The four most common types of compressors are tube, optical, FET, and VCA compressors. Tube compressors are one of the oldest types of compressors and are still used today for their distinct 'vintage' sound. Optical compressors use a light element and a light-emitting diode to reduce the amplitude of the output signal. FET compressors use Field Effect Transistor circuits to mimic the sound of tube compressors. VCA compressors use Voltage Controlled Amplifiers and are popular for use on the mix bus and instrument groups.