Compressor Types - FET , Opto, VCA and Vari-Mu

Detailed explanation of the various types of compressors with processed before/after audio examples to display the differences between compressor designs.


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Compressor Types – FET, Opto, VCA and Vari-Mu video tutorial explains all the different types of compressor topologies and how they work.

With most music genres producers will use parallel processing for treating a number of the recorded sounds. In Hip Hop working in parallel is routine and knowing how to set up a parallel channel and maintaining and controlling the gains across the dry and parallel channels is as important as the compressor type to use. However, before we can get funky with the different delicious compressor modes available we need to understand what parallel compression is and what the different compressor topologies are.

Parallel or NY Compression

The idea is very simple: you take a drum beat, copy it onto another channel in your DAW, heavily compress it and mix it with the original channel. The thinking is that the original unprocessed drum beat acts as an anchor to keep the drum beat in check whereas the parallel or copy is used for heavy processing or ‘colour’. The two mixed together will give a denser controlled sound. HOWEVER, this is not the only way to create a parallel copy. You can use the auxiliary feature in your DAW and send the drum beat to that in varying amounts. It is the same procedure as setting up an effects send/return in your DAW. I tend not to use auxiliaries for parallel processing for the simple fact that any process I apply to the original is copied across to the auxiliary channel.

Although most compressors come with a Mix feature which in effect mixes the original signal with the processed one and thus is termed as parallel it is not the same as running two separate channels of the same audio. We might need to apply additional processing to either channel without one interfering with the other.

Parallel compression is only the tip of the iceberg when it comes to parallel processing. Basses, drums, vocals and so on can all benefit from this wonderful powerful technique.

Compressing Hip Hop drum beats and in parallel requires a basic understanding of how different types of compressors process audio. The behaviour of a compressor is defined by a number of factors but it is safe to say that the gain reduction cell is what drives the compressor and different gain reduction technologies result in different behavioural characteristics.

Let us take a quick look at the most common types of compressor topologies and how they behave:

Compressor Topologies (types)

VCA – Uses a Voltage Controlled Amplifier. VCAs are fast and have low distortion which makes them come across as ‘clean’. However, VCAs can sound aggressive if pushed. Because VCAs don’t add colour to a sound they can be used for transparent compression as they sound clean and smooth. Most common VCAs used are the SSL G Bus and the DBX 160.

FET – Uses Field Effect Transistors. FET compressors emulate the tube/valve sound using transistor circuits. They are fast and bright sounding and are popular with rock genres for exactly these reasons. FET compressors add both punch and colour to a sound and can be extremely useful on percussive sounds or sounds that need a snappy transients. The most famous FET compressor is the Urei 1176.

Opto (optical) – Uses a  photocell as a detector and a light bulb to determine the gain reduction. Different light sensor types and illumination sources affect the gain reduction in different ways. The time lag between the photocell and detector makes for a slow attack and release and this makes this type of compressor perfect for processing vocals, performing gentle automatic gain changes and mastering. The most famous Opto compressors are the LA-2A and Avalon AD2044.

Vari-Mu – Uses a valve (tube) as the variable gain reduction. They are slow to react which makes them perfect for ‘glueing’ purposes and the ratio increases with gain reduction which gives this topology such a distinct sound. This type of compressor works best on the master/group bus as it glues sounds together into a single homogeneous sound. It can be used to add warmth and thickness to a sound. The most famous Vari-Mu compressor is the Fairchild 670.

Now that we understand how to run a parallel channel/copy in the DAW and have familiarised ourselves with the various compressor topologies we can process a Hip Hop drum beat confidently.

In the Compressor Types – FET, Opto, VCA and Vari-Mu video, I create a parallel channel using a Hip Hop drum beat. I then run the drum beat through 6 different types of compressors making sure to cover all the compressor types. I explain how each compressor works and what settings to use to achieve different textures. I show you how to manage gains across all the compressors when using both the dry and parallel channels.

Plugins used in this video:

Softube FET Compressor

Softube CL1B Compressor

Slate FG-Grey Compressor

Slate FG-Red compressor

FG-MU Compressor

FabFilter Pro C compressor

Topics covered in this video are:

  • A detailed explanation of the most important types of Compressors
  • Working in Parallel
  • VCA, OPTO, Vari – MU, FET
  • Structuring and Trim
  • ALL Mode
  • Fixed Values
  • Limiting

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