Microphonics, Feedback and Bob Carver

@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.

@atmasphere

The more feedback you have, the less the speaker can affect the amp.

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.

Consider a situation with two amplifiers, their inputs shorted, while music is played in the room from a completely independent source. Maybe even a drum set in the room.

One amplifier has zero feedback. In this case the speaker’s microphonics will not have any effect on the current flowing in a feedback circuit, as it does not exist.  The voltage at the outputs may rise and fall but there's no current.

The other amplifier has some feedback. In this case the speaker’s micophonics will excite the feedback circuit. Now we have a situation where currents are flowing in the amplifier that have nothing to do with the input signal. In this case, the output voltages are forced to zero by the active response of the amplifier.

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?

@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.