Extensive compressor reviews and FAQ

What are the differences between various types of compressor?
Compressors are often described with category names like VCA, optical, FET, feed-forward, OTA, and so forth. These names are for different circuit designs, for the part that actually follows and reacts to your signal level, and the way in which it reacts. Naturally you will wonder "how do they sound", and why would you pick one type over the others.
The first thing I should say is that it's like the difference between lime green, grass green, Kelly green, forest green, etc. They are all different enough that you would have reason to pick one over another if you're painting a room, but you can't make a useful comparison without actually seeing them next to each other, in context; and whether they are "very different" or "very similar" just depends on your perspective. Also bear in mind that while comps certainly can affect your tone, the type of compression circuit doesn't actually tell you anything about the tonal effect.
VCA stands for Voltage Controlled Amplifier. Technically most comp circuits could be described as VCAs in a sense, but in practical usage it means an IC chip that contains transistors that follow your incoming signal level (voltage) to determine how much negative gain to apply. VCAs are typically associated with fast, clean compression with precise controls over attack and release. Symetrix, dbx, Boss CS-3, Maxon CP9Pro+, and Alesis 3630 are examples of VCA comps. Most cheaper rack comps are VCA controlled. The THAT Corporation, originally a division of dbx, produces most of the VCA chips you'll encounter.
FET stands for Field Effect Transistor, and they are essentially a subset of VCAs, with their own particular qualities. FETs are often used as a solid-state emulation of tubes, but the main reason a comp will be advertised as FET-based is so you'll associate it with the famous and coveted UREI 1176. So it's mostly about the marketing, but you can expect more "color" in a FET effect than from a clean VCA. Aside from the 1176, the other examples of FET comps are mostly expensive rack units by brands like Vintech, Daking, Purple, and Chandler. The MXR M87 pedal is marketed as a FET comp, but it sounds just like any regular VCA unit.
OTA stands for Operational Transconductance Amplifier, which is a chip like a VCA except its output is variable current, not voltage. It refers to a specific circuit design based around the CA3080 chip (or a modern replacement for it like the LM13700), which is the basis for the Ross, Dynacomp, Keeley, Wampler, and probably 90% of all comp pedals on the market. Most "boutique" comps, with all their claims about refinements and developments for ultimate tone, are just versions of this one very simple circuit--sometimes with no improvements other than a fancy paint job and a 3PDT footswitch. Its envelope is a bit hard to describe, but it happens to work really well for guitars.
Optical comps contain a light source that gets brighter as your signal gets louder, and a light-sensitive resistor that reacts to the brightness of the light by decreasing its resistance. The resistor works in either a feedback loop or a voltage divider to change the level of your signal. In some older designs these were separate components, but in 99% of modern designs the light and the resistor are encased in one small black cylinder. Optos are generally described as very smooth, slow-releasing, and "organic". They can be very neutral/invisible, or very squashy/effecty, depending on how the circuit is designed. Optical examples include Demeter, Diamond, EHX Black Finger, Retrospec, and Joemeek. ELOP is just a trademark of the brand Manley, for their opto comp design.
Tube compressors, almost always, are really optical comps with a simple 12AX7 tube gain stage added on the end. However vari-mu (short for Variable Mu, also a Manley trademark) designs use a vacuum tube in place of a transistor, where the variable voltage input changes a tube's bias instead of a transistor's gain. It has a somewhat different action, generally claimed to be even smoother and more "creamy" or "organic". Aside from the Manley and some very old Fairchild units, the only other vari-mu-style examples I know of are the Markbass Compressore and the Lightning Boy Opti-Mu Prime.
Feed-forward means the control circuit will receive ("hear") your signal before the signal goes through the amplifier stage; this is the more modern approach. Feed-back means the controller receives the signal after the amplifier stage, and this is more associated with older designs. "How they sound" is a very subtle shade of distinction, you'd have to try both to have a feel for it. Most of the previously-named circuit designs can be built to feed forward or back.
There are lots of other circuits for compression that don't necessarily fall into a named category. The Orange Squeezer is its own thing, based around the JRC4558 chip, with a unique action. The TLC (Trans Linear Conductance) circuit by Aguilar is its own thing, similar to a VCA but not one exactly. Advanced electronic designers like Rupert Neve develop circuits that are either completely original, or they use pieces of earlier designs but in a new way that doesn't have a name yet. So not every comp fits a neat category.
Pressed for a short and easy simplification, you'd probably be OK generalizing that VCA comps are fast, modern, and precise; optical comps are slow, vintagey, and organic; and OTA's are somewhere in the middle with a response that guitarists like. But remember there are plenty of exceptions to those descriptions. And all the rest of the "types" have their own subtle differences in action that are really hard to put into words--you would just have to compare them in person, to get a feel for it. So this still doesn't tell you how they sound, but at least now you have an idea what it all means.

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