Background
For the last forty years I have been working with low feedback and zero feedback amplifiers. Initially I was convinced that audio power amplifiers had to include certain elements (such as negative feedback) to control distortion. After some exposure to Robert Fulton of FMI (Fulton Musical Industries) in the late seventies, a larger picture began to emerge. Robert Fulton was emphatic that the quality of the topology came first, then quality of components, and if everything was right, the distortion would already be low. Negative feedback could be reduced or eliminated.
Back in the days when feedback was being experimented with, it was common knowledge that this was a compromise. In the succeeding decades it became accepted as a fact of audio design. More recently the negative feedback idea has been challenged by elements of the high-end audio community.
My own explorations resulted in an amplifier that was designed to reduce distortion in every way possible without feedback. In this way it was possible to examine the effects of feedback, since the amplifier was very functional without it. During the process, I also learned about common engineering practices that tend to hold back development in fields such as audio. In their recent book "Control Design And Simulation", Jack Golten and Andy Verwer discuss this phenomena in chapter two, with regard to applying mathematical models to the real world: "...mathematical models invariably involve simplification. Assumptions concerning operation are made, small effects are neglected and idealized relationships are assumed."
It is the mark of a good engineer to know when and which things should be assumed, neglected or idealized, and we see this in audio all the time. The problem here is human nature. We tend to stay within the limits thus set by the existing paradigms and to resist changes that threaten one's viewpoint of the world. When someone else creates challenges to the paradigms, it is normal also to try to protect one's world view by preventing the new idea from gaining ground.
As alluded earlier, negative feedback has been found to be an inexact solution to amplifier distortion. This is due to phase shift
in the amplifier circuit (which when significant enough can look like propagation delay on an oscilloscope) which is a normal phenomenon
of amplifiers. You have a variety of phenomean all going on at the same time; the point at which the feedback is fed back into the amp
will be non-linear itself (resulting in some added distortion); if the phase of the feedback does not match the input you will also
get some distortion (higher ordered harmonics added) and in most amps made in the last 70 years, the feedback value will fall off at
higher frequencies due to a lack of gain, bandwidth or both, causing the distortion to rise at some frequency above 1KHz in most
amps. This results higher ordered harmonics which has two effects: the ear/brain system uses to them to sense sound pressure and it
also assigns a tonality to them, which is that of 'harsh and bright'.
In the 1960s, General Electric's conducted a variety of tests, confirming that amounts of barely hundredths of a percent distortion were not only audible but also irritating to the human ear (conversely, they also found that the ear is quite tolerant of lower ordered harmonic distortion). In other words, a disparity exists between the mathematical proof for negative feedback and its actual application, and is an example of the engineering phenomena to which Golten and Verwer refer. Despite this, negative feedback is commonly accepted in the audio world, causing the reigning design, test and measurement paradigm to have a built-in weakness.
