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
11,765 posts
 

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.

cey's avatar
cey
260 posts
 

@erik_squires, i wonder why bob carver

put 15W edcor output transformers in an amp he claims does 80WPC. 

pdspiegel
19 posts
 

I recall a series of articles in AudioXpress on a topic similar to what the original poster raised referred to as “Back EMF.”  Here’s a link to the articles.

After analysis with a lot of math the author determines that Back EMF is not an issue worth worrying much about.  Interesting thought experiment though.  Any unwanted cone movement generates a signal that an amplifier using feedback tries to compensate for.

erik_squires OP
16,020 posts
 

Thanks so much, @pdspiegel !!

 

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