2. amps-preamps
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Microphonics, Feedback and Bob Carver

First, I’m going to present a theory I don’t have a lot of investment in, I’m just curious about.

Bob Carver and I, at different times, have wondered about the ability of an amplifier’s feedback circuit to be impacted by a speaker’s natural microphonic attributes. That is, in a room and unplugged to an amp a speaker and microphone are broadly similar. A diaphragm is exposed to sound which moves a voice coil which generates an AC signal in proportion to the acoustic event. Of course, a speaker is a terrible microphone, but it can be one.

The question I’ve had, and then read Bob Carver also asked (but he’s not known for always being right, nor am I 😁) is whether part of the problem with negative feedback in an amplifier is that it can pick up in room sounds (such as from the other speaker) and react to it in a negative fashion.

While this is testable via equipment I don’t have, not to mention time/energy to do it right, I’m wondering if anyone has ever read any more on this subject they’d care to share?

erik_squires
atmasphere's avatar
atmasphere
11,841 posts
 

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.

erik_squires OP
16,130 posts
 

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.

erik_squires OP
16,130 posts
 

@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. :)

mazian's avatar
mazian
406 posts
 

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.

erik_squires OP
16,130 posts
 

@mazian

While I appreciate your accolades, lets be honest. @atmasphere actually builds excellent and highly sought-after electronics for a living and if anyone should be called an expert it should be him.

By comparison, I’m just a dabbler in a thousand little things.

OTOH, ask me to compare how Dolby Surround functioned in motion picture auditoriums and how it differed from Dolby Digital and DTS and multi-track digital film and I probably have the edge. :)

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