Hands-On Review: Keeley 4-Knob Compressor

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Years after it’s debut, it’s still setting the standard for audio excellence and superb control in a guitarist-friendly stomp.

[This review originally appeared on Harmony Central in a somewhat different form.]

By Buddy Lane
Contributor, Harmony Central

Mention compression pedals and any true gearhead will say “Keeley.” Robert Keeley has become the go-to man for clean, transparent stompbox compression. Though he has expanded into modifying other companies’ effects and creating other original pedals, the Keeley Four-Knob Compressor is where it all started. We thought it was time to revisit this classic to find out exactly why it remains the default squish tool for countless guitarists.

Squeeze me

Keeley Four-Knob Compressor Pedal

Before we get into the pedal, a quick word about how compression works—something that can be confusing, even to professional musicians. Basically, a compressor is an automatic volume control that adjusts the gain level of the signal that is put through it. This means it can lower the attack level to keep the sound from distorting when recording, and/or raise the output level as the original signal dies to increase sustain. Guitarists use it to even out their performances and increase the ring of their notes; electric bassists might use compression to tame pops and slaps; while a keyboard player can keep LFO sweeps from getting out of hand. Occasionally employed for coloration, great compressors are generally preferred by musicians to deliver a clean, transparent signal. It is these qualities that have made Keeley’s reputation.

Learn more about how compressors work with Compressors Demystified.

Parts ain’t parts

Robert Keeley based his compressor on the coveted Ross stompbox, but has refined it even further. To maintain true-bypass switching and still have an on/off light, he needed to use an expensive triple-pole, double-throw switch. His design ensures there are no ground loops. The controls and jacks are point-to-point wired. The output transistors are matched for gain within 1%. Keeley uses the Harris (Intersil) CA3080E to prevent noise and eliminate any tonal effect on the original.

The blue on/off LED uses only 2.5mA of current. The overall circuit draws so little power that the company says that a 9V battery should last four to six months, even with heavy usage.

Control issues

Attack determines how fast the compressor starts acting on the signal. If I want to hear the initial transients loud and clear when I hit the guitar strings with my pick, I set the Attack for slow (clockwise); if I want to squash that initial hit, I would set the knob for a fast attack time (counterclockwise). This doesn’t affect the sustain, which is controlled with the—you guessed it—Sustain knob.

The Clipping control (called Trim in earlier models) lets you set the input volume of the compressor. This way, if you have a Sustain and Attack setting that you like for your Stratocaster with single-coils, you can just lower the Clipping level when you plug in your humbucker-equipped Les Paul to maintain the same compression effect. Nice!

Sustain for days

I tested the Keeley 4-knob Compressor with a single coil-equipped Stratocaster and a humbucker-equipped Jazzmaster into a Fender Blues Junior.

The internal attack and clipping turned out to be set perfectly for my Strat. If I were to use this compressor regularly with a humbucker guitar, I might go under the hood to slow the decay and reduce the clipping level. But used with single coils, it squashed the initial attack just enough to even out my playing and make me sound better than I am. Or at least as good as I could be, tonally speaking.

With the Sustain up around two o’clock, chords seem to ring endlessly. Even wiggling the whammy bar, which can often kill sustain, was unable to dampen the chime.

Another way I like to use a stomp compressor is to run it into an overdrive pedal or a slightly distorted amp. This allows me to keep the gain low on the overdrive or amp while deriving my sustain from the clean compressor. I find that the notes cut through the band mix better using this approach. The Keeley was ideal for this, as it had virtually no effect on my guitar tone, sending the amp or overdrive the same frequencies as if I were plugged straight in.

For rock, I slowed the attack, letting me pile on gobs of sustain without squashing the initial strike; for imitation pedal steel licks and ambient work, I liked the volume swell effect of a fast attack.


The Keeley 4-Knob Compressor puts a lot of control at the feet of the tone-tweaking guitarist. Its impeccable sound and precise circuitry means you can put this thing to work in the studio. And adjusting its sensitivity control, you can use it on keyboards, drum machines or any other line-level instrument/device. The internal attack control can be adjusted to work with power-buffering pedal boards as well as bass guitars with extra-hot active pickups.

[Editors note: Since this review originally appeared, Keeley has introduced the GC-2 Limiting Amplifier. Using a three-knob control layout, it’s the ideal stomp for guitarists who want to keep a lid on clipping and potential speaker damage while preserving the musicality of their tone. As with the 4-Knob version, the GC-2 employs high-grade components and benefits from Keeley’s years of compressor development. And like the Four-Knob, it offers the kind of signal quality you’d expect from high-end rack-based gear.]
To explore the entire range of compressors for guitarists and bassists, check out our Guitar and Bass Effects Buying Guide.

Tags: Electric Guitars Effects Pedals Harmony Central

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