Pellucid compressor

An ultra-low-noise compressor for guitar and bass, designed for simplicity, transparency and silence. Two controls only: Sustain and Volume.

The circuit. A feedback compressor. The side-chain is the classic two-transistor peak detector found in the DOD 280, the Dyna Comp, the Ross and most guitar compressors since. The gain cell is an LM13700 in the feedback loop of an NE5532 low-noise op amp, the technique the Valve Wizard's Engineer's Thumb uses to keep noise down. Both halves of the LM13700 run in parallel, with current-sharing resistors, for a further noise reduction, and the OTA is driven with a balanced differential signal to keep control signal out of the audio.

Attack is fast, about 3 ms, and release about 200 ms; fixed, and chosen to make the compression feel alive under the fingers. A larger release capacitor (100 µF instead of 47 µF) suits bass. Gain reduction runs from almost none, a clean boost, to 40 dB. Measured with a 5 kΩ source at full compression, it is about 11 dB quieter than a stock Engineer's Thumb.

Standard parts throughout. The 50 × 55 mm two-layer PCB is laid out for low noise and fits a 1590B enclosure.