Feedback:
Nearly all conventional high Negative Feedback amplifiers operate in what could be called ‘voltage-voltage-voltage shunt global’ mode, i.e. they act as voltage sources, with voltage sensing and shunt-mode voltage feedback, connected globally.
Well designed amplifier’s feedback in spite of relatively high percentage of distortion, are key element for stable and quality sound reproduction. What the most common form of ‘feedback’ achieves, when well adjusted, and whether series or shunt-derived, is well known:
- Input impedance is high. Little current is drawn from the signal source.
- Output impedance is very low, so the signal presented to the speaker at the output behaves almost like a pure voltage source, i.e. voltage does not sag or flinch no matter how much current is demanded.
- Non-linearity is smoothed out, hence % distortion is reduced. This presumes all signal path semiconductors are alive in forward active mode throughout the cycle.
- DC balance at the output is assured and principal DC conditions are stabilized at switch on. Only DC feedback is needed to achieve this, but making the AC and DC feedback proportions different requires unnatural HP (filtrative) capacitors. This may degrade sonic for other reasons, as discussed elsewhere.
It is also well known that global NFB, as practically employed, has limitations. In particular, the amount of it you can usefully apply in practice reduces with increasing frequency. Thus its beneficial effects are gradually or eventually lost with increasing frequency, often significantly within the audio band. Trying to increase the feedback causes RF instability.
‘Zero’ feedback
In amplifiers which operate purely in class A, global feedback may be abandoned altogether. This is plausible with circuits that are suited to open-loop working because they do not require global feedback to maintain near ideal DC conditions of bias and output midpoint centering. Usually, such circuits are simple, with minimal gain build-up. ‘Zero’ really means no overall feedback. It is usually practically necessary in production, and is also no less advantageous to measured linearity and sonic, to add local feedback where it isn’t already used, and to increase the amounts. Generally, local feedback is achieved at signal frequencies by unbiased resistors in line with the emitter. So called ‘local’ (but really global) feedback also occurs when gain blocks comprising more than one active device are created. The connections creating the local feedback, same resistors may double at other tasks. One problem with solely local feedback comes in production, as the various parameters of individual amplifiers that are important to audio quality, such as hum levels, are likely to measure quite variably, mainly as a result of the wide tolerances of many active device properties.
Existing of master feedback which depend solely of input signal (up to 150kHz) and local independent feedbacks for every Mos Fets control (up to 1MHz) are ours solutions.