A compressor turns down the loud parts of your playing and, once you add output level, brings the quiet parts up. The result is a more even volume from note to note, and notes that seem to hang on longer before they fade. That extra sustain is why many players first buy one.
It is a subtle effect at sensible settings. If you can't hear it at first, that's normal: listen to how evenly a picked arpeggio comes out, and how long a held chord rings.
The knobs
- Sustain (or Comp): how hard the pedal squeezes. More means flatter dynamics and longer sustain.
- Attack: how quickly it clamps down after you pick. A slow attack lets the pick click through before the squeeze, which keeps notes punchy. A fast attack smooths the front of the note away.
- Level: output volume. Match it to bypass so you judge the tone, not the loudness.
- Blend (or Mix): mixes the dry signal back in. This is parallel compression: you keep your natural pick attack and add the sustained, compressed signal underneath.
Not every compressor has all four. The MXR Dyna Comp has just two knobs; many modern designs add Attack and Blend.
Two ways to use it
Squashed and obvious. Country chicken picking and funk rhythm often use heavy compression: every note pops out at the same level with a clicky, snappy front. Try Sustain at 2 o'clock, a medium-fast Attack, Blend fully wet.
Subtle and always on. Many players leave a light compressor on all the time to smooth a clean sound and help it sit in a band mix. Try Sustain around 10 o'clock, a slow Attack, and Blend at about 50%. Switch it off and the sound should feel slightly thinner, not dramatically different.
Common mistakes
- Too much Sustain. Your playing loses all its dynamics: soft and hard picking sound the same.
- Noise and hiss. A compressor raises quiet sounds, including hum and buzz. Keep it early in the chain and don't overdo the squeeze.
- "Breathing". Long chords swell back up as they decay. Back off Sustain.
- Expecting it to help high gain. A compressor before a high-gain pedal adds little, since distortion already compresses heavily.
The usual position is near the start of the chain, after tuner and wah, before your drives.