A Future Without Feedback? Page 3
One aspect of sound quality reached a focus recently when I received a sample of the Conrad-Johnson ART for review in the UK magazine Hi-Fi News & Record Review. This preamplifier's $15,000 price tag is irrelevant; what's important in this context is its overall attainment.
The ART is a technically uncompromised design---by which I mean it has no significant weakness observable by established technical precepts---which should gladden the heart of a measurement diehard. Consider its moderate output impedance, minimal noise, negligible distortion, and the wide, flat frequency bandpass. And don't forget the highly accurate volume control. It does invert signal polarity. Then note that it comprises just one stage: a zero-feedback, common-cathode amplifier employing paralleled 6DJ8/6922 triodes. Tube purists might argue that C-J's choice of tube isn't optimal, but no matter. The truth lies in the listening result. The sound is excellent. However, that global superlative encompasses something special, which I have come to understand as the sound of zero negative feedback.
Compared with many of its colleagues, the ART fairly breathes tonal accuracy, dynamic expression, clarity, and natural musical vitality. Conrad-Johnson's designers told me that, during the preamplifier's development, and with their minimalist single-stage objective firmly in sight, they still could not conceive of using zero feedback. Instead, the initial design featured just a few dB of feedback, but a few dB nonetheless. When a circuit idea emerged late in the day that allowed negative feedback to be reduced virtually to zero, with what feedback remaining being merely local degeneration (something generally considered to be harmless), they were forced to concede that the sound quality was improved in precisely the area where the production ART is so admired.
The ART provides a logical meeting place for objectivists and subjectivists. The former cannot accuse the latter of being fooled by measurable errors. The latter may express and explain what they hear without fear of attack.
What they and I hear is an aspect of sound quality that transcends the general experience of reproduced audio; a quality that cannot be specifically addressed by system matching, cables, or speaker substitutions. In the context of the ART, and to a significant degree one or two related solid-state preamplifiers that use a single FET as an amplifying stage (the XTC PRE in the UK and the Pass Aleph P in the US), this quality can be considered as an absence of previously unidentified, almost unsuspected degradation present in much established amplification. I invite you to keep a sense of proportion when I claim that, compared with worthy zero-feedback designs, conventional amplifiers impose a significant "graying" of dynamic expression, a falsification of timbre, a shift of truly natural tonality, and a smearing of temporal definition. There may also be an associated loss of rhythm, a blurring of the delicate nuances of the leading edges of natural sounds.
Then I received review samples of the Cary CAD-805C monoblock power amplifier. I wanted to try the single-ended 805C because it is sufficiently powerful to produce credible loudness and fair bass with my Wilson WITT speakers. Much to my relief, the 805C was a seriously good-sounding amplifier. (See Dick Olsher's review of the earlier 805 in the January 1994 Stereophile.)
Despite this amplifier's obvious competence, however, there were still some allowances to be made. Its intrinsic frequency response is not perfectly flat, especially at the band extremes. In addition, its relatively high output impedance significantly alters the effective frequency response of the speaker. Both of these factors required some mental adjustment and acclimatization. For an old speaker hand, this wasn't too difficult; true, my Wilson WITTs weren't quite the same as before, but they still possessed their trademark qualities of good dynamics, fine clarity, and good rhythmic expression. And in combination, the ART and 805C showed an immediate association---a commonality of expression and harmonic line with no apparent concession in low-level detail, focus, or stage width or depth.
At this point the proceedings took on an educational dimension, as the big Cary offers the fascinating feature of user-adjustable variable negative feedback. In fact, the degree of negative feedback can be reduced right down to zero.