A Future Without Feedback? Letters page 3
I was surprised to find Martin Colloms---and John Atkinson in his footnote---perpetuating the notion that Harold Black "invented" feedback (January '98, p.87). As I recently pointed out elsewhere (Hi-Fi World, April 1997), that honor belongs to the British engineer Paul Voigt, a pioneer of high fidelity, who had incorporated it in his Motional Feedback Patent 231,972, dated January 29, 1924. Over 70 years later, motional feedback as applied to loudspeakers has made little further progress, and Voigt's use therein of selective negative feedback, which he regarded as obvious and not worthy of special attention, is the first reliably documented use of which I am aware---many years before the Black disclosure. I enclose a copy of Voigt's circuit in the patent, and a description of its operation in his own words that shows that he fully understood its application. The latter is taken from papers in my possession, typed and annotated by him in August 1970, and reads as follows:
"As you will see from the circuit, it makes the loudspeaker one arm of an AC bridge. The arm that balances...inductance is a choke, with no parts supposed to move at any frequency. So long as the program frequencies were those on which the 'speaker behaved as a pure choke, the bridge remained in balance and nothing special happened. At those frequencies, however, at which the 'speaker drive portion or the diaphragm or horn resonated, the bridge became unbalanced. The 'floating' audio transformer then fed back into the grid circuit a signal due to the imbalance of the bridge.
"It was possible to make the connection in such a way that the fed-back signal provided negative feedback at the resonant frequencies, thus diminishing the effect of the resonant peak. By suitable juggling of the exact values of the bridge arms, it was possible to unbalance the bridge slightly at both high and low frequencies in such a way that the feedback was positive, thus boosting the ends of the scale where the 'speaker's response was poor.
"The values of the components of the bridge arms were chosen so that they 'stole' only a small amount of the audio energy. By means of a switch, the system could be made to behave like a normal amplifier, and it was enlightening to switch over from 'Normal' to 'Compensated.' Apart from the improvement in frequency response, it had the effect, when several voices or instruments were involved at the same time, of 'disentangling' their sounds (no doubt due to better transient response). The sensation was that the sound came through the loudspeaker rather than originating at its mouth."
Voigt, a month short of his 20th birthday, had joined the English Edison Bell Company (J.E. Hough Ltd.) straight from college on November 1, 1922, to work on "wireless," into which they wished to diversify, as it was then widely believed that broadcasting would kill off the gramophone record. However, on February 9, 1924 he had made an experimental electrical recording on a disused cylinder machine of an evening radio program. Mischievously, he left the result on the boss's desk; this resulted in an immediate instruction to develop this "new" recording method. By 1927, Voigt had invented and produced moving-coil cutters, feedback amplifiers to drive them, and the slack-diaphragm condenser microphone. There followed high-efficiency horn loudspeakers, and later the long-coil MC pickup. By the 1930s he had taken this gear to major middle European cities and sent back some 600 wax masters.
I got to know Voigt in the late 1930s when Edison Bell had gone to the wall (1933) and he had started his own company, Voigt Patents Ltd. Unfortunately, the war, which for us began in 1939, put paid to his business dreams, and in failing health he emigrated to Canada in 1950 and ceased his work in audio.
---Geoffrey Horn, Oxford, England
It was instructive to read Martin Colloms' thoughts on negative feedback after he reviewed his 701st amplifier, the Cary CAD-805C ("A Future Without Feedback?," January '98, p.87). But is it true to say that it is improperly designed amplifiers that are allergic to negative feedback? I discovered this phenomenon for myself almost 28 years ago when I heard a whistle from my first hi-fi system. It later became clear that a generic amplifier has a nonlinear phase response; moreover, the latter varies with the amplitude of the input signal. Such a system is hard to control with negative feedback, as Professor Kalman has said.
The main problem of a real closed-loop system is stability. Any amplifier stage built with an active element (tube or transistor) has always got some negative-feedback loops, either intrinsic and parasitic or a circuit for operating-point stabilization. A resistor introduced in series with the tube's cathode, a bipolar transistor's emitter, or a FET's source causes negative feedback. A cathode/emitter follower is an amplifier with 100% negative feedback. Therefore, all kinds of amplifiers have negative feedback. The Encyclopedia Britannica says that feedback is a basis of nature. Why, then, must designers avoid using feedback? In the two centuries since Watt's use of a governor in a steam engine (1768), and Maxwell's formulation of feedback (1868), negative feedback has been working for mankind.
I am sure that 25 years ago a tube amplifier designed by an "old man" with 45 years of experience sounded better than any transistor design from a novice. But 25 years ago Robert Widlar had already created his legendary LM101 op-amp chip, and fast silicon power transistors were about to appear on the scene. It was time for the solid-state amplifier. (Though CD has recently returned tubes from their dusty shelves.) And designers continue to think about a distortion-free amplifier. I made one in 1984 (theory and circuit diagram published in 1987).
All things are measurable---even light pressure, which was measured by Petr N. Lebedev early in the century. With harmonically rich pink noise as an input signal and using DSP-based modern statistical methods, it's quite practicable to measure any spectrum modifications in an amplifier's output signal.
Martin Colloms is right about there being a "thermal compression" phenomenon in loudspeakers. (Why do good loudspeakers have sensitivities below 90dB?) However, I have observed thermal intermodulation in low-voltage (10-15V) stages with currents of only a few milliamperes. In general, thermal effects are the main difficulty in precision-analog/mixed-circuit design.
Any amplifier stage is a low-pass filter (described with gain, time constant T, and output resistance). In an ideal case, that filter is first-order (T1 only). As the loop is closed with nondelay negative feedback, the amplifier is stable without a load! But since a real amplifier stage isn't a first-order filter and also has to drive a complex load, it becomes clear that the application of negative feedback is far from trivial. Hence, we see "designers...seeking a safe route toward designs with minimal or no negative feedback," in Martin's words.
The coloration mentioned in the Colloms article results from negative feedback being applied to a relatively stable amplifier. A typical negative-feedback circuit consists of two carbon or metal-film resistors, and it can't be nonlinear. Dynamic nonlinearity is caused by transient feedback breaking under varying signal conditions in improperly designed amplifiers. Since any nonlinearity causes intermodulation distortions, coloration appears.
There is no mysticism in amplifier design, just serious science.
---Andrey A. Danilov, email@example.com