Extensive compressor reviews and FAQ

Aphex 661 "Expressor": Among the higher-end Aphex rack comps, the 661 (or its earlier version the 651) is the one you want for a bass rig. It's a mono line-level unit with a really nice set of features. It allows for either automatic ("Easyrider") compression control or a good set of manual controls; the Easyrider setting sounds/acts very smooth like an optical comp, and the manual settings allow a more dramatic range of control. There is an enhancer/maximizer function labeled "SPR" which really does bring out a tighter and more energetic tone; I wouldn't use it all the time, but it sounds really great for certain applications. The "Tubessence" tube/valve circuit, using a single 12AT7, is a pretty subtle thing- the overall tone is clear, warm, and natural, but not exaggeratedly "tubey". Another nice feature is the high frequency expansion. In order to counteract the loss of highs associated with strong compression, this circuit dynamically un-compresses an adjustable range of high frequencies in proportion to the amount of overall compression applied. It's subtle, but very nice sounding on fingerstyle playing.
 
Overall, this comp has no rolloff of highs or lows, almost no artifacts, and the noise level is very low. It's a fantastic unit, one of the best and most useful I've tried. I do have some criticisms of it: it's deeper front-to-back and heavier than many other rack comps, and it runs very hot. Also, while the special features are quite good, they are honestly much more subtle than the ad copy would suggest. For many players, the features might be more than they need, but for the finicky user this one is a really nice choice. Because it runs so hot it might not be a great choice for a gigging rack; otherwise, the 661 sounds better (to me) for clean/natural sound than any other comp I've tried. These units are a bit expensive new, but you can find much more reasonable prices for used ones. Just be careful though, because it seems like the majority of those Aphex bargains 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 without any issues. If you get a good one though, in my opinion the 661/651 is one of the best-sounding compressors ever made.
 

 
 
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