I am no expert in the subject but would guess that the damping factor on an amplifier plays a part whenever external or internal vibrations reach a driver and excite it, wanting to send electrical impulses back into the amplifier, same as a microphone does. I would guess that the higher the damping factor of an amp the less affected by this problem it would be. I never really thought of this until reading your post although on a couple of occasions I did connect a couple of little speakers to the microphone input of a cassette deck and recorded my voice while experimenting as a teenager! I removed them from some old handhelds AM radios. This was before I could afford a real microphone!
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?
- ...
- 16 posts total
@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.
|
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. |
- 16 posts total