A Future Without Feedback? Letters
Many thanks to Martin Colloms for his most interesting article on negative feedback and amplifier design in the January '98 issue (Vol.21 No.1, p.87). Stereophile readers, I am confident, will be interested in reading a similar article by Jan Didden that was published in the Six:1997 issue of Audio Electronics (published by Audio Amateur Publications).
---Ray Segura, New Orleans, LA, Segurar@nctamslantdet.nola.navy.mil
Mr. Didden's article mentioned research by the later Peter Baxandall that showed that while loop negative feedback applied to a simple amplifier circuit reduced the level of the second and third harmonics as expected, it actually increased the level of the subjectively harmful high-order harmonics compared with the circuit used without feedback.-
Feedback & Martin
After reading Martin Colloms' article on amplifiers without feedback in January, I must comment.
Without feedback, a power amplifier becomes load-sensitive. Stereophile's own measurements show the frequency response into a simulated load. These measurements show the audio expansion/compression that zero-feedback amplifiers exhibit---often as much as 2dB. This is indeed audible, and while it may enhance perceived performance, it cannot be considered accurate. There are plenty of products out there to "process" audio---expanders, compressors, bass-impact enhancers, spectrum harmonic enhancers, spatial enhancers, etc. However, an amplifier should never behave as a signal processor!
Feedback, used properly, is a good thing. However, it should never be relied upon to compensate for poor power-supply design, nor can it be trusted to control radical nonlinearity, as in class-B operation. Unfortunately, it is called upon to do this more often than not. And that, too, is audible.
There is a lot more to this debate than simply feedback or no feedback. Audio design is more complicated than that. It would be a shame to see high-end designs distilled down to this level.
---Kevin A. Barrett, President, KAB Electro-Acoustics, www.KABusa.com
Feedback & Audio Research
Martin Colloms' recent article in Stereophile described the history of negative feedback in amplifier design and detailed in some length the effect that it can have on component sound. Intrigued by this, I did some research of my own. The Audio Research VT100 was recently auditioned by Stereophile (March '97), and the reviewer was quite taken by its sound. The VT130 was also recently auditioned (November '96), but the reviewer was not so impressed by its audio merits.
The VT100, VT130, and VT150 [reviewed in August '94---Ed.] share similar tube complements and internal architectures. One area where they differ is in their use of negative feedback. The VT100 uses half the negative feedback of the VT150 and one third less negative feedback than the VT130. The higher level of negative feedback in the VT130 would appear to have an effect on listeners.
I hope that manufacturers will begin to take to heart the effect that negative feedback has on their creations' sounds, and will begin to design components that use less, or ideally zero, negative feedback.
While the 1980s were an era of less than notable improvements in the audio realm, with the hopefully soon-to-be-adopted improvements in digital standards and landmark components such as the Pass Labs two-gain-stage amplifiers, the '90s will turn out to have been a great decade.
---Dave Brown, email@example.com
Feedback & Measurements
Martin Colloms raised the question of measurements vs sonic performance in his interesting article on negative-feedback amplifier design in the January issue. I keep reading about this subject, and I begin to be bothered that the real issue is not that measurements cannot reliably predict the sound of a piece of equipment, but that the wrong measurements cannot do so.
We know that there is no magic involved---these are all physical devices whose physical properties can be known and described, if only we take the care to first figure out what the complete set of relevant properties are. For example, I have auditioned various speakers whose measured high-frequency responses extend out to 20kHz, 26kHz, 35kHz, and 40kHz, yet I have found that one sounds dull at the top of the spectrum and another sounds bright, with no correlation between these observations and the HF specs...
The audiophile community ought to be devising new and more meaningful measurements that we could then use to better predict how gear will sound and how it will interact with other gear. It is not enough simply to say that measurements don't always correlate to sonic results---we need to focus on taking the right measurements. Why doesn't Stereophile solicit suggestions from its readership, and try some out in the lab to see if they correlate with the sonic results?
---Agim Perolli, firstname.lastname@example.org