Compression 101: Types of Compressors

The other day, I did my best to explain how compressors work, but despite the length of the post, I still didn’t answer my dad’s question: “Why do you need more than one?”

So with this post, I hope to answer that question!

Initial Thoughts

My first answer to the above question is that it’s very useful to have multiple channels of compression when tracking. My personal philosophy when it comes to recording is that it’s better to do as much processing up front as possible so you can spend less time worrying about it later. It’s for this reason that I’m investing my time and money into analog gear to track through and also mix through. Well, that and also because hardware is way more fun than plugins. Plus, I get to build a lot of the hardware myself. Anyway, onto the main purpose of the post!

Types of Compressors

There’s a surprising amount of diversity in the world of compressors, especially if you’re new to the concept. I read a great comparison the other day that types of compressors are a lot like shades of colors. Shades like forest green, yellow green, lime green. You know which colors you want to use when painting, but it can be kind of difficult describing them to people. Compressors are generally named after the specific component that applies the gain reduction. So, without further ado, let’s get started.

FET Compression

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You again…

FET stands for Field Effect Transistor, which is just a type of electrical component that can apply gain reduction to a signal. Almost all FET compressors are based off of the classic 1176 circuit that was developed by the Urei company back in 1967. FET circuits are very popular for their introduction of subtle harmonic distortion to the audio signal. Also for their sheer versatility. There’s a reason this design has remained almost unchanged since it’s inception. To a sound engineer, the 1176 is basically the Frank’s Red Hot of compressors. You can put that shit on everything. It’s one of the reasons I’m building a stereo pair of them despite already having a mono unit in my rack. They’re just that usable. Also, the stereo pair that I’m building is based off of the Revision D, whereas the Blue stripe unit is based off of the Revision A, but that’s a topic for another post. Functionally, FET compressors tend to have a very fast attack, and a fast to moderate release.

VCA Compression

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The GSSL is a VCA compressor

VCA stands for Voltage Controlled Amplifier. Technically just about any compressor circuit could be described as a ‘VCA’ circuit if only because of the nature of the components used. VCAs are really just a collection of transistors that follow the incoming signal and apply negative gain. VCA compressors are very similar to FET compressors in that they have a very fast attack time and fast to moderate release time. However, as FET compressors add a certain amount of color to a signal, VCAs tend to compress very cleanly. One of the reasons I added the Cavendish mod to my GSSL was to give it a little bit more character, as the stock very was a little too clean for my tastes.

Opto Compression

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The most famous opto compressor: The LA-2A

Opto compressors are pretty cool. As you might have guessed by the name, the opto compressor uses light and optics to apply gain reduction to a signal. The way it works is that the incoming signal is sent through a light. The light shines brighter or softer depending on the amplitude of the signal flowing through it. The light then shines on a light sensitive resistor, the resistance of which changes depending on the intensity of the light. Opto compressors are characterized by their very slow attack and release times. Even the slowest attack setting on an 1176 will be faster than an opto compressor. These types of compressors excel on vocals, and chances are that if you’ve listened to music recorded within the last half-century, you’ve heard this compressor.

Tube Compression (Variable Mu)

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The Fairchild 670 is considered by many to be the ‘Holy Grail’ of compressors.

Tube compression is like a mix between FET/VCA and Opto compression. The circuit is much more similar to the FET/VCA style, but instead of using a transistor for gain reduction, they use vacuum tubes. This small change makes a big difference, since it takes much longer for a tube to respond than a transistor, attack times for these types of compressors are usually pretty slow. Most people who’ve used them claim that they have a very smooth character and impart a lot of color to the source signal. I’ve never used one personally, but Drip Electronics sells PCBs for a clone of the 670 that I’d love to build some day.

Diode Bridge Compression (or Zener Compression)

 

ZENER
Diodes!

Zener compression circuits are based of of zener diodes. Diodes generally are used as voltage regulators. The only allow electrons to pass across them in one direction. LEDs are a common type of diode. I’m no expert when it comes to the working of various electrical components, but from my understanding, Zener diodes differ from other diodes in that they can and will allow certain voltages to pass through them in reverse to a certain degree without failing. It’s most likely this characteristic that is utilized to apply gain reduction in an audio circuit. One of the first zener compressors, EMI’s TG-1, was built to replace the Fairchild 670. Functionally though, the zener circuit can achieve a much wider range of attack and release times, from as slow as an opto to as fast as a FET. Once I’m finished with my stereo 1176 build, I’m going to start slowly putting together a stereo zener compressor based off of the TG-1 circuit. Can’t wait!

PWM Compression

PWM stands for Pulse Width Modulation, and it a term that should be familiar to any synth fans reading this. Basically it refers to the frequency of a square or pulse wave signal. Believe it or not, these compressors essentially work by feeding the audio signal through an on/off switch triggered by the square wave. Essentially what it does is turns the audio on and off hundreds or thousands of times per second to apply gain reduction. It seems like kind of a counterintuitive way to do it, but the fluctuations happen so fast that you can’t actually perceive the audio switching on and off. All you hear is a reduction in volume.

That’s every type of compression circuit that I personally know of. Considering though that I’ve only just learned about the last two within the past year, I think it’s safe to say that there may be even more types out there. I’ve learned a lot about how compressors work from building them, and I’m always excited to learn more. Hopefully the parts I’m waiting for for my 1176 build come in the mail soon so I can get back to work on it. In the meantime though, I hope you enjoyed learning about compressors. I might come back and write more about them in the future, so stay tuned.

 

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