Product: Model 742 Audio Mixer

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.