Microphonics, Feedback and Bob Carver

That is, the resistance to my fingers or external sources is causing the amplifier to respond with enough current to keep the voltage at zero, so it's not a zero sum game, I think.

@erik_squires You exert far more energy with your fingers on the cone than sounds in the room will! To me this seems a non-issue. Again, if the amp is on, the speaker is harder to move due to the output impedance and feedback having nothing to do with it. To show this for yourself, put a 0.5 Ohm resistor across the speaker terminals and you'll see what I mean. The resistor has no feedback yet the speaker is much harder to move.

When I'm at audio shows and there are multiple speakers in the room, I short out the terminals of the unused speakers so they won't resonate.

Sounds like we're talking about the same thing but using different semantics.

Since the amp makes the speaker hard to move, it inherently also reduces any audio signal picked up by the diaphragm. Since that motion would be a tiny amount compared to the actual output of the amp, its easily rejected. I'd regard it as a non-issue.

We've been making zero feedback amps with a high output impedance for decades now. I've noticed with them that the room size can affect the deflection of the driver- that in fact the zero feedback amps react to the room size a bit. You can see where I'm going with this- with an amp using feedback and having a low output impedance this effect would not be measurable.

I think it is worthwhile to get very specific. The more feedback the less the speaker’s impedance can affect the amplifier’s frequency response. That is, it behaves closer to an ideal voltage source. OK, that part I agree with, but this question is a little different.

The more feedback you have, the more the speaker’s movement is likely to create the amplifier to respond.

@erik_squires Your first paragraph is correct, the second is not. Again, feedback allows the amplifier to reject that which is not the signal. So if the speaker is caused to move by outside forces, the amp will actively work to stop it and that work will be quite minimal; the speaker cone won't respond easily to forces outside that of the amp.

Is this a matter of semantics?

@erik_squires The more feedback you have, the less the speaker can affect the amp. Feedback allows the amplifier to reject that which is not the signal and microphonic input from the speaker is an example of that. With more feedback also comes greater damping, which means its much harder to get the cone of the speaker to move from forces outside that of the amplifier power driving it.

FWIW feedback is a part of electrical theory known as 'control theory', which is used extensively elsewhere in the electronics industry and whose rules are well understood. For some reason I can't discern (if tradition alone isn't it) feedback has been poorly applied in audio 'as if' the designers were ignorant of control theory.

To this end, rather than producing an error signal and using it for correction, the error signal (feedback) is treated to a bit of non-linearity along the way, which might be the cathode of a tube or the base of a transistor; a thing that does not happen elsewhere in the industry.

The exception to this is feedback around opamps. Why this technique has not been used in regular amps is a mystery to me, unless its been tradition and so created a blind spot for designers.