Extensive compressor reviews and FAQ

Keeley: Among guitarists, this is probably the most positively-reviewed pedal ever. At least it was when I first wrote this review, around 2007, but it has held on to a great deal of popularity even years later. It is a boutique version of the standard Ross/Dynacomp circuit, and while it performs better than the originals, it still has their basic action and tone characteristics.
It comes in two-knob and four-knob models; they are actually exactly identical except that in the two-knob one the "other two" knobs are hidden inside the housing as small trim pots. The two knobs they both have on top are the same as you find on any other Ross/Dyna type: Sustain (a gain boost across a fixed threshold) and Level (output volume). The two other controls are Attack time and Trim (which cuts the input signal level). On recently-produced units the Trim knob is labeled "Clipping", since the point of trimming a signal is to reduce the amount of clipping (distortion caused by signals that are too strong) in the circuit. The only reason to buy the two-knob version is if you know you will set up the pedal once, for one guitar only, and do not want to adjust it again. But the Trim/Clipping knob is super important for adjusting between different guitars.
It has a punchy boost to the mids and high mids, but there is some noticeable rolloff of the lows. Also there is some noise; it's not too bad, although it does seem to amplify preamp noise from my active bass more than some other comps. The higher you turn up the Sustain, the higher the noise floor. With a four-string bass, live on stage, the lows would be sufficient, and the hiss would not be noticed. But if you play a low B, or detune, or record with it, those issues are audible.
It's meant for sustain, which means strong compression, but the envelope shape makes it not so great as a peak limiter. Also I found the overall sound had a more "natural" quality at the lighter settings. The action is very smooth. Just as with its many cousins, it suits most guitar-playing styles, but only a few bassists will like it. For example it might suit a fretless jazz player for whom shiny high-end articulation is important. Some people find this type of circuit can be made to work better on bass by replacing the input and/or output capacitors with higher values, but personally I did not get great results when I did this mod on the Keeley.
The housing is the small MXR size, and it has excellent construction quality. It runs on standard Boss-type 9V DC. There is no LED to indicate signal over the threshold. The footswitch is "true bypass".

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