WHAT COMPRESSOR SHOULD I USE, AND WHEN?
Compression is one of the most common tools available to audio engineers, one that we tend to reach for all the time – and yet it is one of the deepest and sometimes least understood topics out there.
Like everyone I’ve been using compressors since my first days in the audio world, but I have to admit it took me years to start really understanding their intricacies. Not only they are powerful tone shapers, featuring an array of settings that need to be understood (attack / release, knee etc.) but they also come in a myriad of different brands and models, which we will try to cover here (non-exhaustively).
So, what kind of compressor should I use, and for what purpose?
There are four common categories you will be likely to encounter, all associated with different sounds and uses:
VCA compression stands for “voltage controlled amplifier”, and utilizes an IC chip that contains transistors to determine how much negative gain to apply. It is one of the most common compression type, and VCAs are associated to a “clean” sound, with fast to very fast attack timings. Also it is relevant to note that their detection circuit is generally based on PEAK levels (not RMS). Hence VCA compressors tend to react a lot to transient material, making them super useful to shape the micro-dynamics of a source and keep transients from being overly loud.
When to use them: for clean compression, controlling peaks on percussive materials (guitars, drums etc.), and sometimes adding some “smack” when pushing particular VCAs harder like the traditional dbx 160 and its modern counterparts.
Personal favorites: dbx 160X or 160A (the best hardware compressor for the money I feel), UAD dbx 160 (plugin), Vertigo Sound VSC-2.
The Vertigo Sound VSC-2 (and its excellent plugin emulation from Brainworx) is one of the most powerful VCA compressors, featuring premium, fully discrete VCA chips.
Opto (or optical) compressors are very different as they use a photocell and a light bulb in the detection circuit to determine the amount of gain reduction applied. Generally speaking, those circuits are slower to react than previously mentioned VCAs, making opto compressors less sensitive to transient materials and peaks. Since the light bulb reacts more to the “average” level of the program source, opto compressors tend to behave more musically and can apply greater ratios with less artifacts. This can be achieved in the digital domain by using “RMS” detection rather than PEAK.
When to use them: opto compressors are often used with vocal tracks, for their smooth response and great transparency. For years one of the most sought after classics has been the Teletronix LA-2A, but this unit features a tube in its amplification stage so it’s also used for coloration. Optos are somehow better than VCAs for managing macro-dynamics, doing a good job at keeping levels at bay when heavier parts of the song kick in.
Personal favorites: Universal Audio LA-2A (both hardware and plugins), Brainworx bx_opto (one of the best compressor plugins out there!), Tube-Tech CL1B.
The LA-2A has been emulated by so many plugin manufacturers… I am a big fan of the UAD versions, but many others sound great too.
Note: a quick way to change the behaviour of Logic Pro’s Compressor is to choose between PEAK or RMS for the detection circuit in the Side Chain menu. Experiment with it!
Tube Compressors (or Variable Mu)
As mentioned earlier, some optical compressors like the LA-2A are equipped with a tube gain stage, but they are not to be confused with Vari-Mu designs (or Variable Mu), which are using a vacuum tube to achieve the compression itself. Variable Mu and tube compressors are actually the oldest compressor designs, but they are still used today because of their smoothness and musicality. Their compression curve is far from being linear, hence the name “variable”, and the more you push them, the higher the ratio is going to be. The most famous tube compressor is probably the original Fairchild 670, that was used on countless sources by Geoff Emerick (engineer of The Beatles), but this unit is almost impossible to find now and you can achieve comparable results with the Manley Variable Mu, or plugins like the Klanghelm MJUC.
When to use them: when placed on a mix bus, Vari Mu compressors are great to control the macro-dynamics and glue things together in an unparalleled way. But they also shine on individual tracks thanks to their time constants – they take their time and never over-react!
Personal favorites: Manley Variable Mu (hardware and UAD plugin), UAD Fairchild 670, Klanghelm MJUC.
The UAD Fairchild 670 is an excellent plugin recreation of the original unit used by Geoff Emerick.
FET (or “Field Effect Transistor”) compressors are quite close to VCAs, but they differ by using a particular type of transistor to control the gain reduction. FET compressors are usually a bit more colored, the most ubiquitous model being the classic Urei/Universal Audio 1176, and its stereo cousin the Urei 1178. FET compressors are known to be lightning fast (the slowest attack being usually faster than the fastest one on a Vari-Mu) so they are great for quick transients control, but in a more aggressive way than VCAs. And once you start increasing their attack time, they will let some transients pass through and achieve a great punch. The FET time constants are generally so fast that they distort more quickly than other compressors, but in a musical and useful way.
When to use them: “1176” compressors work on almost everything, whenever you are trying to add punch and a sense of excitement, and they are indeed among the punchiest! No wonder they are often associated with that typical “rock snare” sound. They also work extremely well on bass and vocals, which might seem counterintuitive given their tendency to distort, but it might be exactly what you need to make the source pop out more in the mids region.
Personal favorites: Urei 1178 (hardware), UAD 1176 Classic collection (plugin), Slate Digital FG-116 series (plugin). The Warm Audio WA76 is also a great value for the money in my opinion!