The Unseen Variable Page 4

These anecdotal tests only go to show the remarkable complexity of sound-quality changes that can result from using an amplifier with a significant source impedance. Even experienced critics—sorry, I do place myself in this category—find it very difficult, if not impossible, to separate these physical and acoustic interactions from the overall perception of sound quality. We have to accept that moderate tone-control effects—unfamiliar response changes over limited sections of the overall frequency range, allied to changes in level—can significantly alter perceived sound quality in all its many aspects. And, in my view, it isn't possible with any confidence to separate the sound of an amplifier from the changes it may impart to the frequency response and bass damping of a specific loudspeaker.

A tubed SE amplifier may be of very high intrinsic quality, but its sound will always be hard to pin down, due to this unavoidable load interaction. The one exception will be with loudspeakers that have flat impedance characteristics. However, as such speakers tend to be 4 ohm designs, there will be an undue loss of overall power when they are driven by single-ended tube amplifiers—something that is hard to accept given the limited power delivery available from such amplifiers.

Evaluations of SE amplifiers tend, therefore, to degenerate into a search for that one special speaker—generally an off-the-beaten-track design—that has exactly the right response and impedance characteristics that its sound will be optimized when driven by an SE amp. Thus, the critic is rewarded, his skill in "system matching" vindicated, and the SE amplifier transports him to audio heaven.

A case history
To close, I offer a real-life example of an experienced critic who got the right musical result with an SE amplifier for the wrong reasons, and in the process unknowingly corrected a speaker problem.

My colleague Ken Kessler and I recently co-wrote a review of the Wilson WATT/Puppy V speaker system for the English magazine Hi-Fi News & Record Review (footnote 2). We ran into some trouble in our assessment of the speaker. The test samples had been drawn from early production, which had generally been well-received by customers in the US.

Though the speaker's balance was fine in large listening spaces—especially in US-style timber-frame houses—a moderate flaw in the form of an excess in upper bass centered on 90Hz became apparent, even obvious, in smaller rooms—especially in apartments and houses of concrete or brick construction.

Ken bravely fought the fight for system matching, discovering that he could optimize the sound of the Wilson System V. His intuitive solution was to drive the speakers with single-ended amplifiers: the Cary 805, a Unison Research from Italy, and the Paravicini-designed EAR 954. Using these amplifiers, Ken was amazed to find that, contrary to his expectations, the upper-bass excess of the Wilson speaker had been tamed. "Amazed" because, given the intrinsically good sound quality of these SE amplifiers, we know that the WATT Puppy is a pretty rough load, and that historically it has favored tight, gutsy amplifiers—generally solid-state.

I found the answer to this apparent paradox while performing the test mentioned in this article. It turns out that, while the new WATT is close to a 6 ohm load and is fairly constant over the main frequency range, the new Puppy is a tougher load than the old, falling to around 2.5 ohms in the 90Hz region, and typically averaging 4 ohms. Now Ken's results could be explained! The high SE source impedance acted as an attenuator/damper for the Puppy, depressing its errant upper bass. In addition, the upper range covered by the WATT was now attenuated less overall, changing the balance between the two units.

The figures are instructive: assuming a 3.3 ohm SE source, the WATT is attenuated by 3.8dB. The Puppy, however, is stepped back by an average of 5.2dB, and by 7.2dB at 90Hz. Given tolerances for the exact value of source impedance in the frequency range concerned, you have exactly the -3dB correction this speaker requires (footnote 3).

Ken's story was concluded with Wilson Audio supplying the factory-specified 15 ohm damping resistors for the Puppy woofers. With these connected across the outputs of the woofer crossover feeds—an easy retrofit—the system now sounded bass-light and lacking in slam when driven by the SE amplifiers, mandating Ken returning to a gutsy, solid-state amplifier to complete his evaluation.

It is easy to extrapolate from this tale to other circumstances. Consider, for example, a neutral speaker that is not optimally placed in a room. A high amplifier impedance may, in conjunction with that speaker's characteristic load impedance, deliver a better tonal result. Most of the credit will go to the change in amplifier, not to the unseen variable of source impedance.

Footnote 2: HFN/RR, January 1995, p.28.

Footnote 3: Since October 1994, the Puppy V has been fitted with a resistor to the crossover. This damps the Puppy's motional impedance interaction with the crossover, thereby giving 3dB of control at 90Hz—the offending point.

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