A Future Without Feedback
Stereophile Magazine, January 1998, pp.87-

Martin Colloms asks whether negative feedback, almost universally applied to amplifier designs, is a good thing.

Over the years as a reviewer, I have tracked the swings of opinions and popularity of various audio ideas and technologies. Amid a sea of advanced designs that achieve powerful technical performance and laudable specifications, I'm reminded of a major blind listening test of 18 power amplifiers that I set up for the long-since-defunct UK magazine Hi-Fi for Pleasure back in 1975. We had "advanced technology" then: the transistor amplifier had matured and was well accepted by audiophiles. Prices of the review samples ranged from $300 to $3000 (equivalent to $1000-$10,000 in today's dollars). The auditioning sessions were graced by the late Spencer Hughes of Spendor, Julian Vercker of Naim, Philip Swift of Audiolab, Alan Harris of retailer Audio T., Bob Stuart of Meridian, and John Wright of IMG (now TDL in the UK).

On the suggestion of Alan Harris, a serious tube amplifier fan, I introduced a ringer to those tests: an ancient (over 10 years old) 25 Wpc tube amplifier, the Radford STA-25 III, worth perhaps $100 at the time on the used market. I used a selection of master tapes as the source. When the results of the blind test were analyzed, the tubed Radford had come in first, despite showing the poorest measured performance. (Needless to say, its secondhand value soared after the review appeared).

This results dramatically illustrated almost a quarter century ago that the associated between measured performance and sound quality is uncertain. However unsuspected at that time was the possible benefit in that test context of the Radford's relatively low level of negative feedback and the consequent effect on sound quality.

Feedback and the Ferry
The dominant problem in the early days of long-distance telephony was that the harmonic and frequency distortion added to the signal both by the line and by the necessary chain of repeater amplifiers made voices unrecognizable and unintelligible. Harold Black graduated from Worcester Polytechnic Institute in 1921 and took a post as a research scientists at what later became Bell Labs. For six years he struggled with the telephony distortion problem. The solution was the use of negative feedback. Black described how he conceived the theory and the equations in a flash one day in 1927 while community to work on the Lackawanna ferry [1].

[1] Black published his work on feedback in "Stabilized Feed-Back Amplifiers" in the January 1934 issue of Electrical Engineering, published by the American Institute of Electrical Engineering. See James E. Brittain "Scanning the Past: Harold S. Black and the Negative Feedback Amplifiers," Proceedings of the IEEE, Vol. 85 No.8, August 1997.

To understand the revolutionary nature of Black's idea, consider a device with useful voltage or power gain (u) that may be compromised by undesirable nonlinearity or distortion. It may also have a nonflat frequency response. Prior to Black's flash of insight, all the output of an amplifier was fed to the next stage, be it a transducer or another amplifiers stage (fig.1). But instead, if a proportion of the output (b) is fed back into the input of the amplifier and applied in inverted form (fig.2), the fed-back distortion and frequency-response errors will cancel those generated within the device. In addition, the amplifier's output impedance will be lowered. The price to be paid for these performance gains is that the amplifier's overall gain or amplification is reduced in proportion to the amount of negative feedback. But in theory, if the amplifier, operated "open-loop," has a surplus of gain above that which is required, closing the negative feedback loop allows its errors to be reduced to negligible levels.

The concept of negative feedback is hugely valuable both in electronics and in control systems, and is firmly entrenched as a powerful design tool. Many audio engineers see it as a panacea for the ills of practical amplifying devices, using feedback --often with great skill-- to engineer amplifiers with superb linearity and consequently low levels of measurable distortion. By and large, negative feedback works. It has made a vast variety of audio products possible and manufacturable. It is hard to conceive of the world of audio engineering without Harold Black's negative feedback.

Fun?
Engineering can explain much about the audio world. However, it is when it can't explain something that the real fun begins. Some aspects of perceived sound quality are not explained by established theory. There is a growing suspicion that some of these aspects --a loss in natural timbre; a duller, less expressive performance; increased aural fatigue; and missing life and energy in reproduced sound-- may be consequences of the application of negative feedback.

It would be a mistake to demonize any particular philosophy. To do so forces people into entrenched positions and encourages the adoption of unhelpful defensive reactions, thus missing the opportunity for constructive dialog. Consider some of the contentious subjects that have been debated recently in these pages: analog vinyl and tape vs digital; one-box CD players vs separate transports and DACs; tubes vs solid-state; single-ended vs push-pull amplifiers; monoblock vs stereo amplifiers; class-A vs class-A/B or any other class of amplifier operation; pentodes vs triodes; and integrated amps vs pre- and power amps. Audiophiles have debated the merits or otherwise of cables ad nauseam. And in the high country of the tube purists, discussions rage about the virtues and vices of various types of triodes, even individual brands of triodes.

In my opinion, such debates have been valuable, though in some cases the importance of what differences do exist has been blown up out of all proportion. But the fact that a given difference or sonic error is detectable and audible doesn't mean that all is lost, that you can't adjust to ane live with some of these transgressions.

Measuring
Many of us working in the audio industry have long been aware that measurements do not fully describe sound quality. Moreover, it seems that measurements fail to describe some of the more important aspects of subjective perception. For example, we may guarantee that an amplifier will have a perfectly flat frequency response under normal conditions of use, yet we cannot explain why it may still sound duller or brighter than another comparably "flat" amplifier. We can measure crosstalk, channel separation, distortion, and noise to incredibly low levels, yet we cannot explain why some amplifiers have greater perceived stereo depth, resolution of detail, and low-level ambiance than others. While we know that 0.3-0.5% of third-harmonic distortion is just audible the midrange, how can the overall sound of a tube amplifier be judged "just fine" when we can measure 1.5% of second harmonic and 0.8% of third at moderately high listening level?

Still more intriguing is the matter of dynamics. Some electronics sound flattened and dulled in terms of musical expression; other may be wonderfully revealing of this quality even at quiet sound levels. Or consider rhythm and timing: One power amplifier gets your foot tapping, another leaves you reading the sleevenotes. I can identify no measurement associated with rhythm or musical dynamics.

How can an amplifier produce superbly low measured treble distortion, yet give the aural impression that there's sand in the tweeter? How can an amplifier that in theory should have great low frequencies (for example, it has a DC-coupled topology, a big power supply, and a high damping factor) have soft, slow-sounding bass?

Some engineer have been developing an awareness of how we've gotten some of it wrong. Glimpses of audio heaven have been observed and reported with a number of exotic single-ended creations. More precisely, the SE units' sound over the broad midrange --in point of fact, over most of the significantly audible frequency range-- reaches a level of purity and intrinsic musicality that inspires near-religious fervor. Such quality shows the rest of the industry what they're missing.

This isn't the place to expand upon the SE power-amplifier technology's strength and, in some instances, audible weaknesses. Suffice it to say that the problems, often located at the extremes of the audio range, are in this context relatively harmless, and won't confound the following argument.

Art . . .
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 pre-amplifier'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 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 zerofeedback. 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 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.

Science . . .
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 Disk 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 a 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.

With the Cary's control set to its maximum of "10 dB feedback" --when measured, this in fact turned out to be 6dB, a factor of 2x for the 8 ohm output and with the 8 ohm feedback switch position-- you could certainly hear a more accurate frequency response, the benefits of a greater damping factor, tighter bass, and a midband more like that of the familiar Wilson WITT. However, much of the magic was lost. That particular degree of perfectly timed involvement, of convincing transient edges, of natural tonality and expression, was lost. Now what we had was just another very good tube amplifier with a particularly pleasing midband.

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 1.4dB 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?

Unmusical?
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, the 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 characteristics 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 common called "midrange glare": a hardening of an 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.

Consider the proposition that a pure input signal is subjected to the usual nonlinear amplification and is then applied with all the subsequent errors back to the input to be amplified again. In theory, the errors are subtracted at the feedback connection, but there is inevitably some error in this subtraction. No problems, says the textbook: the wide bandwidth of the closed-loop amplifier will ensure that the signal and errors, and their errors, will go many times 'round the loop, reducing the distortion to below audible levels.

Or will it? Audiophile pundits know only too well that making a single audio stage perform to a truly high standard is not a trivial matter. Almost by inspection you can see that the feedback amplifier has the capacity to go on compounding its error residual. When an amplifier is processing a complex, harmonically rich input signal --music-- and not a steady-state single sinewave tone in a lab test, something could well go wrong. That cascade of residual errors will intermodulate at low levels, but it will intermodulate in a fantastically complex manner.

Subjectively, the effect of increased negative feedback is generally that of increased compression, in addition to the midrange coloration noted above. This loss of dynamic expression suggests that additional energy is indeed filling in the natural spaces in the original spectrum and thus blurring musical expression.

A future without feedback?
When Black proposed negative feedback in 1927, he was trying to solve a specific problem: the deep cascading of imperfect, transformer-coupled tube amplifiers. But has anyone explored the implications of negative feedback for reproduced sound quality in the absolute sense?

Based on my experience of the pairing of a zero-loop-feedback preamplifier and amplifier, and supported by the evidence that amplifier designers consciously or unconsciously attempt to reduce negative feedback to improve subjective quality --even if this means worsening the measured performance-- we need to reconsider the subject.

It is possible that engineers need to rethink how audio systems should be designed. Before the introduction of transistors limited to low voltages, which forced the speaker industry down to 8 and 4 ohm impedances, speakers were typically 8 to 16 ohms, did not need thick cable to write them up, achieved maximum electromagnetic utilization at good efficiency, and were well matched to tube amplifiers. High efficiency spells better dynamics, reduced thermal compression, and the potential of using smaller, more perfect power amplification. But if speakers were designed to have smooth impedance curves, to be relatively uncritical of amplifier or cable matching, and to offer higher sensitivity, we would have greater freedom to examine the feedback question and the validity of the low-power, short-path, zero-feedback approach.

We still have much to learn about the art of sound reproduction; ultimately, our responsibility is to our ears, not to established precepts.