Sonic Euphoria PLC passive line stage
Because "gain" seemed to be the tricky part, and most CD players could drive an amplifier directly, some designers simplified even further by eliminating the gain stage altogether, and just combining I/O with an attenuator. Voilà! The "passive preamp," or "passive line stage," was born. I liked the logic, but was inevitably disappointed with passive units' dynamics and bass performance. Plus, I found it difficult to live with the short cables they required to sound their best.
Passive line stages continued to evolve, however, and when I started shopping for a preamp a couple of years ago, a few were reputed to be quite good. After a bit of snooping around, I decided to audition the Placette Remote Volume Control (see my review in the June 2004 Stereophile) and the model reviewed here: the Sonic Euphoria PLC. Like Placette, Sonic Euphoria is a direct-sales operation, and offers a 30-day, money-back trial period.
I expected the Sonic Euphoria PLC to adhere to the "simple inside, simple outside" philosophy, and so was pleasantly surprised by its luxurious look and feel. The chassis is brushed stainless-steel and the thick, aluminum front panel is pure high-end elegance. The three large knobs—for input selection, volume, and ±1dB offset—are nicely textured and move with smooth precision. The rear panel is similarly well done, with solid, high-quality jacks (my unit had the optional WBTs, which cost $40/pair) and a very clever grounding system.
Inside, the PLC is simple, but its build quality and layout reflect attention to detail. All internal wiring is point-to-point, as you'd expect, but carefully routed to keep signal paths short. Swiss Elma switches are used throughout, and the PLC's heart, the pair of autotransformers, are custom-built to SE's specifications. All internal wiring, including the transformer windings, is by Cardas.
Autotransformer, autoformer, tapped transformer—there are several names for the devices that the PLC uses as a volume control, and that distinguish it from the Placette and most other passive devices. In the Placette, attenuation is achieved by placing a resistor in the signal path, which is simple but inefficient. Dropping the voltage means dumping power by converting it to heat. Another limitation is that resistor-based attenuators can only attenuate. They can't increase the voltage.
With an autotransformer, things work a little differently. This is a transformer with only a single winding, but in addition to being connected at two points for the input signal, an autotransformer is also tapped at several specific points (24 in the PLC) along its length. The output connections of the autotransformer are the same ground reference point as the input, and whichever of these intermediate taps is selected by the position of the volume knob. So a fixed portion of the winding is the transformer's primary side and a variable portion of it is also the secondary. However, the magnetically induced current in the secondary portion of the winding opposes the primary current, so the output is the difference between the two. Simple, eh? I didn't think so either, but trust me, it works.
The autotransformer approach has a few inherent advantages. One is that since the ratio of primary to secondary turns can be whatever you like, it can produce voltage gain as well as attenuation. A second is, theoretically, greater efficiency. An autotransformer attenuates by converting voltage to current rather than dissipating it as heat. For example, a resistor attenuator drops 20dB by cutting the voltage by a factor of 10, keeping the current constant. With the autotransformer, 20dB attenuation drops the voltage by a factor of 10 as well, but actually increases current by the same factor. [It is fair to point out that in a conventional system, the amount of current available to feed a power amplifier should be irrelevant to performance.—Ed.]
Another benefit is that while a resistor attenuator always increases the output impedance of the source/attenuator combo, the autotransformer can actually lower the output impedance, better enabling the combo to drive long cables and power amps. On the other hand, some people would argue that no transformer will be as linear, precise, or transparent as a high-quality resistor.
The PLC is available in single-ended ($1295) and balanced ($1995) configurations, the latter adding a second pair of autotransformers and replacing two inputs and three outputs with XLR connectors. Other I/O configurations are available, should the customer desire, as are a remote control and upgraded RCA jacks. It's worth noting that the PLC does not and cannot convert single-ended signals to balanced; it simply shunts pin 3 to ground. To get a balanced signal out, you must put a balanced signal in.
Designer Jeff Hagler explained that though Sonic Euphoria stocks a few units in the most popular configurations, most PLCs are built to order. Once a customer decides on a configuration and places an order, he said, the unit is built and shipped within three to four days. My review sample was the only balanced demo unit available when I put in my request; it had the optional WBT RCA jacks but no remote control.
In theory, theory and practice are the same. In practice...
The PLC's tenure in my system overlapped some pretty impressive gear: the Simaudio Moon Rock and VTL S-400 amplifiers, and digital units from Burmester and Cary. After a week of round-the-clock burn-in, I installed the PLC in my system, cued up Sarah Vaughan's The Lonely Hours (LP, Roulette SR 52104), and sat back to listen. My first thoughts were that the system sounded awfully good—in fact, not a lot different from how it had sounded through VTL's superb TL-7.5 line stage. Vaughan's vocals were husky and soulful, and her unique, powerful vibrato and dynamic transients were vivid and arresting. She had a wonderful presence through the PLC, with a great sense of her body behind the vocals.