PMC: Poor Man's Compressor

An exercise in thrift: a high-quality guitar compressor made from a handful of penny transistors, with no op amps, no LDR, no JFETs, no VCA chip and no OTA.

Each usual gain element has its drawback: THAT VCAs are expensive, vactrols scarce and inconsistent, JFETs need selecting and trimming, and OTAs are noisy unless wrapped in a feedback loop.

The circuit. In effect an OTA built from discrete transistors. The transconductance of a bipolar long-tailed pair is proportional to its tail current, so varying that current varies the gain. A complementary feedback pair buffers the input. The long-tailed pair forms the current-controlled amplifier, with the guitar padded down to about 100 mVpp to stay clean. A three-transistor Wilson current mirror loads its output; a plain two-transistor mirror let the bias wander with signal level and thump. A half-wave peak detector pulls down the tail current through a second current mirror as the output rises. Emphasis and de-emphasis, and a high tail current, keep hiss down, and two transistors smooth the supply.

Attack is about 10 ms and release just over 200 ms, set by ear with a Strat. Conceptually it is a Dyna Comp with the CA3080 replaced by discrete parts and a half-wave side-chain. Everything mounts on the PCB, including pots, sockets, LED and the true-bypass footswitch, and it fits a 1590BB.