Symmetry ACS-1 -- John Curl's Perfectly Coherent Electronic Crossover (Vol.2, No.1: Winter/Spring 1979)

John Curl's Perfectly Coherent Electronic Crossover

Symmetry Audiophile Systems, 101 Townsend Street, San Francisco, CA 94107. ACS-1 Active Crossover, 8650. Tested #0573, on loan from manufacturer.

We have two misgivings about reviewing any separately available, all-purpose electronic crossover like the Symmetry. One is that there's no such thing as an all-purpose filter slope, whether it's 6, 12 or 18 dB per octave.

The correct choice of slope depends both on the passband and on the out-of-band characteristics of each driver used, as well as on the total speaker system design concept. For example, the 12 dB per octave low-pass slope of the Symmetry would be much too gradual for the Janis W-1 woofer, which needs to have some rather ferocious out-of-band peaks attenuated as sharply as possible. The other reviewing problem is that the sonic flaws, if any, of the cross over are virtually inseparable from those of the speaker system whose signal traffic it directs-are the motorists or the corner police man responsible for the traffic snarl? A simple bypass or substitution test is just about impossible in the case of an active crossover; we have no “straight-wire” filter networks for comparison, nor even a 'reference' electronic crossover.

Those reservations out of the way, we can report that the Symmetry ACS-1 is a truly excellent unit, adding little or no subjectively perceivable coloration to the remembered sound of various electronic chains into which we've inserted it. Perhaps an occasional touch of barely evident hardness or brightness, perhaps not even that; as we've said, these aren't genuine A B tests. And on the best bench the ACS-1 is nothing short of amazing. You can actually feed a square wave into the input, sum the signals from the low-pass and high-pass out puts, and get a perfect square wave back. We were able to do this regardless of the selected crossover frequency, which is continuously vari able from 45 Hz to 4.5 kHz.

John Curl, who designed the ACS-1 (as well as the Mark Levinson JC-1 and JC-2, back in the days when he worked for Mark), believes that such waveform coherence is every bit as essential in a crossover circuit as in any other section of the audio signal path, whether up stream or downstream. We wholeheartedly agree; indeed, we can't see how anyone can disagree (except certain Bostonians). Interestingly enough, no other separate electronic crossover known to us exhibits this kind of total coherence, not even the vastly more expensive Mark Levinson LNC-2. As a consequence, you'll be way ahead with the ACS-1 in any attempt to achieve even a semblance of coherent alignment in a biamped or tri-amped speaker system.

On the other hand, since the ACS-1 is most likely to be used for biamping systems with separate subwoofers, it should be pointed out that coherence becomes less and less audible as you go lower in crossover frequency. In our experience, it makes absolutely no difference at 100 Hz, for example. From a few hundred Hz on up, however, and especially in the kHz range between midrange and tweeter, it becomes quite significant and shouldn't be neglected.

Our overall recommendation, then, is this: If you feel sufficiently qualified to “roll your own' biamped or tri-amped speaker system (are you sure?) and if you've determined that you don't need very steep crossover slopes (such as 18, 24 or possibly 36 dB octave), we don't see how you can go wrong with either one or two Symmetry ACS-1 units. It's a well-engineered piece of equipment that will perform exactly as its makers claim.

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[adapted from TAC]

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Also see:

Box 392: Letters to the Editor

Various audio and high-fidelity magazines