The Aphex 651 and 661 models are 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.
The Aphex 661 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 we'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.
The primary difference between the two models is that the 661 includes a tube gain stage and the older 651 has no tube. The audible difference is small, however. The tube in the 661 may add a bit of subtle richness to the signal, but the 661 is known more for transparency than coloration; the 651 is even a little more transparent yet.
The Gear Rack