A Future Without Feedback? Letters page 2
I read with interest Martin Colloms' discussion of negative feedback in the January Stereophile, and something occurred to me as I digested the article. I was wondering if there has been much thought given to the differences between steady-state vs transient response. It is relatively easy to think about the positive effects of negative feedback---sorry---during steady-state conditions, such as reproducing a sinusoidal waveform. The distortion components are somewhat static, and can be canceled out in real time.
However, during transient events there could be instantaneous conditions when the distorted feedback is not canceling anything out, but simply building on itself. If all the measurements are made with swept sinewaves, then this phenomenon would be missed in the measurements---which might account for the disparity between the subjective perception and the objective.
I have seen plenty of measurements using squarewave responses. While these provide a useful evaluation of overall dynamics, they excite frequencies only at odd harmonics of the fundamental, and the measurements do not give any indication of linearity, just dynamic response.
This leads me to a question: Does the audio industry ever measure frequency-response functions with white-noise excitation? If so, are partial and multiple coherence analyses performed to provide direct measures of linearity (and hence distortion) within and between channels under more realistic conditions?-
--Nigel A. Linden, Burnsville, MN, Nigel.Linden@MTS.com
Feedback & IM Distortion
I think Martin Colloms, in his January article, touched on a neglected subject: intermodulation distortion. It has long been recognized that spot frequency measurements, such as second- or third-order harmonic distortion, while indicators of amplifier linearity, are insufficient to explain audible performance. Similarly, two-tone difference tests for intermodulation also may fail to correlate with subjective tests.
I would like to suggest a more sophisticated intermodulation test that might go a long way toward correlating measured performance with audible effects.
Years ago, when the world was analog, designers and users of wideband telephone transmission equipment (of the type Harold Black worked with when he developed negative feedback) recognized the failure of single- and two-tone frequency tests to fully characterize their systems. Eventually a test method was developed that correlated well with system performance, a method known as "white-noise loading." This test, while simple in concept, takes into account all orders of intermodulation products that may be present.
Noise loading may be described in the following way: The test signal consists of white noise, limited to the bandwidth under test. The amplitude is chosen to remain just under hard clipping. A narrow frequency segment of the test signal is removed with a band-stop filter. At the output of the amplifier, a narrow bandpass filter is used to measure the signal present within the stop band of the test signal. Any energy within this passband is due to the presence of a mixture of idle noise and intermodulation products. The IM component can be removed by removing the test signal. The difference between these two readings is an indication of the level of intermodulation products.
I don't know if the noise-loading technique has ever been implemented for audio amplifiers, but it seems to me that it should be. Perhaps this technique could help explain why some amplifiers sound poorer than others that measure, by conventional means, as good or better.
---Lee Stephens, firstname.lastname@example.org
Both Belcher at the BBC and the late Deane Jensen have suggested using a signal with a comblike spectrum to test amplifier performance, the idea being that any component that appears in the gap between the discrete test tones is due to intermodulation and noise. I have done some testing using this kind of technique---Stereophile's Test CD 3 contains some suitable signals---but have come up with enigmatic results.
Amplifiers & Feedback
The following question surfaced after reading Martin Colloms' thought-provoking article about negative feedback in January ("A Future Without Feedback," Vol.21 No.1, p.87).
How bad can feedback designs be when Mr. Colloms could achieve a "dawning" of such fine sounds from zero-feedback amplifiers when he used recordings bearing the imprint of any number of feedback circuits? Most likely the sound survived all those feedback circuits in the recording chain. It seems unlikely that a zero-feedback amp would erase the imprint.
I guess the next step is to make a live recording using all zero-feedback electronics as an experiment.
---Tony DeLuca, Frankford, DE
Feedback & Sympathy
I read Martin Colloms' indictment of feedback (Vol.21 No.1) with great interest and more than a little sympathy.
Martin's article was not based on any scientific "research" into the use of feedback in audio amplifiers. Rather, it was a simplistic generalization based on MC's exposure to a limited number of commercially available products. I am familiar with the products MC has mentioned in Stereophile and HFN/RR and can understand his feelings.
I'm sure that MC is well aware of the weakness of such an ad hoc argument, and surely this is why he offers his experience with the variable-feedback Cary 805C amplifier in an attempt to buttress his argument with something a little more rigorous.
At first glance, comparing the same amplifier with various amounts of feedback applied appears to be reasonable. However, a quick look at the test reports of the Cary 805 in the January 1994 issue of Stereophile reveals that when feedback is applied, this amplifier exhibits gross ringing that would be considered unacceptable by any standard (Vol.17 No.1, p.108, fig.2). (There really is a place for such reports in high-end magazines.)
This is about as fair as comparing the sound of a push-pull amp as supplied by the factory with the sound of the same amp with half the output tubes removed and claiming that this is a valid comparison between push-pull and single-ended! Or how about taking a low-feedback amp (like the CAT JL1) and simply removing the feedback to "prove" the reverse of MC's assertion? Indeed, the loss of directness, vibrancy, life, and harmonic purity when decreasing feedback is similar to MC's claimed losses in the Cary when increasing feedback!
Additionally, even if the feedback were properly compensated to produce a good-looking squarewave, it could still be a sub-par feedback representative. Every circuit has an optimum amount of feedback, and sometimes this amount is zero. Further, as one who has experimented extensively with feedback and nonfeedback circuits, I can tell you that there are numerous ways to go about phase-compensating a feedback amplifier to produce equally good measuring results, but each way sounds very different. Would you believe that a 10-picofarad compensation capacitor can have profound sonic consequences across the entire audio band, including the bass? It doesn't make textbook sense and can't be measured by any standard technique, but it sure is easy to hear!
Martin Colloms' "A Future Without Feedback" is the kind of article that drives certain types of audiophiles, the type who thinks too much and listens too little, to make very poor purchasing decisions. Fifteen years ago this type was convinced that super-low THD was the answer. (Who believes that now?) Ten years ago they thought slew rate and TIM distortion was the answer. (Does any manufacturer even specify TIM distortion these days?) Five years ago we were told that fully balanced circuitry was the answer. (And now now we have the backlash: fully single-ended circuitry!) Now MC would like to usher in a zero-feedback craze! The trouble is that we've had zero-feedback practitioners, both tube and solid-state, with us for many years, and they have made both very good and very bad products, just like their feedback-using colleagues. (Although it seems that both the very best and very worst designs use feedback.)
Yes, Martin, I hope you are unsuccessful, but I know that the audio press has to latch on to some "new" idea even if it's recycled, so why not "no feedback"? Maybe it will be followed up by a "high-feedback" craze! The "think-rather-than-listen" audiophiles will lose either way.
Now I don't want Martin to get mad at me for accusing him of making "simplistic generalizations." Perhaps he'll feel better if I do the same thing. How about this one: "High-voltage tubes don't have any magic." To prove my point, first I should say that I've heard many amps that use high-voltage tubes, including some that cost more than a nice house, and none have had "magic."
But MC might want something a little more rigorous than my personal experiences, and so I encourage him to compare his beloved Cary 805 (which uses high-voltage tubes) to the cheaper, albeit lower-powered, Cary 300SE (which uses medium-voltage tubes). Both amps are made by the same manufacturer; presumably, both are made to the same quality level and designed by the same person, who, I'm sure, must be equally adept with both high- and medium-voltage tubes. If this doesn't prove my point, I don't know what will!
--Ken Stevens, President, Convergent Audio Technology