Microphonics, Feedback and Bob Carver

Thanks so much, @pdspiegel !!

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, i wonder why bob carver

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

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.

Some people can sit on a small rise above a serene valley on a beautiful spring day and just marvel at the wondrous view. Others never get to really enjoy the view because they get too wrapped up in trying to figure out why it's so wondrous. As Bonnie Raitt sang, "Life gets mighty precious when there's less of it to waste." In my golden years, I much prefer to sit back and enjoy the view, or in this case, the sound of my system, for what it is. I'm happy to let all the young inquisitive minds take my place wondering why it is. 

@erik_squires you said recently "welcome to AG where you get what you pay for and a whole lot less".  I thought that was brilliant (comeback) but I'd say this is far from worth-less.  And like the prior,  I'm barely hanging on past, the speaker acts as a microphone!   Carry on. 

@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. :)

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.

@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. :)

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.

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.

@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.

@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

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.

@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 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!