Extensive compressor reviews and FAQ

Keeley GC-2 (and Bassist) Limiting Amplifier: This is a new design from Keeley, completely different from his previous comp pedals, based around a VCA chip in the same style as dbx uses. So these new Keeleys are competition for other VCA pedals such as the Carl Martin Comp-Limiter and the Maxon CP9Pro+, particularly of interest to anyone wanting a "dbx 160A in a pedal".
The only difference between the GC-2 and the Bassist is one capacitor that causes the GC-2 to react less to peaks in the lows. This means the Bassist will be a more effective hard limiter, while the GC-2 will be a bit less restrained sounding. Apart from that they have the same tone, frequency range, and effect.
The knobs include Compression (ratio), Threshold, and output Gain. The range of ratios goes all the way from 1:1 to infinity:1, so it covers every function from clean boost to light and medium compression all the way to hard peak limiting, which it does reasonably well. The threshold knob also has a very wide range, so it will respond well to instruments with very low or very high output.
It also has an LED that indicates whether your signal is over the threshold. However it does not indicate the amount of compression with varying brightness; it's just green for "under threshold" and red for "over threshold".
The action is smooth and even, with no "dip and swell" or other artifacts. The attack and release times have no controls, but they adapt themselves to the peak levels of your signal. Higher peaks get faster attack times and slower releases, which is a good basic premise. Most users won't have any practical issue with this arrangement.
It has very low noise (almost none) at most moderate settings. It only becomes noisy if you choose a very heavy setting, which is to be expected with most comps. There is zero loss of either highs or lows, and no humps or scoops in the middle. The tone is quite neutral and close to transparent. It doesn't add any "tone magic". The highs can sound a bit congested or dulled during heavier compression, which is very common with this type of comp design; I've commented on every model made by dbx having the same effect on the highs. Aside from this, the tone is just what you feed into it.
The construction quality is excellent, and it looks sharp and modern. The housing is the small MXR size. It runs on Boss-standard 9V DC. The footswitch is "true bypass". The text around the knobs is too tiny to read; I mentioned this to Keeley and he agreed, and says they will be using larger type in the future.
Earlier I mentioned the Carl Martin Comp-Limiter and the Maxon CP9Pro+; I'd say this new Keeley does what they do just a little bit better, in a smaller package. I spent a few hours just A/B comparing it to the MXR M87, which is probably the modern competitor most people would debate buying versus the Keeley. The M87 has a slightly fuller or slightly less sterile tone, but the GC-2 does a better job of peak limiting. Also the M87's input knob, which affects the threshold, is notoriously finicky and hard to set in its sweet spot for your instrument--while the threshold knob of the GC-2 is very easy to dial in. The M87 has attack and release knobs, but I don't think they make a big difference compared to the GC-2's automatic timing. The M87 has better metering, but many faulty M87's have gone out to customers, and I doubt Keeley would have allowed that. In short, I liked the sound of the M87 a bit better, but the GC-2 is easier to use and seems more reliable.
Price in USD: new $199, too soon to say what the used price will be.
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