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Feedback, no feedback and selective feedback

Driven by the strong criticism of many expert inmates about negative feedback I have personally verified (by experimenting with my DIY PP amplifiers) that, depending on the number of parallel output tubes and the amount of maximum feedback, frequency selective feedback can get the best relative performance in subjective listening tests.

By frequency selective feedback I mean that feedback is fully applied only at mid-low frequency, with no feedback at all above 20-40 KHz.

The simplest way to implement this kind of feedback is to place a small capacitor in parallel to the standard (feedback) cathode resistor.

From the frequency response point of view such selective feedback is the same as a gentle high boost, but it sounds different from a standard high tone control, because the high emphasis is the result of reduced feedback rather than due to a LC resonant circuit.

The likely rationale is that low frequency feedback is certainly beneficial to obtain optimal speaker damping at low frequency and that above about 5 KHz the perceived benefits of negative feedback (if any) become vanishingly small.

I think that, due to common hearing impairments due to age (most tube lovers, like myself, are quite 'seasoned') the key factors in assessing subjective audio performance are:

- perceived quality of the critical audio band 200 Hz - 5 KHz;
- perceived tonal balance of this critical band w.r.t. low (50-200 Hz) and high band (5 KHz - 10 KHz).

Perceived tonal balance explains why low level listening is quite unimpressive and we need to adjust volume to a sufficient level for a pleasant listening experience.

When removing feedback from a commercial amplifier you likely alter tonal balance and sensitivity (but I suppose that you are not influencing so much the quality of the critical audio band, expecially if the 'basic feedback' is low).

In fact, in the lower spectrum you reduce damping and increase low cut frequency (and reduced damping can be even perceived as stronger bass) whereas in the upper spectrum you introduce some gentle high frequency roll-off. Overall, the increased sensitivity allows a lower volume setting for the same listening volume.

This change is often perceived (and reported) as a 'spatial improvement', 'micro-detail' or 'enhanced soundstage'.

The obvious conclusion is: as the ear is the final judge......

Best Regards
Luca








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