Quote: gainreduction Originally Posted by The short answer is what kind of circuit or component(s) performs the actual gainreduction.



Basically like this:



Optical - optical component used for GR (example Teletronix/UA LA-2A)



Vari-mu - tubes used for GR (example Fairchild 670, Manley Vari-Mu)



FET - FETs used for GR (example Urei/UA 1176)



VCA - VCA chip/circuit used for GR (example dbx 160, SSL bus comp)





Among newbies there's usually a bit of confusion about tube compressors. Tubes are used in many compressors at the output stage but that doesn't mean that the tubes do the actual compression. That's the case only in true Vari-Mu designs. For example the LA-2A is an optical compressor with tube output stage.



All of this is a generalisation, to give the big picture.



Google-ing will probably give you more in depth answers.

I am not disagreeing with anything here, but I did want to add a couple of things.In an optical compressor there is a photo resistor that is doing the GR. The photo resistor is a resistor that is sensitive to light. The brighter the light, the higher the resistance; the dimmer the light, the less resistance. This light source is controlled by the side-chain.The side chain can be either an external source or a sample of the audio; either way the audio is rectified, which is an electrical term that describes how AC (alternating current) is converted to DC (direct current). After the side chain audio is rectified it passes through a series of resistors and capacitors that sets the attack and release times.The side chain is then fed to the light source; so as the side chain voltage fluctuates the light intensity also fluctuates, and yes, the resistance of the photo resistor fluctuates (bingo…we have GR).A Variable-Mu® (you can also use the term delta-mu) compressor uses a tube, usually a remote-cutoff, for the GR. The GR in this tube is controlled by a DC voltage that comes from the side chain (sound familiar); this is referred to as the control voltage. As this voltage fluctuates the flow of electrons in the tube also fluctuates; the higher the voltage, the less flow of electrons, and the higher the gain reduction. The opposite can also be said the lower the voltage, the more flow of electrons, and the less gain reduction.The side chain in delta-mu compressor also comes from either a sample of the audio or as an external source. It is also rectified and it also passes through a series of resistors and capacitors, which sets the attack and release times. The side chain in this type of compressor is then fed to the remote-cutoff tube via the center tap of the input transformer.The reason a remote-cutoff tube is used is due to the manner in which the tube is constructed. Remote cutoff tubes differ from other types (such as sharp cutoff) of tubes in the way the grid wires are spaced. Instead of the grid wires being uniformly spaced as in a sharp-cutoff tube, the wires in a remote cutoff tube are closer together on the ends and further apart in the middle. What this means is that the control voltage required, and which is supplied by the side-chain (the DC signal discussed above), to stop the electrons from moving to the plate is different for each area of the grid. So, instead of the grid voltage increasing to a certain voltage and all electrons stopping (which is what happens in a sharp cutoff tube), in a remote cutoff tube only a portion of the electrons will stop. In fact, in a remote cutoff tube there will never be a total stop to the flow of electrons, the flow will become very small. For the person listening to this compressor it means that the GR will be really smooth, as opposed to an on/off feel.These descriptions are severally simplified, but hopefully it helps the conversation.