Summary

Although it isn't cheap, the Aphex 661 is a powerful and very well-executed compressor/limiter. The preliminary manual was thoughtful and well-written and didn't leave very much to the imagination of us users. The unit will easily interface with just about any console or gear in the outboard rack, and it does a lot, with well-chosen values and rigorous attention to accuracy and repeatability. Although the "tube" sales argument doesn't do much more me, the tube was measurably there and it may ring your bell in a way that missed me. And they sure didn't compromise on other aspects of performance just to get the tube in. The unit really handles! Me, I want two or three!!

Happy levels!

Dave Moulton has been trying for gain reduction ever since Thanksgiving.

Follow-up

After Aphex read this review, they commented that the tube was in the output stage and that its qualities are best evoked by really cranking the output - something I didn't do while testing the unit. I tried it and they've got a point. When the output level is running around +10 VU or above, the sound quality definitely changes: rounder, to my ears, and a little thicker or fatter, which is also to say less transparent. Measured harmonic distortion, at +20 VU (just below clipping), shows the second harmonic about 45 dB down, the third harmonic 65 dB down and 4th and 5th harmonics dropping off to -90 dB. The higher harmonics (my system measures the first 20 harmonics) don't get louder as they get higher, which is more typical of solid state stuff.

All of this supports Aphex's contention and description of "tube-like" sound. So, if you want that particular non-linear quality, with or without compression, just crank the output. The unit behaves in a very benign way - the only caveat is that you've got to turn down the signal somewhere else in the chain after the compressor, and you need to know that you are using up the signal headroom right here. In fact, that's the purpose of the exercise: to substitute the particular non-linear headroom quality of a hard-driven tube gain stage in place of the more conventional headroom associated with operational amplifiers.

Aphex also sent along some explanatory notes about the Spectral Phase Refractor circuit. They refer to a psychoacoustic quality they call the "Leading Phase Effect." I've not run across this elsewhere in my studies - some of what they allege seems reasonable to me and some runs counter to my experience (and direct research). Physically, the device works pretty much as I described: it adds phase shift to low frequencies. This is in effect a frequency dependent time delay (around 3 ms. at 90 Hz., 7 ms. at 60 Hz.). Without getting into a debate about what good it is or does, some of the suggested applications make sense (correcting frequency-dependent phase shift that occurs as a function of analog recording) while others seem peculiar ("helping" live acoustic recordings). The argument that such frequency-dependent time delay inherently helps our auditory system seems debatable at best.

Whatever, the circuit is there for you to use and have fun with. If it works for you, go for it. My only concern would be what I watch out for in all time-domain devices: be sure to check that it doesn't damage your mono mix. After that, let 'er rip!

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