As we explored in our previous article, Sequence Selector, a compressor pedal decreases dynamic range. What that means is that the compressor is compressing (or reducing) the peaks in the signal, e.g. turning down the loudest passages of your playing automatically. The quiet passages don’t actually get any louder until you increase the output level, but the net result is that there’s less difference between strumming soft and strumming hard. It’s not an entirely artifact free process, but the sonic fingerprints left behind by compression are actually what we think of as the effect. A closer look at the Keeley 4 Knob Compressor, and compressors in general, illustrates how compression can shape both the envelop of your playing as well as the tone.
First off, all compressors have a threshold setting, which effectively controls how much signal the compressor has to see before the compression circuit kicks in. Also important, is the release setting, which is essentially how soon the compressor stops compressing once the signal drops below the threshold. A third important factor is the compression ratio, which controls how much compression is applied once the signal crosses the threshold.
By combining threshold, release, and ratio, the Sustain knob on the Keeley Compressor adjusts those factors in combination to result in increased sustain. One way to think of this is to think of strumming a single open G chord. As you allow that chord to ring out without compression, it has a fairly natural, linear decay. Add heavy compression and now the decay of the note doesn’t begin until the signal drops below the threshold setting and the release time comes to an end— resulting in the kind of ringing sustain you might associate with an overdriven tone, but without the overdrive/distortion characteristic.
The Attack setting determines how fast the compressor starts to work once you cross the threshold. On faster settings, the Keeley 4 knob reacts relatively transparently, maintaining the envelope of the note. However, set the attack extremely slow, and you may notice that the compression kicks in just after the note if plucked or strummed, creating an exaggerated effect.
The Clipping setting adjusts the input gain of the 4 Knob, allowing the user to increase or reduce the level on input to best match their instrument to the pedal. High output pickups, for example, might clip the input stage at certain setting and this knob allows you to adjust accordingly.
Lastly, the Level setting allows you adjust the output of the Keeley Compressor, either at “unity,” so that there’s no appreciable change in level when engaged. Or, use it as a boost, should you want to cut through for soloing, or to drive the input of a tube amp harder.
There’s no wrong way to use the Keeley 4 Knob Compressor, so make sure you experiment with different settings, there are several useful starting points outlined in the owner’s manual, but the golden rule is this: if it sounds good, it is good.