Xotic AC Comp Booster: Xotic's AC Booster is an overdrive pedal, not normally the sort of thing I review here; however their "Custom Shop" line includes a couple of models with variable amounts of compression, including this AC Comp.
The original AC pedal has a certain amount of compression inherent to its drive, so for the Comp version they added a three-position toggle switch which selects the amount it compresses the signal. The position labeled "norm" is the medium amount of the original pedal; the middle position compresses very little, hardly at all; and the position labeled "comp" totally flattens the signal. There really is a dramatic amount of change between the three settings. Since the resulting effect just changes the dynamics of the distortion, and is not a separate dynamic control for your signal, it doesn't make much sense to talk about attack time, or whether it works as a limiter. I would not advertise or recommend this pedal as a compressor per se, that's not what it's for. But the low and high compression settings really do make a huge difference in the performance and value of the overdrive. Personally I like the "no compression" setting best.
It has knobs for Gain, Volume, and Treble and Bass EQ. In OD and distortion pedals, Gain controls the amount of boost into the clipping circuit, so it sets the amount of distortion. There is no "clean" setting, it's not a clean booster. It covers a very wide range of drive, from light grit to heavy scream. The clipping has a bright, cutting, and grindy sound. It offers a huge amount of output volume boost, if you want, for driving your amp harder.
The Treble and Bass knobs each cover a wide, shallow slope of cut and boost, for very useful musical results. You can tame the brightness of the distortion with the Treble knob, however it's tricky finding a sweet spot where you are not also darkening your instrument's tone too much. For bass guitar, I liked Treble at 12:00 and Bass at 2:00. There is no loss of lows or highs generally; however the brightness of the distortion, plus the way strong clipping inherently squashes low-frequency peaks, means the lows can sound weak by comparison. In fact when I first plugged in to this pedal, with the EQ knobs at noon, I thought it was cutting my lows badly--but it's an illusion, the lows are there. To get them to sound as full and strong as the distortion, you need to turn up the Bass EQ, or -for best bass results- use the "no compression" setting.
There is a little bit of background hiss at most settings, but overall the noise level is not bad, and it does not get nearly as hissy as a lot of other compressors or distortions.
The footswitch is "true bypass". The construction quality is excellent, with my only complaint being the tiny toggle switch is difficult to set to its middle position. The housing is the small MXR size, and it runs on standard Boss-type 9V DC.
Price in USD: used $100 to $150, new $196.
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