A Future Without Feedback? Page 5

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 problem, 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 wire them up, achieved maximum electromagnetic utilization at good efficiency, and were well matched to tube amplifiers. Higher 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.