by Craig Anderton
The studio world is not experiencing a compressor shortage. Between hardware compressors, software compressors, rack compressors, and whatever other compressors I’ve forgotten, you’re pretty much covered. But there may be a useful compressor that you haven’t used recently: one of the stompbox persuasion.
With most DAWs including inserts so you can integrate external effects easily, interfacing stompboxes isn’t very difficult. Yes, you’ll need to match levels (likely attenuating on the way into the compressor and amplifying on the way out), but that’s not really a big deal.
But why bother? Unlike studio compressors, which are a variation on limiters and whose main purpose is to control peaks, guitar compressors were generally designed to increase sustain by raising the level as a string decayed (Fig. 1).
Fig. 1: The upper waveform is an uncompressed guitar signal, while the lower one adds compression to increase the sustain. Both waveforms have the same peak level, but the compressed guitar’s decay has a much higher level.
In fact some compressors were called “sustainers,” and used designs based on the Automatic Level Control (ALC) circuitry used to keep mic signals at a constant level for CB and ham radio. The gain control elements were typically field-effect transistors (FET) or photoresistors, and had minimal controls—usually sustain, which was either a threshold control or input level that “slammed” the compressor input harder—and output level.
Some guitar players felt that compressors made the sound “duller,” so a few designs tuned the compressor feedback to compress lower-frequency signals more than higher-frequency signals—the opposite of a de-esser.
Many guitarists patched a preamp between the guitar and compressor to give even more sustain because higher input levels increased the amount of compression. Putting compressors before octave dividers often caused them to work more reliably, and adding a little compression before an envelope-controller filter (like the Mutron III) gave less variation between the low and high filter frequencies.
Some legendary compressors include the Dan Armstrong Orange Squeezer (Fig, 2), MXR Dyna-Comp, and BOSS CS-1. But many companies produced compressors, and continue to do so.
Fig. 2: Several years ago the classic Dan Armstrong Orange Squeezer was re-issued. Although it has since been discontinued, schematics for Dan’s original design exist on the web.
Bass. Not all compressors designed for guitar could handle bass frequencies, especially not a synthesizer set for sub-bass. So, it’s usually best to patch the compressor in parallel with your bass signal. With a hardware synthesizer or bass, split the output and feed two interface (or amp) inputs, one with the compressor inserted. With a virtual synthesizer or recorded track, send a bus output to a spare audio interface output, patch that to the compressor input, then patch the compressor output to a spare audio interface input. Use the bass channel’s send control to send signal into the bus that feeds the compressor. Synthesizers are particularly good with vintage compressors because you can edit the amplitude envelope for a fast attack and quick decay before the sustain. Turn the bass output way up to hit the compressor hard, and you’ll get the aggressive kind of attack you hear with guitar.
Drums. Guitar compressors can give a punchy, “trashy” sound that’s good for punk and some metal. As with synth bass, parallel compression is usually best to keep the kick drum sound intact (Fig. 3). Adding midrange filtering before or after the compression can give an even funkier sound.
Fig. 3: This setup provides parallel compression. The channel on the left is the drum track; the one on the right is a bus with an “external insert” plug-in. This plug-in routes the insert effect to your audio interface, which allows patching in a hardware compressor as if it was a plug-in. The drum channel has a send control to feed some drum signal to the compressor bus, whose output goes to the master bus.
Bus compression. You wouldn’t want to compress a master bus with a stompbox compressor (well, maybe you would!), but try sending bass and drums to am additional bus, then compressing that bus and patching it in parallel with the unprocessed bass and drums sound. This makes for a fatter sound, and “glues” the two instruments together. What’s more, many older compressors had some degree of distortion, which adds even more character to any processing. Vintage compressors with relatively short decay times (most stompbox compressors had fixed attack or decay times) give a “pumping” sound to rhythm sections.
EMULATING STOMPBOX COMPRESSION WITH MODERN GEAR
Don’t have an old compressor around? There are ways to come close with modern gear. If your compressor has a lookahead option, turn it off. Set the attack to the absolute minimum time possible. Decay time varied depending on the designer; a shorter release (around 100ms) gives a “rougher” sound with chords, but some compressors had quite long release times—over 250ms—to smooth out the decaying string sound. Set a high compression ratio, like 20:1, and a low threshold, as older compressors had low thresholds to pick up weak string vibrations. Finally, try overloading the compressor input to create distortion, which also gives a harder attack.
Craig Anderton is Editor Emeritus of Harmony Central. He has played on, mixed, or produced over 20 major label releases (as well as mastered over a hundred tracks for various musicians), and written over a thousand articles for magazines like Guitar Player, Keyboard, Sound on Sound (UK), and Sound + Recording (Germany). He has also lectured on technology and the arts in 38 states, 10 countries, and three languages.