Diamond CPR-1: This is Diamond's original model comp pedal, an optical design with simple EQ. The "Comp" knob is an input gain control which increases the level of your instrument signal into a fixed threshold of compression. There are a few other comps that use this method, such as the Aphex Punch Factory. Generally speaking that system only benefits low-output instruments, but the audio circuit here is well-designed enough that I have no complaint about it in this case.
The ratio is also fixed, and is very low--about 3:1. Great for fattening and smoothing, but not usable for limiting strong peaks. There is almost no noise at all, it is impressively quiet even with a noisy instrument, and even with the EQ turned to the treble side.
The EQ control is a "tilting" EQ, meaning if you turn to the left it boosts lows and cuts highs, and turn to the right it boosts highs and cuts lows. I generally don't like tilting EQ's, because I want full lows without losing any highs. Fortunately, with the knob turned to around 10:00 or 11:00, the lows are quite good-sounding and the highs are still pretty decent. It's nowhere near as extreme as the tilting EQ on the Ashdown and Moen pedals, which is a good thing. There is no loss of highs with the knob at 12:00 or higher. The very lowest frequencies are cut off a bit even if you turn the EQ to the low end. However the cutoff point is low enough that a 4-string bassist shouldn't have any problem, and even a low B still sounds OK. The loss of low end is not quite as noticeable as with the Keeley and similar pedals.
The input gain control allows this pedal to be useful with an instrument of any output level, which is a valuable feature. However, a very high-output instrument (or very aggro playing) can cause clipping at the input stage. This can be completely fixed by powering the Diamond with a supply of higher than 9VDC, up to 18V. But a more normal-level signal will not have problems even with 9V powering.
Really where this pedal shines is in making your tone bigger, fuller, more "present". I love the tonal improvement it gives. So if your main interest in a compressor is just beefing up your tone, and helping your signal "pop" through the mix, then the Diamond will give that to you better than most other pedals. It's not so great at keeping unruly signal peaks in check, balancing high and low string levels, or other "utility" uses of compression. But again though, for adding oomph and shine to your tone, it's pretty much unbeatable.
The construction is solid, and the footswitch is true bypass. The green "on" LED turns orange to indicate the amount of compression, which may not be the best metering around, but it is better than no metering.
Price in USD: new $200 to $250, used $150-$175
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