Diferent kind of compressors. Whats The diference?

[SISKO]

Could anyone point me the diefrences between VCA, LDR, OTA, FET, etc based compressors?
Also, what makes the compressor suitable, say, for vocals or guitar? Is it the detector?

I only know the diference between feedback or feedfoward compressor as Mark Hammer pointed out on his site, and remeber seeing a comparison beetween VCA, LDR and transistor on R.G. Keen`s page. But i can not find it anymore. 

[Mark Hammer]

This older article in EM magazine is a good review of compressors: http://psbg.emusician.com/ar/emusic_big_squeeze/index.htm

You can make some of the following distinctions in stompbox compressors:

• Do they change gain or change the signal level receiving a fixed gain?  The Dynacomp/Ross adjusts the gain downward in response to a signal peak.  The Orange Squeezer applies the same gain, all the time, every day.  What changes is the level of the signal you feed the gain stage.  You can use an LDR and FET to do the second kind, and the OTA/VCA (the same thing, really) is generally used to adjust gain level.  The LDR/Fet route is essentially varying a single resistance somewhere, which can either be used to adjust gain OR attenuate signal level.  Active stages like OTAs are generally used only to vary gain.
• LDRs essentially HAVE no clipping point.  They are resistors, plain and simple.  FETs and OTAs can be faced with signal levels thatresult in distortion.  You can manage this with circuit tweaks and other compensatory strategies, but the fact remains that they can be MADE to distort, whereas LDRs can't.
• You can get LDRs with fairly fast response times, but a FET or OTA will generally beat the best time an LDR has to offer, hands down.  Consequently, for ultra-fast attack time, and being able to keep a signal peak from pushing the VU meter into the red, FETs and OTAs are better.    FETs and OTAs can also have much faster fallback or recovery times, compared to LDRs.  In the world of compression, however, that isn't necessarily a great thing, since recovering faster can also mean greater susceptibility to envelope ripple and the little "burr" one sometimes hears at the end of sustained chords/notes when compressing.  LDRs provide a much smoother transition in that regard and help to smudge out some of that ripple.
• Ultimately, the rectifier/follower circuit is as important as the control element.  If the rectifier is able to eliminate the ripple then you can use a FET or OTA  and reap all the benefits in the attack time, without having to suffer the downside of the decay problems.  As well, the way in which input signal is translated into control signal is important in fostering a "feel" in the nature of the compression.

A lot of this is outlined in the EM article.  Even though it deals with high-end rackmount compressors, what makes a compressor good for drums vs vocals, etc., can be shared in common with stomp units.

[SISKO]

Thanks Mark, that was REALLY helpfull to me.
Ive decided to design a variable mu or an optical compressor. Its for vocals and/or bass.

Ill show my results when i get em