COMPRESSION TYPES

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.