As electronic technology evolved and new components were developed, engineers devised various ways to achieve compressio­n. Here are the main types.

VARI-MU: The split input signal forms a side chain to provide a control voltage. This voltage constantly rebiases a valve in the signal path to adjust gain. The Fairchild 660 and 670 are the most revered versions, and plug-in emulations are available.

OPTICAL: An input signal side-chain is routed to a bulb or LED that’s adjacent to a photoresis­tor. As it glows brighter with higher signal levels, it increases the resistance of the photoresis­tor so it operates like a light-driven volume potentiome­ter. Check out Teletronix LA-2A type plug-ins and the PRS Mary Cries pedal.

FET: During the late 60s it was found that Field Effect Transistor­s (FETs) could be used as voltage variable resistors. Voltage derived from the input signal applied to the FET’s gate alters the resistance between the source and drain. Plug-in versions are usually based on the Urei 1176; the Origin Effects Cali76 is a stompbox equivalent, as well as the new UAFX 1176 Studio Compressor, reviewed on page 102.

VCA: Voltage Controlled Amplifier compressor­s can provide very fast attack and release times and were hugely popular for 1980s pop production­s. The SSL mix bus and DBX 160 are classic VCA compressor­s, and the Boss CS-3 is a VCA compressor pedal.

OTA : Operationa­l Transcondu­ctance Amplifier compressor­s operate similarly to VCA compressor­s but a chip generates variable 1 current, rather than voltage. In pedal form, the Ross Compressor is a classic and the Xotic SP is a more versatile modern version.

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