Sonic Euphoria PLC passive line stage Art Dudley February 2006
The first time I heard the word autoformer was more than 30 years ago, when a friend bought a brand-new McIntosh 6100 amplifier, the output section of which had them in abundance. I learned then that an autoformer was a sort of a vestigial output transformer that McIntosh still used when they began making transistor amplifiers, in much the same sense that a coccyx is a vestigial tail that God still uses when he makes us—the attitude being, I suppose, It can't hurt.
Today my understanding has deepened enough to accommodate a few complexities: An autoformer is just like any other transformer, in the sense that current applied to its primary coil induces a complementary current in its secondary—but here, the primary and the secondary are actually different portions of the same winding. As with a normal trannie, an autoformer can be used to invert the relationship between voltage and current in a circuit; but because its function depends on a series of multiple taps—thus creating a secondary coil of varying length—an autoformer can also function as the inductive equivalent of a potentiometer in which each tap represents one of a series of attenuation steps, like the discrete resistors in a ladder of a different sort. And whereas resistive attenuation works by throwing away a portion of the input power, an autoformer circuit can actually be configured to provide a modest degree of gain.
So it goes with the remarkable Sonic Euphoria PLC passive line stage, which Brian Damkroger wrote about in the January 2006 Stereophile. As BD observed, the PLC provides fine control of signal attenuation and source selection for a reasonable number of inputs, yet still can provide up to 10dB of gain—without a single battery or AC cord in sight. And unlike passive line stages based on resistive attenuation devices—which is to say, most of them—the autoformer-based PLC keeps the output impedance so low that cable length is a nonissue. (But as John Atkinson confirmed in his measurements of the PLC, care should be taken that the output impedance of the source component isn't too high.) Ohm's law is more than merely poetic: It is just.
Some folks will balk at the tradeoff, suggesting that the real price of the PLC's ostensibly free gain and impedance matching is in forcing the fragile signal to travel through all that wire. But on a theoretical level, at least, I don't share that concern. Ever since Herb Reichert turned me on to the writings of the late Norman Crowhurst, I've been a bit prejudiced toward the use of transformers in audio circuits, while noting that prevailing late-20th-century attitudes seem prejudiced against them—and I suppose I can respect that.
Knowing my taste for that sort of thing, BD gave me the heads-up toward the end of his time with the Sonic Euphoria PLC; after JA made his measurements, the review sample was diverted to my place for a few weeks. I plopped it into my regular system and sat down to listen—casually, at first, just to get a handle on whether the PLC had a decent enough way with notes and beats.
I needn't have worried. The PLC ($1295 single-ended, $1995 balanced) was every bit as good as my Fi Preamp ($5000) in its steadfast refusal to distort pitch relationships and the subtleties of musical timing. Musical flow and momentum were seriously good. And it did all that with absolutely no dynamic compression, and no bass attenuation that I could detect—two things that make the PLC so much better than every other passive pre I've tried in my own system. Not long after JA sent the PLC my way, I used it to play some of the Japanese Victor XRCD versions of those great recordings of Fritz Reiner and the Chicago Symphony from RCA's golden age. I was surprised at how the very wide dynamic contrasts in the Beethoven Symphony 7 (JMCXR-0006) were preserved.
The Sonic Euphoria PLC won't suit every installation, and even at its best, it wasn't perfect. With the best active preamps I've heard, such as the Lamm LL2, it's somewhat easier to mentally "follow" the sounds of notes—by which I mean their shapes, from attack through decay. With the PLC, that degree of insight wasn't quite there. But what I heard instead was the same degree of musical immediacy, of drama and presence, that the best active units I'm familiar with can provide. Some folks might even get more of that sort of thing from their PLCs, depending on their installations. I wouldn't doubt it a bit.
I put the PLC's theoretical level of impedance matching to the test by using it to drive my Lamm ML2.1 monoblocks through the 6m Nordost Valhalla interconnect pair I usually use, then shuffling the components around to accommodate a 1m pair of the same cable. There was no difference at all—not even an imagined one.
I undid the PLC's cover, and on a slow day in November counted 370 individual solder joints in my balanced sample of the PLC—and there were still a few I couldn't quite see. Considering the balanced version's $1995 price, that works out to a little less than $5.40 per joint—assuming, of course, that the chassis, Cardas connecting wire, autoformers (wound with Cardas magnet wire), high-precision Elma selector switches, jacks, front panel, knobs, and shipping materials are free, which is unlikely. Hell, I wouldn't even do that kind of work for the $5.40/joint, and I don't know anyone else who would, either.
I like Jeff Hagler, who designs and builds the Sonic Euphoria PLCs, but I'm beginning to suspect there's something wrong with his calculator. Until he gets it cleared up, consider the PLC's $1295 price (single-ended version) to be the audio equivalent of the "Bank Error in Your Favor" card in Monopoly—and jump on it.—Art Dudley