Types of Compressor

   By Lisa F   Categories: Audio EquipmentGeneral

Distinguishing between these different designs – either in software or hardware – is essential in picking the right tool for the job. It also highlights the historical evolution for compressors, and how technical innovation might have ushered in some significant extra flexibilities, but also occasionally losses in respect to character and sonic interest.


By comparison to today’s digital signal processing, an optical compressor is a relatively crude form of signal processor, but also one with a surprising degree of musicality. As the name suggest, the input of an optical compressor is fed to a light bulb or LED, which glows brighter or darker in response to the incoming signal levels. In turn, the strength of the light is read by a photocell, which is then used to control the amount of gain reduction applied. Put simply, as the light glows brighter, a greater amount of compression is applied.

What’s so interesting about an optical compressor is its unique response times in respect to the photocell gain control element. In effect, the light has an inherent lag to it – being sluggish in its response to transients, and slow to fully dissipate its energy after long periods of high-energy signal. As a result, optical compressors have a unique slow-acting attack time, while the release almost has a two-stage quality to it, being initially quick, but then slow to completely return to its null state. Although technically inefficient, the response of an optical compressor is incredibly ‘musical’ and sympathetic to many of the sources you pass through it.

Optical compressors are often slow to react, but they provide a surprisingly musical form of gain reduction.

While older-style optical compressors see plenty of use in tracking and mixing, their application in mastering needs a degree of caution, as there’s often no total control over the attack and release settings. However, there are some examples of hardware master compressors featuring optical stages, such as the Shadow Hills Mastering Compressor, and of course PrismSound’s Maselec Master Series MLA-2 Precision Stereo Compressor is based on an optical gain cell.

Variable- MU

Variable-MU compression is another older design of gain control, this time using a re-biased vacuum tube as the gain control element. What’s interesting about a variable-MU compressor, particularly to the application of mastering, is the lack of a traditional ratio control. In effect, a variable-MU compressor could be compared to an ultrawide soft knee, with the ratio effectively becoming stronger as the compressor is pushed harder. Rather than working with threshold and ratio, an engineer needs to finely tune the compressor’s input and output controls – either driving the compressor hard for a more extreme compression, or in a more reserved setting if you want the compression to be subtler.

Variable-MU compressors don’t offer the fastest transient response, but they compress in a musical way.

Although attack and release controls are provided, it would be fair to say that a variable-MU compressor isn’t the fastest-acting compressor in the world. As such, you’ll probably want to avoid using variable- MU designs for any kind of aggressive peak limiting activities. Instead, use a variable-MU compressor to massage the body of the track into place, often using deliberately graduated attack and release settings so that the compression is smooth. Of course, as the compression is tube-based, it’s also likely that the variable-MU design will offer a touch of extra colour to your master, which may be as beneficial as the compression itself!


FET compressors, which use a ‘field effect transistor’ for their gain control, where originally developed as a snappier alternative to variable- MU and optical designs. Even to this day, FET compressors are famed for their ultra-fast attack and release settings, making them a much more suitable choices for peak-style compression, and for more extreme ‘colorful’ compression.

FET compressors are fast-acting with plenty of character. Not the subtlest compression in the box, but they have a great colour to add in the right situations.

Despite having superior attack and release times, FET compressors are often more coloured than other types of compression, and aren’t widely used in mastering. That said, if you’re looking at techniques such as parallel compression (which we’re taking a closer look at later on in this chapter), the distinctive fast-acting and colourful sound of an FET compressor may well be an interesting avenue worth exploring. However, if you’re look for smooth transparent compression, you’re in the wrong place!


In many ways, VCA compression represents the principle of technical and sonic development in respect to the design of a compressor. By using a ‘voltage-controlled amplifi er’ for its gain control, a VCA compressor can provide plenty of smooth gain reduction and full control over its attack and release settings. In short, VCA compressors are versatile – adaptable enough to provide smooth and sumptuous ‘glue’ to a master, or aggressive enough to clamp down on transients. For a mastering engineer, this versatility is an important quality, especially if they’re working with a huge variety of musical styles, each needing a slightly different approach to dynamic control.

A good mastering-grade VCA compressor is one of the most flexible and dependable tools to have at your disposal.

However, before we crown the VCA as the king of compressors, it’s worth pointing out that there are some huge variations between the quality and performance of different VCA designs. Given the relative cost efficiency of producing a VCA compressor in comparison to optical, FET or variable-MU designs (which all require expensive discrete components), a lot of VCA compressors have been built to provide ‘serviceable’ audio performance, rather than audio excellence. Rather than using any VCA compressor, look at designs that are specifically aimed at mix-buss processing, such as the Neve 33609, SSL XLogic G-series, or the Vertigo Sound VSC-2.

Excerpt from Practical Mastering: A Guide to Mastering in the Modern Studio by Mark Cousins and Russ Hepworth-Sawyer © 2013 Taylor & Francis Group. All Rights Reserved.


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