Extensive compressor reviews and FAQ

Coldcraft Crimson: This pedal, from a small boutique builder, packs a lot of features into a small package. At the time of this writing the Crimson is not in production, but the builder may bring it back sometime. Also there may be some different versions made, but I don't know of any markings to distinguish them.
It has separate engines for compression, limiting, and noise gating (downward expansion), and these operate independently at the same time. Knobs include Ratio, Volume, Gate, Limit, and Resp (response); these require some explanation.
The ratio knob controls the compressor stage, and it is actually gain boosted into the comp circuit. So while it does control the ratio, it only does so by changing the strength of your signal over the threshold. This boost has a very wide range, which means it is capable of an extreme amount of smashing at the higher end of the knob turn, and also means that you have to keep the knob set fairly low if you want moderate or light compression. Also, while the comp doesn't add much noise of its own, this gain stage can amplify your pre-existing noise floor significantly. So for most guitars and basses the only way to have low noise is to use a low setting of the ratio knob. The compression action is smooth, with no noticeable artifacts, though again at a higher setting it can be an extreme smashing effect.
Limit controls the threshold of a limiter stage with a fixed 15:1 ratio. Turning the knob clockwise lowers the threshold. It is quite effective at catching and controlling most peaks, though 15:1 is not quite the infinity:1 of a true hard limiter. There are no click artifacts when the limiter engages, which is nice since that's a problem many limiters have. Lowering the threshold also lowers your output volume, so you need to compensate by turning up the volume knob; so in some cases stronger limiting can mean increased noise floor. Overall I got the best results for my taste, for moderate compression with a bit of limiting, setting both ratio and limit knobs fairly low.
Gate controls the threshold of a downward expander, which turns down the volume of the signal as it falls below the threshold. That's how most noise gates work. So as you rest between notes, it cuts the volume, silencing noise--but only while you are not playing. This is how guitarists using high-gain distortion are able to control noise while palm muting, for example. This particular gate is very jittery and spitty. It works, but it only works "well" if you are playing with minimal dynamic range and very distinct rests, and ideally playing loudly to cover up the sputtery sounds it makes. Where this gate "made sense" and became useful was when using higher ratio knob settings that naturally have higher noise. Turning the knob clockwise raises the threshold, allowing more of your signal through the gate, but even the max setting does not turn off the gate.
"Resp" controls the attack and release times of all three engines together. It's difficult to figure the actual time ranges by ear since the effects differ with each stage, but there does seem to be a useful difference in action at each end of the knob turn.
The tone is clean and mostly uncolored, although turning up the ratio knob can dramatically increase fatness and harmonics just from the very strong compression. There is no loss of highs even at strong settings, and no peaks or valleys in the mids. The lows get attenuated a bit, but probably not enough to bother most people. Expect tight lows instead of boom at the bottom end.
The noise level is tricky to describe, because the circuit is not at all noisy by itself, but the gain of the comp stage plus the sputtering reactions of the gate mean that the very thing that kills this pedal for me is how it handles noise: badly. I hope future editions of this pedal will either nix the gate completely, or improve it and ideally give it an "off" setting. Apart from that, the comp and limit stages are really nice, and it's a treat to have both of those operating independently in a small sized pedal.
It runs on Boss standard 9V DC, with no battery option. The housing is the small MXR size, and the construction quality is pretty decent for handmade, although two of the knobs are plastic-stemmed trims that might not take abuse. The footswitch is a relay-based true bypass. There is no LED to indicate the amount of compression, limiting, or gating.

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