Audio Compressors: VCA, OPTO, FET, And Other Compression Circuit Types

A circuit board showing capacitors and transformers

One of the most common topics that I get asked about is compression. People often ask “Should I get X or Y plugin?” without realising that they just compared two completely different audio compressors. When it comes to the audio engineering world, it seems compression takes the close second on a list of difficult things to comprehend. Second only to mastering of course.

Most people have an understanding of the basic principles of compression. However, when it comes to the more subtle differences between audio compressors, a lot of you might feel a little lost. This article aims to rectify that. We’re going to delve into the depths of circuitry and find out what makes a compressor right for the job.

Audio Compressors? Circuit Types?

So you may now be asking yourself what these circuit types are. Most audio compressors operate using either VCA (Voltage Controlled Amplitude), FET (Field-effect Transistor), OPTO (Optical), or VARI-MU (Variable-MU). By learning to differentiate these circuit types, you will gain far more control over your mixes. Let me break each of them down for you so you can see what key aspects make them each so unique.

VCA – Voltage Controlled Amplitude

An image of one of many VCA audio compressors
Native Instruments VC160

Perhaps the most famous VCA compressor of all time is the DBX160. Practically every VCA style plugin that you’ll come across today will be modelled, in some way, on this device. Voltage controller amplitude compression acts on PEAK signal input and offers a fast attack and release. For this reason, it’s fantastic at taming transient content.

To use it in a practical scenario, let’s say you’ve got a snare top recording where the drummer gets a little happy during transitions. Here and there, you are likely to have some hits that stick out. Due to its fast acting nature, VCA compression will help to act on those hits almost instantly. This can draw them in line to an acceptable dynamic range for your mix. Of course, there can be a tendency to go overboard with this. VCA audio compressors are fast and generally quite sensitive. Make sure to keep their usage minimal or you risk squashing the life out of your recordings.

I very rarely use a compressor to even out dynamics. Dynamics are something that I just can’t get enough of. – Dave Pensado

On a contradictory note, VCAs are brilliant for parallel drum compression. Being so fast and heavy handed, they can help to produce fantastic snap and punch. Dial them in strong (I’m talking 10dBs of reduction) and listen to your drums smash through the mix.

As you are probably now aware, there are some pretty obvious times not to use this. If you are looking to add colour, warmth or a sense of smoothness to anything, this isn’t the circuit type for you. Due to its fast nature, VCA compression has no regard for long frames of average volume.

FET – Field-effect Transistor

Moving on to our next circuit type, the FET has played a major part in the music business. The likelihood of finding a studio that doesn’t have a 1176 (or other FET style audio compressors) today is incredibly low. If you’re looking for some punch and aggression, you’ve come to the right place.

One of many FET style audio compressors
Softube FET Compressor

Whilst it isn’t the most transparent sound, this is hardly a problem. The FET is often treated more as an effect than a process. It’s something you want to be able to hear doing its job. If you can’t, you’re not using it right! Typical usage will be on anything that you want a bit more character out of. It’s known for its use within the typical rock band sound (think big drums, driving guitar, and aggressive vocals) but shouldn’t be limited to this. The transformer technology offers warmth to anything that passes through it. In fact, many great records have been passed through these units with the compressor turned off. Why? To capture the rich sound and subtle harmonic distortion that it adds.

Obviously, the time to avoid FETs is when you want simple, smooth, and transparent compression. Running a beautifully recorded violin piece through a FET to even it out is probably the worst thing you could do to it. It’s also not fantastic as a bus compressor. Whilst it’s great at adding character to individual sounds, it can lead to muddy and trashy sounding mixes when used as a glue compressor.

Optical

One of many OPTO audio compressors
Tube-Tech CL 1B Optical Compressor

Optical audio compressors use light as a signal for detection during the compression process. The audio signal is passed through the bulb, causing it to glow brighter or dimmer, representing the signal strength. However, as the strength of the light is subject to the speed at which the filament of the bulb can adjust, we see a much slower effect. Think of optical (or OPTO for short) as average compression (RMS) rather than peak.

Unlike our previous circuit types, optical compression is smooth and colourful. It has a tremendous sense of musicality to it. Due to its slow nature, it works far better than the previous two as a bus compressor to average out dynamics over time. It is also brilliant at tightening up individual sounds without throttling them. I almost exclusively use optical audio compressors for things like synth bass or bass guitars. With slow attack and release settings, you can really thicken up bass tones whilst keeping the dynamic and playing style intact.

As is probably very clear, the times not to use optical audio compressors will be when dealing with spikes and frequent peak signal data. It simply will not catch these fast enough and due to its averaging nature, can let them pass altogether. Another thing to consider is their usage in bus scenarios. Whilst the gentle nature of subtle compression over time can really help to balance out dynamics, they can be susceptible to pumping caused by excessive bass or large, short-term blocks of loud content.

VARI-MU

Whilst not as common as the previous three, the VARI-Mu has cemented its position in the audio world. The Manley VARI-MU is possibly one of the most sought after pieces of gear ever. These fantastic bus audio compressors are smooth, colourful, and incredibly clever. VARI is short for variable. The reason behind this name comes from its ability to actually adjust its own ratio based on gain reduction. This creates a nonlinear reduction line capable of ducking only as much as is necessary based on the content of a whole song.

One of many VARI-MU audio compressors
Waves PuigChild 670 Vari-MU

For example, a track that features an average dynamic range of 10-14dB may only need 1-3dBs of compression at any given time. The VARI-MU is capable of applying little to no compression when the threshold is just being touched (close to 1:1) whereas a sudden spike will trigger stronger compression (much higher than 1:1) for just that instant.

This circuit type is best used when your mix (or instrument group) has achieved a near perfect dynamic balance. It’s subtle and self-adjusting nature provides the perfect glue as well as some warmth to provide that final stage of unison. It’s my go to for mastering.

Whilst VARI-MUs can be used on individual elements, the likelihood you’ll achieve a better sound than if you used one of the previous three is slim. This thing doesn’t do snap or aggression and to me, it doesn’t offer as much musicality as a smooth OPTO can. Keep it on your buses and treat it like glue.

To Summarise

VCA – Fast, peak based compression that offers you snap and punch. A fantastic tool for dealing with transients. Think drums and percussion. Also, great for parallel compression.

FET – Aggressive, warm, and punchy. The transformer stage can be utilised like a pre-amp without the compressor (in hardware) and it’s a great tool in the classic big rock sound. Not to be used if you’re wanting to keep things transparent.

OPTO – Musical and colourful. RMS based compression that will act slowly to smooth out sounds over time. If you are trying to deal with spikes, this isn’t the one to use.

VARI-MU – Brilliant self-adjusting bus compression. Generally not one to use on individual sounds. Works as a great way to glue mixes or buses together. Very helpful for creating rough mixes.

TL;DR

There are four main circuit types within compression. VCA, FET, OPTO, and VARI-MU. Each of these offers a distinctly different effect and should be considered as individuals. It is important that you understand the difference between each in order to use them correctly. Misuse of compression is one of the easiest and fastest ways to destroy a mix. Take some time to experiment with all of them. Hear how they differ and the outcomes they can offer you. Once you can remember a basic understanding of each, you’ll find your mixes come together a lot easier.

I understand that there is a lot to comprehend in this article and it’s quite technical. I hope that what I’ve said has helped you to understand better but if not, do drop me a comment below and I’d be happy to answer your questions.

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