Thanks so much, @pdspiegel !!
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?
Showing 6 responses by erik_squires
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. :) |
@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. :) |
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 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. |
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. |