Negative feedback, voltage and output impedance

Traditionally the accepted notion is that adding negative feedback will decrease voltage gain in an amplifier and also decrease the output impedance.

However it is interesting to note that the addition of negative feedback will not alter how much power the amplifier is capable of into a given load. When you think about it, the reason is obvious. The power generated by the amplifier for a given load comes from the output devices and power supply in the amp, and not from how much signal is returned to its input.

Negative feedback will change how the amplifier reacts to the load, having the effect in some cases of reducing the amplifier's power into certain impedances, and increasing it into other impedances, so long as this power increase is within the limits of the amplifier in the first place.

Unfortunately, negative feedback also increases odd-ordered harmonics in the range of the 9th, 11th and beyond- a place where the human ear is very sensitive. The result is that amplifiers with negative feedback will exhibit an unnatural sheen and/or harshness in the high frequencies.

In the world of high end audio, this effect is audible enough that many companies have eschewed negative feedback for other means to the same end. We live in an interesting time.

Hi Marakanetz,

Just FWIW, as a manufacturer I have to be careful what I say as this forum is moderated. At the same time what I am expressing is actually the way I see it and that has served me pretty good so far for the first 26 years or so anyway :)

I hope you don't consider it a sin that I try to walk what I talk. OTOH, our early amps for the early 80s did use feedback, so its not as if I've not given it a fair shake. Every now and then we re-try some of the stuff we abandoned, just to see if we are still on target. FWIW, amps without feedback are harder to set up as you have to be more careful about the speaker choice. This means our market is probably a bit more limited. As a result it doesn't take the ability to download and chew gum at the same time to figure out that I'm not in it for the money...

Another difference between us and Mac is that our warranty is transferable and we will reactive that warranty on any product that we have fully updated.

The idea that:

>Amp's input and driving stages do definitely have a feedback so the statement "no feedback" isn't truthful in any case.

-is incorrect. It is quite possible to build an entire amplifier without feedback of any kind.

There are two types of feedback that are both considered 'negative' (in that the feedback can be used to lower distortion): Loop and degenerative.

Usually Loop feedback is the type that is considered malfesent, as it is the type that increases high odd-ordered distortion. This occurs because there is a very finite time that it takes for the input signal to propogate to the output of the amp, where the signal is then tapped off back to the input. What this means is that as frequency increases, the feedback signal becomes increasingly out of sync with the input signal. Hence the addition of odd-ordered harmonic content. Esentially at higher frequencies the loop feedback results in ringing unless steps are taken to control it (usually by tailoring the feedback circuitry).

Degenerative feedback occurs in real time and so does not contribute to odd ordered harmonics, however the output impedance of the circuit is increased.

Audio Note (UK) made amps that lacked either type of feedback, back in the early 90s, at least according to the designer whom I met at CES.