Optical compressors, or “opto” compressors, rely on a light-dependent resistor and a light source to dictate the way in which compression is applied. An optical compressor’s input signal illuminates a light source—the more powerful the input signal, the brighter the light will shine.
FET or Solid State compressors, or field effect transistor compressors, make use of transistor circuits. They respond quickly to the signal you run through them, and they’re also quite punchy, colourful, and bright.
VCA compressors, or voltage controlled amplifier compressors, use a control signal to dictate whether or not gain reduction is applied to an audio signal. These are some of the most common types of compressors and often include all the controls you’re used to seeing on plugin compressors, like attack, release, threshold, ratio, and sometimes a knee.