Microphonics, Feedback and Bob Carver

@atmasphere Thanks for taking the time to work through this with me.

Not semantics, but what it is we are looking at.

Negative feedback causes current to flow to maintain the output voltage at zero. It "rejects" the input by doing work. It's not a passive act, though the output voltage should remain fixed.

Related to your point, one way you can tell if an amplifier is on or not is to push against the woofer. With most amplifiers (with negative feedback) when you push against a cone you are met with resistance. When the amp goes off the speaker cone gets soft. (don't try this at home kids, and certainly not with anything other than your woofer).

In this sense the amplifier is _working_ to reject the motion.

It is that work which I am curious about.

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.

Since the amp makes the speaker hard to move, it inherently also reduces any audio signal picked up by the diaphragm.

Ah, but (apologies to Shakespeare) there's the rub.  In order to keep the diaphragm from moving, current must flow.  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.

@atmasphere To put this another way, the amp must produce an equal and opposite reaction to the incoming room noise (or fingers poking).  That means that the current flowing through the output stage must be an analogue of that pressure. The voltage is zero, but not the current. :)

Fascinating to see two technical experts to and fro on an issue even if I have no understanding of the technicalities that they are debating.