A Future Without Feedback? Page 4
Reduce the feedback to a factor of only 3dB (as measured)---negligible by the standards of the majority of most modern amplifiers---and the sound improves a little. Reduce it to 1.5dB and the light begins to dawn. Turn it completely off (0dB) and musically you know where you ought to be.
Without feedback, both I and my friends and colleagues who shared the listening found that reproduced sound could really be different from the usual expectation, that a pervasive grayness of expression and false tonal color had been swept away without dire consequences for other important aspects of sound quality.
I can hear the arguments already: "This amplifier is probably so wrong that it can't use feedback successfully...it's one of those rare cases where negative feedback makes it worse."
Somehow I don't think so. An analysis of the approximately 700 amplifier reviews that I've undertaken over the years indicates that, if there has been any trend associated with improving sound quality, it has largely been associated with reductions in global negative feedback. Even the majors---Mark Levinson, Krell, Audio Research, Conrad-Johnson---have consistently moved toward more elegant, more linear circuitry, allowing lowered feedback levels for the same closed-loop linearity. Are these designers unconsciously and instinctively seeking a safe route toward designs with minimal or no negative feedback?
The combination of the Conrad-Johnson ART and the Cary CAD-805C forcefully supports the contention that there is a region of aural perception that is not quantified by measurements---a region powerfully related to musicality, to how real audio replay sounds in terms of vividness, expression, microdynamics, rhythm, and timbre. It was almost uncanny how this zero-feedback pairing allowed more of the natural vitality and characteristic signatures of notes to be replayed, especially their beginnings and endings. It's as if other components blur these nuances. Well, they may be nuances, but they somehow tell us so much more about the quality of the instrument and of its playing.
Let's consider the outrageous proposition that corrective feedback is fundamentally unmusical. In my reviews, I have observed that high-feedback amplifiers---which have an inherently limited open-loop bandwidth---suffer what is commonly called "midrange glare": a hardening of and forwardness in the upper midrange. Amplifiers with wider open-loop bandwidths have less of this, or their "projection" moves up to the mid-treble. Low-bandwidth, high-feedback designs can end up sounding "dark," even significantly colored in the midrange.
A typical amplifier with feedback disconnected may have 20% of complex distortion. Closing its negative feedback loop---60dB of feedback is not unusual---will reduce the level of that distortion to a level suitable for the printed specification, but perhaps not for sound quality. Investigation has suggested that the open-loop break frequency is involved---the point at which, without any negative feedback, an amplifier will filter out the upper frequencies (fig.3, top trace). Without feedback, the open-loop break frequency could be as low as a few hundred Hz; these days it is typically 500Hz to 1kHz, and may be as high as 5kHz in wide-band designs. Normally you can't see this low-pass "filter," as it's buried by negative feedback: with its feedback loop closed, the amplifier may have a measured bandwidth 100 times greater (fig.3, bottom trace). Yet I reckon that the buried filter comes back to haunt us in the form of "glare"---a coloration centered around the amplifier's intrinsic open-loop, low-pass function, perhaps due to the nonlinearity of feedback itself.
Fig.3 Gain plotted against frequency of a typical solid-state amplifier with 40dB negative feedback and an open-loop "break frequency" of 500Hz: operated open-loop (top), closed-loop (bottom). Note that, without feedback, the amplifier's output would be -35dB at 20kHz. With feedback, it is flat to 480kHz. (10dB/vertical div.; 10Hz-1MHz frequency scale.)