"In the days of analog hardware compressors, there were four different electronic building blocks that could be used to build a compressor. These were:
- Optical: A light bulb and a photocell were used as the main components of the compression circuit. The time lag between the bulb and the photocell gave it a distinctive attack and release time (like in an LA-2A). Optical compressors don't react very fast to the oncoming signal, but that actually makes then sound pretty smooth, which is why they've become a favorite on vocals and bass.
- FET: A Field Effect Transistor was used to vary the gain, which had a much quicker response than the optical circuit (a Universal Audio 1176 is a good example). FET compressors are often used on drums because of their quick response.
- VCA: A Voltage Controlled Amplifier circuit was a product of the 80s and had both excellent response time and much more control of the various compression parameters (the dbx 160 series is an example of a VCA-type compressor, although some models didn’t have a lot of parameter controls). VCA compressors can be very aggressive, which is why the dbx 160 series have long been a favorite on rock kick and snare.
- Vari-Gain: The vari-gain compressors are sort of a catch-all category because there are other ways to achieve compression besides the first three (like the Fairchild 670 and Manley Variable Mu). You might think of a vari-gain as the ultimate smooth sounding compressor because it was originally made for a radio signal chain, something that had to be as transparent as possible. That said, it's hard to beat a vari-gain compressor across the mix buss for the added "glue" that's difficult to get any other way.
As you would expect, each of the above has a different sound and different compression characteristics, which is the reason why the settings that worked well on one compressor type won’t necessarily translate to another. The good thing about living in a digital world is that all of these different compressor types have been duplicated by software plugins, so it’s a lot easier (not to mention cheaper) to make an instant comparison on a track and decide which works better in a particular situation."
To read additional excerpts from The Mixing Engineer's Handbook and my other books, go to the excerpts section of bobbyowsinski.com. Also check out The Audio Mixing Bootcamp video course on Lynda.com.