Extensive compressor reviews and FAQ

Aphex 661 "Expressor": This is a one-channel rackmount line-level comp with a versatile set of features. Among the various Aphex rack units, the 661 (or its earlier version the 651) is the one to choose, ideal for controlling a single instrument or voice channel.
It has a full set of manual controls over the compression, with a wide range of results. It also has an automatic ("Easyrider") setting, which works better than most auto systems I've tried, with a smooth action similar to an optical comp. There is an enhancer/maximizer function labeled "SPR" which adjusts the phase of the low frequencies; this can tighten the tone and reduce "mud" in some cases, but it doesn't work with everything. The "Tubessence" circuit, using a single 12AT7 tube, is a pretty subtle thing--the overall tone is clear, warm, and natural, but not especially "tubey" or fat. It is NOT meant for dirty, driven tube sounds. Another nice feature is the high frequency expansion, which dynamically un-compresses an adjustable range of high frequencies in proportion to the amount of overall compression applied. This brings back some of the articulation and upper-end tone that can be lost during strong compression. A "low cut" switch reduces the amount that low-frequency peaks trigger the compression, without cutting lows from the audio path.
At most settings the 661 does not roll off any highs or lows, it has no clicks or other artifacts, and almost zero noise. I do have some criticisms: it's heavier, and deeper front-to-back, than most other rack comps; and it emits a lot of heat from the top surface. Because it runs so hot you have to make sure your rack has adequate ventilation. Also, while the various special features are quite good, they are honestly much more subtle than the ad copy would suggest.
It has both XLR and 1/4" in/outputs, and can run either balanced or unbalanced. It will not work well with a direct input from a guitar or bass, but it has a switch for both +4 dBu and -10 dBu input, suitable for any sort of preamp or fx loop. The "process" switch is a hardwired true bypass. It runs on AC power, using a standard IEC cord.
Be careful when buying any Aphex rack unit, because it seems like the majority of the ones on Ebay were pulled from bar/club PA's where they've been abused, and are sold "untested, as is" with no warranty. Don't fall for that--only buy one where the seller asserts it has been tested and works like new. If you get a good one though, it's worth the effort. The 661 is one of my very favorites for clean, clear sound with highly effective range of control.

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