At the heart of the CP101's circuit sits an HTV #P873 CdS (Cadmium Sulfur) Photocoupler. This light-sensitive component varies resistance based on the input signal to control the amp gain, increasing gain as the input signal weakens. Most other current-production compressors accomplish this task using a VCA (voltage controlled amplifier), but the multiple transistors packed into a VCA inevitably lead to high noise levels, which means lots of "hiss" from your rig. The CP101's optical circuit keeps the noise-floor low - so low that you can run the unit after a distortion and still retain near-silent operation. With its ultra low-noise level, transparent operation and natural playing response the Maxon CP101 is the perfect compressor for players who hate using compressors.