There are four main types of hardware-based compressors. All the types have been emulated in software. Each type has its own character and purpose.
- Vari-Mu – Tube
- Opto – Optical
Vari-mu or Tube
Vari-mu or Tube compressors have a smooth sound. They are not fast compressors and as such are used more on average program material such as bus compression. They respond well to acoustic instruments and acoustic styles of music. They sound great on jazz and folk music. The most desired vari-mu compressor is the Fairchild 670.
FET (Field Effect Transistor)
FET can respond quickly, but not quite as fast as a VCA compressor. FETs work well on most sources including drum busses. They can be aggressive sounding when pushed hard which may explain their dominance in rock music. The most famous FET compressor is the UREI 1176.
Optical compressors operate by turning sound volume into light intensity. An optical sensor reads the light intensity and the amplifier’s volume is adjusted accordingly. Optical compressors are rather slow to respond and work on average sound levels instead of peaks. They work well on vocals, bass and bus compression. Most opto compressors colour the sound and are not linear in their response. The way they colour sound and the non-linear response makes them desirable on certain sources. One of the most familiar opto compressors is the Teletronix LA2A. A modern opto example is the Tube-Tech CL 1B.
VCA stands for Voltage Controlled Amplifier. It is perhaps the most transparent and least coloured of the compression types. It is also the fastest of all the compressor types. These are used to control the dynamics of almost all sources without adding to the character of the sound. Their fast response also makes them ideal for shaping the envelopes of sounds to get more punch on percussion and such. VCA compressors of note are the DBX 160 and the SSL G Bus compressor.