Understanding compressors and their differences
I would like to give you a short glimpse on compressors and their differences, I think this can be a little help to everybody who don't know which compressor to choose for which task.
First I have to mention, that it's about analog compressors, which are often digital modelled and available as various plugins.
2nd I want to make clear, that you allways can do what you think is best for your tracks and your music.
VCA compressor (voltage controlled amplifier)
The processing of the compression is made by transistors. This very versatile compressor reacts fast and give you a good control on transients. The sound is transparent and without any coloring - that's the reason why you can use it on every track for every purpose. If you don't know what kind of compression to use, you can't go wrong with this one.
Tube compressor (also delta mu or vari mu)
This type of compressor is well known for the rich tube saturation and its harmonics, and it's possible to add distortion to the audio. Usually it comes without a ratio control, because the compression is caused by the level of the audio you'll drive to it's tube. You can get great glue effects with it on busses, but it's not that great on drums or percussively instruments at all. The attack of this compressor is not that fast as a VCA compressor, but a lot faster than the opto comp can do.
Optical compressor (opto)
This compressor gives you a natural and soft sound with a great coloration and the famous harmonics. There are 2 main components to create the amount of compression: a light and a photoelectric cell. The level of the audio will make the light shine, the higher the level, the brighter the light. The amount of light will be noticed by the photo cell, which will adjust the amount of compression. This compressor reacts very slow (not faster than 10 ms), the release starts fast and gets slower in a nonlinear way. The more it compresses, the slower the release will be. But it can give you a smooth sounding result, if you use it in a gentle way.
FET compressor (field-effect transistor)
This compressor gives you the sound of a tube, generated by transistor circuit. It reacts much faster than the tube or opto compressor, honestly, this is the roadrunner of available analog compressors: the fast attack of 0.02 ms(!) gives you full control on any transient in your audio for sure. It's a fantastic tool for drum tracks and vocals and it does also a hilarious job as a parallel compressor. It can add a lot of punch to your audio, so maybe it's not the best choice for your mastering chain.