The Unseen Variable
Provided that very high sound levels and gut-thumping bass are not required, Tim de Paravicini's small SE amplifier, for example, works just fine with the relatively kind and uniform 6 ohm impedance presented by a Wilson WATT V loudspeaker (used without the Puppy woofer), aided by the speaker's 91dB/W/m sensitivity. Yes, the WATT V is slightly altered in character, and wide-band rock music at realistic levels is out of the question. However, with the right kind of music, the result is true high-fidelity, and can often be most beguiling—in several respects reminding me of my first experience with the original Quad electrostatic: the marvelous lack of "box" and mechanical "cone" sounds, the inaudible midrange distortion, the lightning-fast transients, and the low levels of grain.
However, while electrostatics—and, for that matter, good ribbon speakers—do some things exceedingly well, ultimately they cannot always paint a complete audio picture. I wonder whether SE amplifiers are destined to occupy a similar niche in the field of high-quality audio.
In using the term "niche," I am not being derogatory—there will be many who hear the siren call and will be drawn in. For them, the purity and fluidity of the amplifier's midrange will be all that they desire. It's hard to deny that appeal if you have yet to experience a truly great system.
A number of recent products have pointed to the future—in particular, the Wilson X-1/Grand SLAMM loudspeaker system I reviewed for Stereophile last December (Vol.17 No.17, p.115). Exposure to the X-1 was a revitalizing experience. It does so many things that I didn't think were possible from reproduced audio that it defines a totality of performance—a standard of excellence and listener satisfaction over the entire audible frequency range, not just the midrange.
While it is SE-compatible in that its 95dB/W/m sensitivity and 6 ohm load are surprisingly amplifier-friendly, the Wilson speaker is also kind to average-quality electronics, in that it gets the best from them. At the same time, however, the X-1 is highly revealing. Once you have heard X-1 bass in full song, you will never forget how that disc you're listening to can sound. You are driven to find amplifiers that are capable of this, but you don't have many choices—try to name the tubed SE that's up to the task!
The X-1's tonal balance remains true over a wide power range, provided that the source amplifier remains tonally stable. The X-1 has low distortion; at powers over a few watts, it produces less distortion than an SE amplifier—and this is audible!
Then there's dynamic range. While the Wilson X-1 can whisper like the thoroughbred it is—crystal-clear at milliwatt input levels—there's no doubt that it thrives on sheer power. And can it take it! Clean, undistorted power elevates the X-1/Grand SLAMM to previously unexpected regions of dynamic range, dynamics, and drama. A few hundred watts coupled with linearity and control allow the X-1 to redefine musical dynamic range in domestic replay. This is a thrilling dimension if you can get it, though it's impossible to achieve with a tubed SE design.
I've used the Wilson X-1 as an example, because its performance is audibly limited by even the finest SE tubed amplifiers—despite its nominally compatible load factor. In my own work (I don't own a pair of X-1s), I continue to find correlations to my X-1 experience. My Quad ESL-63s are sufficiently low in distortion to expose SE amplifiers once the output level rises above a few watts. They also need at least 50W to get acceptably loud. My WATT Vs do play louder than the Quads, but they're also low-distortion designs: single-ended amplifiers played reasonably loud sound increasingly thinner and harder tonally, and the second- and third-harmonic products become increasingly audible.
I have had more success with higher-distortion loudspeakers averaging, say, 0.3-0.5% THD in the midband—particularly if they have smooth impedance characteristics and fairly limited bass extension. With such speakers, the sonic virtues of SE tube amplifiers stand out more clearly, confirming my view of their niche-market application.