the factory
the meter is set to monitor the signal level
of the main audio bus. A technician can
change the configuration so that the meter
monitors the source selected by the monitor section’s 3-position switch. In this way
the level of the main audio bus, as well
as the two external monitor sources, can
be displayed by the meter.
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 740’s front panel allows the tone
to be connected to the main audio bus.
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,
and without interfering with normal operation of the main audio bus.
Mounting, Connectors, and
Mains Power
The Model 740 requires one space in
a standard 19-inch rack. Industry-standard XLR-type and ¼-inch 3-conductor
audio connectors are used for all audio
interconnections. To maximize reliability,
all connectors were selected to be premium grade. For example, the XLR-type
connectors feature metal shells and are
manufactured by Neutrik. AC mains power
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is connected directly to the Model 740
by way of a standard 3-pin detachable
IEC cord set; no external power supply is
used. AC mains power is factory configured for 100, 120, or 220/240 V, 50/60 Hz.
Configuration Review
In this section you will be reviewing the
configuration of the Model 740. Configuration changes can be made if required. Any
changes should be made prior to installing the unit.
As previously mentioned in this guide, four
Model 740 operating parameters can be
configured to meet the needs of specific
applications. These include compressor mode, monitor output mode, meter
source, and reference tone frequency.
While simple to perform, all changes do
require the skill of a technician. This person must ensure that AC mains power
is disconnected prior to removing the
unit’s cover. Then the locations of one
or more of the configuration jumpers can
be moved. In the case of the reference
tone frequency, a calibration trim pot
must also be adjusted.
Compressor Mode
The compressor can be configured to
operate from one of three modes:
• From the factory the compressor is
set to have an operating threshold of
6 dB above the main output’s nominal
+4 dBu operating level. In other words,
the compressor will begin to function
when the level of the main output reaches about +10 dBu. This is an excellent
general purpose setting that is appropriate for most applications.
• Alternately, the compressor’s threshold
can be changed to be 2 dB above the
+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 compressor can
also be configured to be disabled.
With this configuration, no dynamic
range control will take place. For most
on-air applications it’s unlikely that this
would be appropriate.
Monitor Output Mode
The monitor output can be configured
to operate from one of three modes:
• From the factory the monitor output is
set to follow the position of the frontpanel rotary level control. This mode
of operation is sometimes referred to
as being “post” the level control. In addition, plugging a pair of headphones
into the jack on the front panel will
cause the monitor output to mute.
• An alternate configuration is available
that makes the monitor output follow
the rotary level control’s setting. But
plugging headphones into the front
panel doesn’t cause muting of the
monitor output. This allows the monitor
output to be active, whether or not the
headphones are plugged in.
• A third mode is specifically provided
for use in applications where an amplified loudspeaker with built-in user level
control is connected. This “pre-levelcontrol” mode allows the user to select
the source to be monitored on the Model 740’s front panel. But only the level
control on the amplified speaker is used
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to adjust its volume. This prevents the
confusing situation of having two level
controls—the Model 740’s level control
and the amplified speaker’s level control—both impacting the speaker level.
In this case the Model 740’s front-panel
control will only impact the headphone
output level.
Meter Source
The meter source can be configured
to operate from one of two modes:
• From the factory the meter is selected
to always monitor the level of the main
output bus. This ensures that the level
of the active or “on-air” output is always
displayed on the meter. Changing the
monitor source will not impact meter
operation.
• An alternate jumper setting allows the
meter to “follow” the source selected for
monitor output. This could prove useful
if level metering of the external monitor
input signals is desired.
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, configuration jumpers can 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
have been performed. For specific details
on how to make configuration changes,
refer to the Technical Notes section of
this guide. The details provided should
have made the process simple and
relatively painless.
Installation
In this section, the Model 740 will be
installed in an equipment rack. Input and
output connections will be made using
the Model 740’s multitude of jacks. Finally,
AC mains power will be connected.
System Components
The shipping carton contains a Model 740
Audio Mixer and user guide. Units destined for North America and Japan also
include an AC mains cord. Your dealer
or distributor will provide an AC mains
cord for non-North American destinations.
Locating the Unit
Now that the Model 740’s configuration has been reviewed and, if required,
revised, the unit can be mounted in an
equipment rack. Physical access and
viewing angles are the primary issues
to be considered when choosing a mounting location. By its very nature as an audio
mixer, an operator will need easy access
to all the controls. Also, a good sight line
to the 10-segment LED level meter, located on the right side of the front panel, can
be important.
As a device that contains high gain audio
stages, hum and noise pickup is possible
by way of the chassis and associated cabling. Locating the unit away from devices
that radiate strong AC fields is an excellent
idea. While not likely, it is also possible
that the Model 740’s toroidal power transformer could radiate energy into adjacent
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