Article Preview - The SOS Guide To Mix Compression

Compressing the stereo bus can give your music coherence, smoothness and, above all, punch — but there are plenty of pitfalls for the unwary. We lead you through the minefield that is mix compression.

Will Haas

Mix-bus compression can be a confusing topic for inexperienced mixers and engineers. After all, some great mixers swear by using mix-bus compression, while others prefer to keep the mix bus pure. There are also many misconceptions about how and why mix-bus compression is used, and if used incorrectly, it can diminish a lot of hard work. In this article, we're going to explain what consequences compressing the mix bus will have for your music, what its sonic advantages may be, and how best to set up a compressor in this context. The main question to be answered is: Is mix compression right for your mixing style?

To set the record straight, there is a huge sonic difference between mixing into a stereo compressor from the outset versus slapping a stereo compressor on the mix bus just before you print a mix. For our purposes, using 'mix-bus compression' means mixing into, or through, a stereo compressor that is inserted onto the main mix bus before the signal passes to the master recorder and monitor speakers. The whole idea behind this technique is that you are mixing through the compressor from the beginning of the mix process; you are carving your mix, dynamically speaking, through the compressor, and monitoring the compressor's output.

The point is that the same set of fader settings will actually produce a different mix balance depending on whether you apply compression, and how much. Applying compression after the mix is complete thus risks changing the balance you have carefully set up, so unless you mixed into a compressor from the start, compressing the mix is best left to the mastering engineer. Experienced mastering engineers know that compression and limiting after the mix is complete can result in negative mix balance issues, so they do so with the utmost care.

Used appropriately, mix-bus compression can be another tool for adjusting the overall balance of your mix, giving you 'glue' to help meld sounds together and make the overall mix balance sound more cohesive. Obviously, compressing any signal way too much will cause negative effects, but mixing through a compressor does allow you to achieve positive compression effects with balance in mind.

Which Compressor?

The most common questions regarding mix-bus compression seem to be: are there sonic benefits? Does the experienced mastering engineer want me to use it? What sort of compressor should I use? What are common compressor settings when starting a mix? How will my use of mix-bus compression affect the mastering process? How does mix-bus compression affect the overall balance, compared with not compressing the mix bus?

There are many sonic and dynamic differences between different types of compressors that are suitable for mix-bus compression. In general, there are two main reasons for these differences: the type of circuit used to achieve gain reduction in the compressor itself, and the range of time constants and other controls that the design affords the user.

Four different types of analogue compressor are widely used for mix-bus compression: optical, variable-mu tube, VCA (voltage-controlled amplifier) and FET (field-effect transistor). Digital processors, either in hardware or software plug-in format, can also be used for mix-bus compression.

Optical compressors such as the Millennia CL2A have a distinctive character that can work well in a mix-compression role.

Optical compressors are very popular in modern music production, and are generally felt to sound both transparent and musical, partly because the photo-optical detector circuit that controls the amount of gain reduction can be designed with a minimal number of components. Popular optical compressors for this application include the Avalon AD2044, Joemeek SC2, Tube-Tech CL2A and Millennia Media TCL2.

Classic tube compressors such as the Fairchild 670 and Manley Variable Mu offer a unique, 'expensive' sound!

Many modern compressors contain vacuum tubes within their signal chains, but a true 'variable-mu' tube compressor uses a valve to actually achieve the compression, by re-biasing the tube to control the amount of gain reduction — in hi-fi language, 'mu' means gain. A good variable-mu compressor has the characteristic sound quality often associated with well-designed tube equipment — that is, slight but pleasing harmonic distortion and a warmth and smoothness that is hard to find with other compressor designs. Some famous and terrific tube compressors suitable for the mix bus include the Fairchild 660 and 670, Manley Variable Mu and Pendulum ES8.

VCA compressor designs are perhaps the most widely used in mix-bus applications, with popular choices including the Neve 33609, SSL XLogic G-series and API 2500.

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