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Audio Compressor Types Explained: VCA, FET, Optical & Vari-Mu

beginners compression Aug 05, 2021

There are four main compression types that will most likely sound familiar to you as soon as you begin using a compressor in music production.

  • VCA
  • FET
  • Optical
  • Vari-Mu

What do they all stand for and what are they doing inside the box? Which one is the best?

Well, in this video I explain the basics of audio compression and dynamic range control, as well as the different compressor types available and how they effect the sound of your music.

Each type has its own signature sound and once you know what that is you can use that knowledge to pick the best type of compressor for your track or mix.

A Subscriber has asked me to explain the different audio compressor types used in music production today. So in this video taken from my upcoming course in compression, I do just that.

Audio Compression in Music Production

Choosing the best type of compressor for your track is yet another decision to make that can make the whole topic of audio compression get over complicated, particularly for people just starting out in music production.

Thankfully, there aren't that many different types of audio compressor to discuss. And in this lesson taken from my soon to be released compression course, I'll explain in simple terms what the top four types are, how they differ and where best to use them when mixing your music. Let's dive in.

Let's examine how a compressor actually works, what it does to an audio signal, and then let's look at the four main types of compressor available to us in our DAW. Let's look at the characteristics of each and what might be the most suitable application for each one.

In this lesson, we're going to talk about what was compressor does to an audio signal, as well as how compressors actually function, and we'll also look at the four main types of compressor and find out why they all sound different to each other and where you might consider using each one to take advantage of its characteristics.

After watching this lesson, you'll be able to quickly choose the best compressor for the sound you have in your head. You'll be across all the major designs of compressors and the sonic characteristics of each one. And you'll no longer be mystified by all the acronyms that up until now were just plain confusing.

What is compression?

So what is compression, how does it control dynamic range? Well, compression is, put simply, an automatic volume control. Like a remote control for your fader. A compressor controls dynamic range based on settings we apply for the outcome we want to achieve. Once we know what the settings do, we can make better decisions about how we want a particular source signal to sound in order for it to best fit in the mix.

We use compression to balance the very loud with the very soft sounds of an instrument or voice to have it sit in the mix more consistently and importantly audibly. The compressor controls dynamic range by manipulating amplitude, using a predetermined set of rules in order to achieve the goal of consistent level coming into this course, and partly the reason for taking this course was probably to get your head around all the different types of compressor out there, particularly the swathes of vintage analogue compressor emulations that quote terms like FET VCA and opto.

What does it all mean??

It's important to understand these differences because understanding them helps us make quicker, more efficient and in the end, better choices. Now, I don't want to go too far into the weeds of how the insides of these compressors differ, because that's not really important. But each type uses a different set of electronic components that determines the sonic quality of the incoming signal, which becomes part of the reason you might choose one over another and ultimately find your own favourite.

These electronic components are transistors, tubes or valves, if you're in the UK. Chips and LEDs, all of these components affect the way the gain reduction circuit sounds inside the compressor is not important that you know how they actually work, but it is important to understand what specific designs do to our sound. One compressor uses tubes to control the gain reduction and another one uses a controlled voltage. What kind of options do these different compressor's give us? And again, what does that mean for our sound?

So let's dive into that next.

The four most common types of compressor are VCA, FET, optical and Variable Mu, the first one we'll look at is VCA.

VCA Compressors

VCA stands for voltage controlled amplifier and VCA compressors are found on SSL channel Strip's, API compressors the dBx 160 and the Shadow Hills Mastering compressor, amongst others. Many engineers love them for their predictability and versatility. And you'll quite often see them on the mix bus on groups of instruments like guitars, bass, drums and vocals. In use,

they are very fast and clean sounding. The dBx 160 and the API 2500 here are both commonly used as a drum bus compressor because of the fast, clean nature of their operation. The next is FET and this stands for Field Effect Transistor. These, like the VCA compressor, are very fast acting compressors and are great on transient heavy material like drums, acoustic guitars and bass guitars. They have a more colourful sound to them than VCAs, and when pushed add a pleasing musical sounding distortion that gives the signal more life and energy.

FET Compressors

One of the most famous, widely used compresses of all time is a FET compressor, and that is the Urei 1176 with only two knobs because of having a fixed threshold design, they are also incredibly easy to use. If you want punch use a FET compressor. It's a type of compression that works really well on aggressive performance. A FET compressor adds a lot of colour to the sound, which can be quite pleasant on drums, vocals, bass and, well, anything really.

Optical Compressors

Optical compressors utilise an LED in their operation that shines on a light sensitive resistor, resulting in a very clean, smooth sound with little to no distortion. But they're not as fast acting as a VCA or the FET designed compressor. This makes them great for vocals and any other sound that needs some rounding off or smoothing out. Typical models here are the Teletronics LA2A, the LA3A and the tube tech CL1B. The shadow hills. Mastering compressor has separate optical and a discrete VCA dynamics section, giving you access to two stage serial dynamic's control, making it a really versatile compressor for taming any transient heavy material.

Variable Mu Compressors

Next, we have variable mu. These rely on tubes to tame the dynamics and are the reason these are such a colourful compressor to use. They have a slower reaction time than the VCR and the FET types mentioned earlier and are very characterful as the transients tend to be handled more musically instead of assertively as, say, a VCA or FET compressor, the ratio in these units actually increases with gain reduction, which means the louder the transient is, the more it's going to compress.

This is referred to as non-linear compression and they sound great on a mix bus to add a gluey rich character. And the famous models of note here are the Fairchild 670 and the Manley Vari-mu. The extra warmth you get from this compressor also works brilliantly on drum bus overheads or room mics and can help to really open up the drum sound. You shouldn't, however, use this compressor when you want punch or to solve particularly wild dynamics.

What you'll notice is that what separates these types of compressors, apart from their sound, is the way they handle the input signals.

Some of the compressors don't have threshold controls, some don't have ratio controls, and some don't have attack or release parameters. VCA compressors are the standout here, as they often have all of the standard controls available to a compressor. All of the compressors discussed so far have been analogue compressors that are world famous and have been used in recording studios for decades, and they're all now available in plugin form so we too can enjoy them in our DAWs without limitation.

As discussed, each has its own unique character to impart to an audio signal adding colour or vibe, often interpreted when listening as having more depth, sheen or width. One compressor I haven't talked about so far is the digital compressor, the stock compressor that usually comes with your DAW or a clean digital compressor from a third party plug in manufacturer that's been designed for a more transparent, colourless operation, like, for example, when mastering or when you're happy with the sound captured during recording and just want to control it.


As you start to learn about compression, it's best you start here with a digital plugin that has all the controls at your disposal, using a clean sounding compressor means you'll also hear what compression is actually doing to your audio rather than any sonic changes that might be enhancing the sound as a result of the circuitry rather than the act of compression itself.

So, that's the top four audio compressor types explained and if you'd like to get on the waiting list for the full compression course, check out this link to sign up and get notified when it's released.

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