Path Not Taken
A compressor can degrade or ruin an audio signal as well as enhance it; therefore, one of the most useful features on any compressor is the bypass switch, which lets you compare processed and unprocessed signals. After using the output-level control to balance the levels of the processed and unprocessed signals—a critical step because louder signals sound brighter and fuller—you can judge whether your control settings actually improve the sound by switching the compressor in and out of the circuit.

Universal Audio reissued the legendary Teletronix LA-2A Leveling Amplifier—a favorite for processing vocals.

Fortunately, most compressors provide a bypass (a notable exception is the Universal Audio Teletronix LA-2A). Typically, this is a switch that disables the compressor circuitry; ideally, it also disables the input- and output-level controls. A hardwire bypass is usually the best design because it routes the input directly to the output and bypasses all compressor circuitry, such as input and output amplifiers and gain-control devices.

Actually, on some high-end units the signal is kept in circuit even in bypass mode—a viable design as long as the signal path is pristine enough not to color the original signal. The advantage of that design is it avoids the use of bypass relays and audio-path switches—elements that inevitably degrade during a period of time and compromise audio quality. The bypass on the Millennia TCL-2 Twincom Opto Compressor/Limiter ($2,995), for example, defeats the unit’s sidechain control only, thus preventing compression from taking place. (I’ll discuss sidechains later.)

Don’t Need Your Input
Many compressors also provide an input-level control, but those are often superfluous—if not undesirable. For one thing, a compressor with a wide-ranging threshold control can handle almost any input level. So an input-level control is necessary only if the threshold range is too high or too low to act on the input signal as is.

For example, if the threshold’s highest setting is +2 dBV and you’re feeding the compressor +12 dBV levels, you’ll compress most or all of the time unless you somehow lower the level at the compressor’s input. That’s one instance in which an input-level control comes in handy. Conversely, if the threshold range’s minimum setting doesn’t go down very low, the compressor may not kick in when fed low levels. In that case, an input-level control is necessary to boost the input to an appropriate level.

The reason an input-level control can be thought of as undesirable is it adds yet another amplification/attenuation stage to the circuitry, thus degrading signal quality. For that reason, high-end, minimalist compressors, such as the Millennia TCL-2 Twincom, typically omit the input control to maintain a pristine signal path.

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Reprinted with permission from Magazine, February, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved