Reverb’s Guide to Audio Compressor Types

Compressors cover a wide sea of designs and applications in the music world, and trying to determine where to get started with all these various compressor types can be daunting even for the seasoned professional.

The compressor world can generally be sliced into four different categories: optical, variable-mu, FET and VCA, based on the way that gain reduction is determined and applied. While all these devices effectively do the same job, not all compressors are created equal. Certain types excel in certain applications, and understanding their function and design can be the key to choosing the right compressor for the job.

Optical Compressors

Some of the first purpose-designed compressors were optical compressors. Relatively simple in their design, these compressors utilize a photocell and a light source, traditionally a simple lightbulb-like design and in some more modern designs, an LED. The general principle is that an incoming signal’s voltage causes a variation in the intensity of the light source; the intensity in the light source as received by the photocell will determine the amount of compression that is applied. What makes this so unique is the response in the photocell to the light source. As there is some inherent delay in the response time to trigger the attack and the release of the compressor, there is a natural smoothness to the response, one that in its native tendencies lends itself to musical material. The design results in a compressor that latches onto a signal quickly, initially releases quickly, but tapers off in its release rate as it decays.

Perhaps the most famous example of an optical compressor is the Teletronix developed LA-2A, a tube-based design that uses the military designed T4 photoresistor. The T4 is a cell that has nearly infinite resistance in the dark, but light hitting the surface of it causes a drop in resistance, allowing it to be the control in the circuit for gain reduction. By the original specs, this allows the LA-2A to have a moderate attack (10 milliseconds) and a gradually sloping release that can last up to five seconds depending on the program material. This simple design also lends itself to a simple control schematic. The LA-2A has a two-knob setup, with the right knob controlling the level of the side-chained signal that will ultimately control the compression, and the left controlling the makeup gain to keep the perceived level consistent. This design also has selectable metering and a switch that alternates between a “Compress” and “Limit,” effectively changing the compression ratio.

While sought after by many engineers during mixing and especially tracking, these tend to see limited — if any — use in mastering work due to their relatively fixed characteristics. In application, these compressors tend to lend themselves to any source you want to compress naturally, and work particularly well with vocals, strings and bass. This is not to say you can’t throw transient heavy material at them, especially as newer units and reissues tend to have quicker response characteristics, and can even find favor on drums and guitars with careful use. That being said, legato material still tends to be the most common candidate for optical compression’s loving touch.

Variable-Mu Compressors

In conjunction with optical compressors, vari-mu style compression can be looked at as one of the O.G.s of the compression world. These tube-based behemoths are known for their tone and warmth. The design itself uses these tubes to control the gain-reduction stage, using input voltage changes to alter the bias of the tubes. This tends to result in a slow acting, harmonically rich form of compression that shares a number of the same musical characteristics with its optical brethren. One of the main characteristic differences is the “soft-knee” style compression inherent in all vari-mu designs, and a ratio that is effectively program dependent, increasing the harder the input hits the circuit. While some of these designs allow for variable attack and release, vari-mu compression will always be a relatively “slow-attack” style of compression thanks to the function of the tube circuits.

The primary compressor one conjures up when thinking of variable-mu style compression is the Cadillac of compressors: the Fairchild, which, as it happens, costs as much as an actual Cadillac. The Fairchild Tube Limitercomes in two flavors: the revered 670 stereo unit, and the 660, a lesser-found mono variant. Weighing in at a hefty 65 pounds and packing an arsenal of 20 tubes and 14 transformers, the Fairchild 670 is a legendary unit that has attained mythical status for mix and mastering engineers alike. The original 670 has two knobs per channel, one for gain and one to set the compressor threshold, and selectors for metering, channel link and “time constant.” “Time constant” is an overall setting to adjust the attack and release times, and the stereo link control can select from independent (Left/Right), and effectively M/S or Sum-Difference (Lat/Vert) for the channels. The 660 has similar controls to its bigger brother, however due to a difference in design has a character and sound of its own.

Unlike optical compressors, vari-mu compressors see more use in mastering, even if strictly for the sound of the circuitry more than its compression. Versions with the ability to utilize mid-side processing can be especially effective in a mastering role. In general studio use, stereo units like the 670 often see use as mix and drum bus compressors, and mono uses can range from vocals to piano and acoustic guitars. Care and consideration need to be given with transient heavy material, especially when functioning as a limiter, as the slow attack can really eat up transients, but this can sometimes be a desirable effect. The vari-mu style compression can, however, be used, especially in layers with graduated attack and release settings, to fold elements into a track naturally and musically.

FET Compressors

Field effect transistor compressors are a newer offering, relatively speaking. Originally designed to be a much more transient-happy alternative to the vari-mu and optical options available when they came onto the scene, these versatile and snappy units utilize the FET component as a variable resistor in its compression circuit. As voltage is applied to the gate, the lower the drain source is set. Therefore the higher the voltage, the lower resistance in the drain circuit will cause gain reduction of the signal. This results in compression with a unique trademark response and sound to it, as FET compressors tend to offer a highly colored version of compression, which has become a very big part of the sound of many recorded sources. Unlike their juggernaut vari-mu counterparts, FET compressors, in general, are a much faster responding compressor making them a perfect companion, not necessarily a replacement, for these other compressors. Subtlety does not tend to be a hallmark of the FET family.

Perhaps the most famous FET compressor, and certainly a candidate for most-famous compressor in general, is the UREI developed 1176 Limiting Amplifier. This studio icon is known for its distinctive bite and somewhat confusing control scheme. While simple, the controls of the 1176 differ from almost every other compressor on the market. Being a fixed-threshold design, there is no threshold control itself and the amount of compression is controlled by the input level, and then balancing the resulting signal with the output level. The input and volume knobs have identical graduations, with unity gain being achieved when the setting of both knobs equal 48. Where things really take a turn is the attack and release controls. These controls are marked 1-7, with 7 being the FASTEST time and 1 being the SLOWEST, counterintuitive to most other designs. The attack knob also has a detent for “off,” which bypasses the compression circuit so you can simply get the sound of the 1176. Ratio controls are a line of punch buttons, progressing 4:1, 8:1, 12:1, 20:1 and also capable of the renowned “all buttons in” mode, which changes the compression characteristics altogether.

Due to the highly-colored characteristics of most FET designs, these see little to no use as mastering or mix bus compressors, though there are always exceptions to the rule. FET compressors play a huge role in tracking and their aggressive and quick attack makes them ideal for parallel compression situations. These compressors see heavy use on drum busses, snares, kicks, and have a desirable tendency to shoot a lead vocal straight to the front and center of a mix. These compressors love punishment, both receiving it and doling it out, but if subtlety is your aim look elsewhere.

VCA Compressors

VCA compressors utilize a voltage controlled amplifier in their compressor circuit, also a relatively more modern concept, that allows for full control of attack and release parameters and a smooth response to the compression itself. This allows for an incredibly versatile design able to handle almost anything you need it for, from mastering applications to knocking down unruly transients. One of the beauties of the VCA chip-style designs is that it also drastically improves the ability to get a highly effective compressor in a relatively small footprint. While many compressors could effectively be described as a voltage controlled amplifier circuit, VCAs traditionally utilize IC chip-based transistors to determine the input voltage that will control the resulting gain reduction, giving you the clean and controllable compression characteristic that VCA compressors users often look for. It would seem that these chameleons of the compression world should be the ideal candidate for all your compression needs, however as their design tends to lack expensive proprietary parts, this can lead to a wide and varying range of quality in VCA compressors that can make the field more difficult to navigate.

While there is a wide range of examples and characters in the VCA world, the compressor that may best illustrate the benefits of a VCA design — with super-hero like acclaim — is the SSL G-Series Console Bus Compressor. Since the release of the 4000 series of SSL consoles, this unit may be responsible for the sound of modern recorded music more than any other piece of gear. Seemingly straightforward with its settings of threshold, attack, release, ratio and makeup gain, the SSL Bus Compressor also boasts an auto release setting that is program dependent. Hailed equally for its transparency and its ability to “glue” a mix together, this has been one of the not-so-secret weapons of many mix engineers to take an already stellar mix to radio ready. With a relatively fast attack, auto release engaged, and conservative gain reduction, you almost can’t go wrong with putting this piece across any mix bus to bring things to life.

In specific applications, you can find VCA compressors in almost all aspects of audio, from tracking to mixing to mastering. Odds are if you’re sitting near any audio gear with a compression stage in it, there’s a good chance you have a VCA compressor within an arm’s reach of you at this very moment. Some famous models like the DBX 160 or the SSL channel strip compressors, despite being VCA based, are noted for the unique character they impart on a sound. For instance the smack the DBX 160 can bring to claps or the signature “spank” of the SSL channel compressor on bass, kick and snare. On the whole, however, VCAs are more lauded for their ability act covertly unless you want them to make their presence known.