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Highlights
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Four mic/line and four line-only inputs
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Setup oscillator with dedicated output
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Studio-quality compressors
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Internal power supply
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Flexible monitor section
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10-segment LED meters
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All operator controls on front panel
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Single rack-space mounting
Overview
The Model 742 Audio
Mixer is expressly designed for use in electronic-news-gathering (ENG)
vehicles, small production trucks, and other specialized mobile
broadcast applications. Many other audio production applications can
also be supported. Model 742 features include four mic/line inputs, four
line inputs, two output buses, LED level metering, and flexible
monitoring. The Model 742 was specifically designed as a dual-channel
audio mixer, rather than as a typical stereo device. This approach makes
it excellent for use in “dual-path” on-air applications.
The Model 742 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 742 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 742 to include all the
crucial features required to meet the needs of fast-paced news-gathering
operations, while still providing the operator with an easy-to-use
product.
The hallmarks of the
Model 742 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 meet the needs of
specific installations. The carefully selected feature set ensures that
the Model 742 will be a “team player” when it is 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 742’s
audio quality is “pro” throughout. The components were carefully
selected to deliver low-noise, low-distortion performance.
Long-term reliability
was also part of the Model 742’s 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.
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, level status LED, and output bus
assignment switch. To support condenser microphones, the four mic/line
inputs can be internally configured to provide 12-volt phantom power.
For ease of use, the 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, like all of the Model 742’s 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 742’s
design. This allows a more positive assignment of an input source to the
desired 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).
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 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 compressor
on/off switches are provided on the Model 742’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 742 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.
Monitor Section
The Model 742’s monitor
section provides two line-level monitor outputs and a “stereo” headphone
output. Monitor outputs 1 and 2 are electrically balanced with output
buses 1 and 2, respectively, assigned as their audio sources. 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 levels. The level of the headphone output is always
adjusted using these controls. Internal configuration jumpers are used
to select the operation of the line-level monitor outputs. They can
either be “post” the controls, allowing an adjustable output level, or
be set to fixed –10 or +4 dBu nominal levels. This allows the monitor
outputs to be compatible with a variety of monitor amplifiers, amplified
loudspeakers, or to even serve as an additional set of main bus
line-level outputs.
From the factory the
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 “pre –10” position 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 jumpers set
to the “pre –10” 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 and enhance performance.
When configured for
“pre +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, dual-color
signal present/peak LEDs are associated with each of the eight input
channels. In addition, two 10-segment LED meters provide an indication
of audio signal levels on the main output buses. The meters use three
LED colors: green, yellow, and red. The LEDs are calibrated such that
the actual output level, in dBu, is indicated.
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 742’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 Mains Power
The Model 742 requires one space in a
standard 19-inch rack. Industry-standard XLR-type 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-type connectors feature metal
shells and are manufactured by Neutrik®. AC mains power is connected
directly to the Model 742 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 volts, 50/60 Hz.
Model 742 Specifications
General
Audio Parameters:
Frequency Response: 20 Hz to 20 kHz, ±0.5 dB, mic/line in to main outs
Distortion (THD+N): 0.03 %, measured at 1 kHz, +4 dBu, mic/line in to main
outs
S/N Ratio: 74 dB, referenced to +4 dBu, mic/line in to main outs, 22 Hz to
22 kHz
Mic/Line
Inputs: 4
Input Sensitivity: switch selectable for microphone or live-level
signals; 43 dB line input “pad” implemented using sealed bifurcated
contact telecom relays
Phantom Power: 12 Vdc, nominal, meets IEC 61938 P12 standard
Operating Level Range: mic input position: –76 to –8 dBu; line position:
–32 to +28 dBu. Range specified to give +4 dBu on main outs, master
level controls set to unity gain.
Type: electronically balanced
Impedance: mic position 1.3 k ohms, line position 15 k ohms
Common Mode Rejection Ratio: 74 dB @ 60 Hz, 73 dB @ 40 kHz (typical),
mic position, ref +4 dBu on main outs
Status LED: dual-color, displays signal present and peak
Line Inputs: 4
Operating Level Range: –14 to +28 dBu. Range specified to give +4
dBu on main outs, master level controls set to unity gain.
Type: electronically balanced
Impedance: 24 k ohms, nominal
Common Mode Rejection Ratio: 85 dB @ 60 Hz, 85 dB @ 40 kHz (typical),
ref +4 dBu on main outs
Status LED: dual-color, displays signal present and peak
Compressors: 2
Function: controls dynamic range of output buses 1 and 2
Type: single-knee, VCA-controlled
Slope: 5:1
Threshold: configurable, choices are: 6 dB over nominal (+10 dBu on main
outs, factory default); 2 dB over nominal (+6 dBu on main outs);
disabled
Attack Time: 2 ms
Release Time: 100 ms
Status LEDs: 2, compressor active
Main
Outputs: 2
Type: electronically balanced, capacitor-coupled, intended to drive
balanced or unbalanced loads of 600 ohms or greater
Source Impedance: 50 ohms, nominal
Nominal Level: +4 dBu
Maximum Level: +26 dBu into 10 k ohms, +25 dBu into 600 ohms
Monitor
Outputs: 2
Signal Sources: output buses 1 and 2
Type: electronically balanced, capacitor-coupled, intended to drive
balanced or unbalanced loads of 600 ohms or greater
Source Impedance: 50 ohms, nominal
Nominal Level: configurable, choice are: +6 dBu, with monitor level
control @ 100%, mode configured for POST level control; –10 dBu when
configured for PRE –10; +4 dBu when configured for PRE +4
Maximum Level: +26 dBu into 10 k ohms, +25 dBu into 600 ohm
Headphone
Output:
Signal Sources: output buses 1 and 2 (“post” monitor level controls)
Type: stereo, configured to drive stereo headphones through 100 ohm
series resistors
Compatibility: intended for connection to headphones with impedance of
100 ohms or greater
Maximum Voltage: 8 Vpp, 100 ohm load
Reference Tone:
Frequency and Wave Form: 400 Hz nominal, sine wave (technician can
revise to 1 kHz)
Direct Output Level: +4 dBu, nominal, adjustable over –1/+3 dB range
Direct Output Type: electronically balanced, capacitor-coupled, intended
to drive balanced or unbalanced loads of 600 ohms or greater
Direct Output Source Impedance: 100 ohms, nominal
Meters: 2
Function: displays level of output buses 1 and 2
Type: 10-segment LED, modified VU ballistics
Connectors:
Mic/Line and Line Inputs: 3-pin female XLR-type
Main Outputs: 3-pin male XLR-type
Reference Tone Direct Output, Monitor Outputs, Headphone Output: ¼-inch
3-conductor phone jacks
AC Mains: 3-blade, IEC 320 C14-compatible (mates with IEC 320 C13)
AC
Mains Requirement: 100, 120, or 220/240 volts, ±10%, factory
configured; 50/60 Hz; 0.2 A maximum @ 100 and 120 volts, 0.1 A maximum @
220/240 volts
Mains Fusing:
Type: 5 x 20 mm, time-lag
Rating: 0.2 A for 100 and 120 volt AC mains, 0.1 A for 220/240 volt AC
mains
Dimensions
(Overall):
19.0 inches wide (48.3 cm)
1.72 inches high (8.9 cm)
9.5 inches deep (24.1 cm)
Mounting:
one space in a standard 19-inch rack
Weight:
8.5 pounds (3.9 kg)
Specifications
subject to change without notice.
Audio Mixers
Which Audio Mixer is right for your application? Review the
Audio Mixers Feature-by-Feature Comparison Chart.
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