Opto vs FET vs VCA vs vari-mu compressors

At a glance

What's the difference between opto, FET, VCA and vari-mu?

Every compressor has two halves. One listens to the level and decides how far to turn the sound down; the other does the turning down. That second half is called the gain element, and the four names on this page are four different gain elements. Before computers, engineers had to build compressors out of whatever part could change its gain when you fed it a control voltage, and each part they found came with its own speed and its own sound.

The type tells you how a compressor tends to behave. A VCA follows its attack and release settings exactly, while a FET reacts faster than any of the others. An opto has a built-in sluggishness that no knob changes, and a vari-mu gets firmer as it works harder.

If you're new to the controls themselves, start with what compression is. This page assumes you know what threshold, ratio, attack and release do, and looks at how the four designs differ in using them.

Reading the lab

The four cards at the top of the lab switch between the types. Each card has a tiny drawing of how that type turns down a held note and lets go of it afterwards. Switching type loads the settings that type is usually used at, the way you'd reach for a different box with its own knobs, and greys out the controls the hardware doesn't have.

The big screen shows the whole loop. The pale shape is the original and the coloured one is the compressed loop, turned back up to the same loudness so you're comparing shape with shape. The amber lane underneath is gain reduction: the deeper it dips, the harder the compressor is working at that moment.

The One hit screen is the most useful one on this page. It zooms in on 800 milliseconds around the loudest moment and draws the gain reduction of all four types on top of each other, so you can see which one grabs first and which one lets go last. Your chosen type is the bright line and the others are faint. The Harmonics screen runs a 110 Hz sine through your current settings and shows what the compressor adds to it, and the Right now panel describes what's happening with numbers measured from the loop.

These are models of how each type behaves, written for this lab. They aren't copies of a particular unit, and real hardware varies from one example to the next.

VCA compressors

A VCA, or voltage-controlled amplifier, is a small chip whose gain follows a control voltage exactly. It does nothing on its own; the compressor's detector works out how much to turn down and the VCA obeys. That's why VCA compressors are the most predictable of the four. A 4:1 ratio really is 4:1, and a 10 ms attack really is 10 ms.

The classic examples are the dbx 160 and the bus compressor built into Solid State Logic's 4000-series consoles. The SSL bus compressor gives you attack times from 0.1 to 30 ms, releases from 0.1 to 1.2 seconds plus an Auto setting, and ratios of 2, 4 and 10:1.

In the lab, the VCA on the drums takes up to 5.2 dB off the loudest hits and lets go in time for the next one. Because its 10 ms attack lets the front of each hit through, the drums keep their snap. It adds almost nothing to the tone. The only bars on the harmonics screen come from the release following the wave of the low test tone, and they shrink as you slow the release down.

FET compressors

A FET, or field-effect transistor, can be wired to act as a resistor whose value changes with a control voltage. Put it in the signal path and it turns the sound down. FETs react extremely fast, and the Universal Audio 1176 made the most of it: its attack runs from 20 microseconds to 800 microseconds, and its release from 50 ms to 1.1 seconds.

The 1176 is laid out unlike most compressors. It has no threshold knob, so you turn up the Input to compress more and turn down the Output to get the level back. Its attack and release knobs are fastest fully clockwise, the opposite of what you'd expect. The ratio buttons give 4, 8, 12 and 20:1, and pressing all four in at once, usually called all-buttons mode, gives a much more aggressive, distorted sound.

Even the slowest FET attack is quicker than most VCA settings, so a FET catches transients other compressors let past. With the same amount in the lab, it takes up to 9.6 dB off the drums, nearly twice what the VCA took. That speed is why FETs get picked for grabbing the loudest syllables of a vocal or for flattening drum peaks into something more aggressive.

The cost is colour. A FET is only clean while the signal across it is small, so its distortion grows the harder it works, and a fast release on low notes adds more, because the gain starts moving with each cycle of the wave. In the lab, with the amount at 12 dB, dropping the release from 100 ms to 50 ms raises the 3rd harmonic of the test tone from −44 to −39 dB. On a vocal or a snare, that edge can be the reason to pick one.

Opto compressors

An optical compressor turns the sound into light. Inside the Teletronix LA-2A, the audio drives a glowing panel, and a light-dependent resistor next to it changes value as the light gets brighter. That resistor sets the gain. The pair sits in a sealed module called the T4.

Light cells don't react instantly, and they take longer to recover than to react. The LA-2A's attack is about 10 ms, and its release happens in two stages: roughly 60 ms to give back the first half of the gain reduction, then anywhere from about half a second to several seconds for the rest. The cell also remembers. After a short peak it recovers quickly, but after a long stretch of heavy compression the slow stage gets slower. That's called a program-dependent release, because it depends on what you've been playing through it.

That's why the LA-2A gets away with having almost no controls. It has a Gain knob, a Peak Reduction knob and a switch between Compress and Limit, and the cell takes care of the timing. In the lab the opto takes up to 4.9 dB off the drums, less than the FET, yet it works on the whole loop because the slow stage never quite lets go. On the vocal, the gain reduction rises and falls with the phrases and hardly reacts to single syllables, which sounds like a careful engineer riding the fader.

Not every opto copies the LA-2A. The Tube-Tech CL 1B is an optical compressor with attack and release controls, for example. The slow, smooth reaction of a light cell is what they share.

Vari-mu compressors

The oldest design of the four uses a valve (a vacuum tube, in American English). Some valves are made so that their gain falls off gradually as you change the bias voltage on them. A vari-mu compressor feeds its control voltage into that bias, so the louder the signal gets, the lower the gain. The name comes from mu, the Greek letter for a valve's amplification factor.

The valve's gain doesn't fall in a straight line, and that's what makes a vari-mu sound the way it does. There's no ratio knob, because the ratio is part of the valve, and it rises as the sound gets louder. In the lab's model it starts bending 6 dB below the threshold, presses at about 2:1 on a peak 6 dB over and at more than 3:1 on one 18 dB over. In effect, a vari-mu has a very wide knee.

The Fairchild 660 and its stereo version, the 670, are the famous vintage examples, and they have a time constant switch in place of separate attack and release knobs. The Manley Variable Mu is a newer design used a lot in mastering. All of them pass the audio through valves, which adds colour of its own. The lab's model gives it mostly even harmonics, and at the lab's usual settings its 2nd harmonic is the loudest bar of all four types, at −34 dB on the test tone. Real units vary here: the Fairchild amplifies in push-pull pairs of valves, a layout that cancels some of the even harmonics.

On the drums, the lab's vari-mu takes only 3.6 dB off the loudest hits but works on 99% of the loop. It presses everything a little and the peaks a little more, which is why vari-mu compressors suit whole mixes. Across the full mix, with a 30 ms attack and a 600 ms release, it takes no more than 2.3 dB off and the parts move together.

Which compressor type should you use?

Pick by the job first and the colour second. Exact control is a VCA's job and speed is a FET's. When you want a sound to sit at one level without hearing it happen, reach for an opto, and for slow, gentle pressure across a whole mix, a vari-mu.

A common pairing on vocals is a FET first, taking a few decibels off the loudest syllables, then an opto after it doing slow, even levelling. Each one only does a little, so neither is obvious, and the vocal ends up steadier than either would make it alone. The vocal chain lesson builds that chain step by step.

How to choose a compressor type by ear

1. Decide what you want to changePeaks that stick out, a level that wanders, or a group of parts that don't sit together. That points you at a FET, an opto or a bus compressor.
2. Set the same amount on eachAim for 3 to 6 dB of gain reduction on the loudest moments, whichever type you try, so you're comparing like with like.
3. Match the loudnessUse the output or makeup control until switching the compressor off doesn't change the level. The lab does this for you.
4. Listen to the front of the notesA FET changes the very start of each hit or word, and a VCA or opto with a slower attack lets it through.
5. Watch the gapsLook at the gain reduction meter between notes. An opto and a vari-mu stay down for longer; a VCA or FET with a short release jumps back.
6. Then listen for colourOn a bass or a sustained synth, a FET pushed hard or a vari-mu adds harmonics you can hear. Decide whether that helps.

Do plug-in compressors have a type?

Not in the same way. A plug-in has no transistor or light cell; it calculates how far to turn the sound down and multiplies the audio by that number. That behaves most like a VCA, which is why a stock DAW compressor tends to sound clean and do exactly what its knobs say.

What a plug-in can copy is the behaviour: the 10 ms opto attack and its memory, the microsecond FET attack, the rising vari-mu ratio and the harmonics each one adds. The four models in the lab do a simple version of that, and emulations of units like the 1176 and the LA-2A do it in far more detail. Logic Pro's Compressor has a choice of circuit types named after VCA, FET and opto designs for the same reason. Since the type mostly describes timing and colour, you can get some of the way with a plain compressor by setting it the way the hardware behaves, which the attack and release lesson covers.

Common mistakes

Choosing a type because of a name

An 1176 on a vocal is a fine choice, but only if the vocal needs its peaks caught. If the problem is a level that drifts from one line to the next, an opto or a slower VCA will do more for it.

Comparing them at different loudness

Different types take off different amounts at the same setting, and the louder one always sounds better at first. Match the levels before you decide, or you'll end up picking whichever compressor had the most makeup gain.

Expecting an opto to catch fast peaks

A 10 ms attack lets the first few milliseconds of every hit through. If a vocal still has spiky consonants after an LA-2A, that's how it's meant to work; put a FET or a limiter in front for those.

Pushing a vari-mu too hard

Because its ratio climbs as it works, a vari-mu taking off 6 dB is pressing much harder than the same unit at 2 dB. On a mix bus that shows up as a dull, squashed mix, so keep it to 1 to 3 dB.

Fast FET release on bass

A 50 ms release follows the cycles of low notes and adds distortion. If you want the bass clean, slow the release to 200 ms or more, or switch to an opto.

Training your ears to tell them apart

The quiz under the lab plays the loop through one of two types at their usual settings, with the screens covered and the loudness matched, and asks which one it was. FET against opto is the easiest pair: listen for whether the front of each hit is flattened. VCA against opto is harder, so listen to the space after each loud hit, where the opto stays down for longer. Your best score is kept in your browser.

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