Damping factor explained - or not?

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I would assume that the back EMF is also fed back through the feed back loop, does this create a correction for an error that wasn't actually there in the first place?

I know nothing about this stuff BTW just adding some fuel to the fire ;)

Tony.

My reaction is that a current fed back into the amplifier node does infiltrate the nfb loop. A speaker is generating all kinds of uncorrelated signals in response to the audio stimulus around it.

No. The effect of a voltage applied to the amp output is fully described by the amp's output impedance.

Lets take an amp we all know, the classic 'blameless'. I've added a 500Hz sinusoidal current source of just 10ma feeding back into the output node. That 500Hz is clearly present throughout the whole amplifier. What is more, that 10ma current source is in series with a 10 ohm resistor. You would expect the damping factor of the amp to 'sink' the unwanted signal to a minuscule residual but apparently it does not.

This shows the FFT as measured at base of the VAS stage. The 500Hz unwanted signal is clearly present 'within' the loop.

FFT.JPG
 
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My reaction is that a current fed back into the amplifier node does infiltrate the nfb loop. A speaker is generating all kinds of uncorrelated signals in response to the audio stimulus around it.

It doesn't really matter whether it is a feedback amp or not. It is the output impedance that is the determining factor - not how you 'make' it. I am not interested in what the feedback does or does not and certainly not whether anyone believes whether it is audible or not - we'vs got enough of those senseless posts, thank you.

What I am trying to find out - see the 1st post - is how this EMF works.

Thanks,

Jan
 
Mooly said:
The 500Hz unwanted signal is clearly present 'within' the loop.
Yes. This is how feedback works. What did you expect to see?

You would expect the damping factor of the amp to 'sink' the unwanted signal to a minuscule residual but apparently it does not.
I would not expect that. What is the output impedance? I would expect to see 10mA x Zout at the speaker terminal, less at the feedback summing point and then whatever is needed within the loop to command the output stage to sink 10mA.
 
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This shows the FFT as measured at base of the VAS stage. The 500Hz unwanted signal is clearly present 'within' the loop.

View attachment 531797

Of course it is present within the loop. That is so that the feedback can do it's job, which is just to generate a compensating output signal. The important question is whether it is present at the output.

Jan
 
jan.didden said:
What I am trying to find out - see the 1st post - is how this EMF works.
I am not sure what you are asking.

There are two ways to consider what happens in the amp-speaker interface. One is via impedance, although this does assume that the system is approximately linear. The other is via back emf. Note that back emf is not something to be added to the impedance model by the more 'sophisticated' audiophiles; it is there in the model so trying to add it on is a sign of confusion. As the system is approximately linear (if not we would not use it to listen to music) it is best to use the simpler model.

Just to clarify: you can use impedance measured in the usual way, or 'impedance with the speaker voice coils fixed, plus back emf'. The former is both theoretically and practically easier, so that is why it is used. Don't be tempted to consider 'normal impedance, plus back emf' as that involves double-counting of back emf and is therefore wrong.
 
True, and often forgotten, although fortunately for many audio amps it has little practical effect.

As my uncle Bud used to say when explaining speakers and back-EMF to his idiot nephew (me), “Well, son, its all pretty complicated. The amplifier is supposed to drive the speaker with an exactly-amplified version of whatever signal is coming from the needle on the record. Real amplifiers aren't exact however. And speakers are positively terrible Rube-Goldberg devices that can drive any competent physicist crazy. But when they're not doing whatever they want, they mostly do what the amplifier proposes they do: accelerate their cones outward when they receive positive signal, and dragging the cones back in when they see negative signal. Between those two, is nothing but conjecture.”

A rather pithy summary of the whole Audiophile World, wouldn't you say?

LOL
GoatGuy
 
I know - the proverbial horse beaten to death. And thought I knew it all.
Yet, when trying to explain it in simple terms I found I didn't know enough detail to do that ;)

Most explanations come up with an example of a kick-drum pulse that all of a sudden stops, and the speaker which continues to move, unless 'braked' by the low internal impedance of the driving amp.
Yes, it will continue to move somewhat, purely by inertia (after the driving pulse has stopped) , so the acoustic waveform produced by it and the electrical waveform coming from a miked up kick drum no longer match, that "blurs" , quite literally, the reproduction of a sound.

In moving, the speaker generates EMF that has to be absorbed by the amp.
True.
The speaker is a generator and will generate a voltage as long as it is moving.
That voltage will produce a curret, depending on load applied, absorbing mechanical energy from the moving speaker and braking it.

The lower the output impedance of the amp, the better it is at braking the speaker (plus a few other issues but this is the gist of it). So far so good.
Of course, a short is the best brake, because it absorbs the most energy.
That said, not even a short will stop the speaker instantly, it will take some time.

But then I thought: the speaker generates EMF because the coil moves in a magnetic field. So I would expect the speaker to generate EMF continuously and not just when the kick-drum stops!
It will generate EMF as long as it's moving, I mentioned inertia above.
Once it stops, no more EMF.

That leads to a realization (I think) that when you fix the speaker moving part and stop it from moving, the amp sees a different impedance then in normal operation.
Yes.
In that case you'll see DCR (coil wire resistance) in series with voice coil inductance (which is noticeable above, say, 400 to 1000Hz, depending on speaker); you will lose the large impedance peak you find at resonance .
You will be "shorting" the *mechanical* "inductance", which comes from the *mass* of the cone (and attached moving parts) , while the mechanical "capacitance" comes from any springiness associatted with it: spider + cone edge + air springiness, if mounted in a cabinet (which is the normal use).

So the amp damping and speaker EMF generation is a continuous process and not just with impulsive signals.
EMF generation is continuous as long as it is moving.
*Why* is it moving is irrelevant.
Applied waveform is irrelevant.
Whether it's applied at the moment or stopped a millisecond ago is irrelevant (as long as it continues moving).

Damping/braking depends on the internal motor power/strength, a big voice coil and magnet will brake more than smaller ones, ad that is noticed in speaker Q ; expensive high quality big magnet, tight gap speakers have low Q (say, around 0.3 or 0.4) while cheap cheesy speakers have Q of 1 to 2 (in real horrible ones).

Guitar speakers all have high Q , on purpose, it's part of the flavour.
 
→ DF96 re: “strictly speaking”… When I read the “short is the best brake”, I too was struck by the fact that in any (electro-)mechanical system, braking must dissipate kinetic energy as heat, either by direct friction (e.g. pads on rotor in car) or the electrical world's version of friction, power dissipation across a resistance (or the springy versions - power entrained in capacitors or inductors!)

Good catch.
GoatGuy
 
Damping factor is a commercial very annoying concept.

The amp ouptut impedance gives a much better overall picture and mainly concerns the driver resonance damping at low frequencies and the voice coil impedance increase at high frequencies.

Back EMF is just a fancy name for saying something is an impedance rather than a bog standard resistance. And also 'back' is a misnomer as it can be negative or positive so if negative is it then 'forward'? :)

Sometimes ago, it has been suggested to me to look at the whole amp-driver as a circuit having the existing easily identified impedances, and two voltage sources, one coming out from the amp and the other one coming out from the driver.

The motional impedance is not a real life impedance, it reflects, in simulation, the voltage due to the moving voice coil in the gap.

Of course, a short is the best brake, because it absorbs the most energy.

This only applies around the resonance, not elsewhere.
A negative resistance amp output does it better.
For a closed driver, the braking action is reflected in the value of its Qtc.
 
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Hi Jan ;)

..........What I am trying to find out - see the 1st post - is how this EMF works.

Thanks,

Jan

You put pen to paper in the first post and said...

But then I thought: the speaker generates EMF because the coil moves in a magnetic field. So I would expect the speaker to generate EMF continuously and not just when the kick-drum stops!

So it was a reasonable assumption (well I thought so :)) to think that you were meaning that the speaker continually produced some continuous emf in response to all the continuous stimuli around it.

Yes. This is how feedback works. What did you expect to see?

Mooly said...You would expect the damping factor of the amp to 'sink' the unwanted signal to a minuscule residual but apparently it does not.

I would not expect that. What is the output impedance? I would expect to see 10mA x Zout at the speaker terminal, less at the feedback summing point and then whatever is needed within the loop to command the output stage to sink 10mA.

Honestly... well I expected to see a fair bit lower than it apparently was... OK, that's my fault for shooting from the hip and not thinking the numbers through.


Of course it is present within the loop. That is so that the feedback can do it's job, which is just to generate a compensating output signal. The important question is whether it is present at the output.

Jan

It is present in the output.

In fact if the simulation is in the right ballpark, then the residual of an 11kHz 10 ma injected current is now the (dominant) unwanted feature in the FFT (main wanted signal referenced at 2.83 volts @ 1kHz 8 ohm)

I am not sure what you are asking....

:) Maybe I'm not either.
 
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Strictly speaking a short cannot absorb any energy. However, it allows the resistance in the circuit (mainly voice coil, plus cables and crossover) to absorb the energy by maximising the current.

I think an ideal short will cause the energy to be absorbed in the other parts of the circuit like the voice coil. It is not required that the amp absorbs the energy.

Jan
 
When I am attempting to discharge an inductance of energy, I have a particular problem..that of where the energy goes.

If I put a short across it, all the energy will be absorbed by the inductor (more specifically, the resistance within the inductor.)
If I use a resistor to discharge the inductor, there will be a terminal voltage, and the resistor will dissipate based on that. There is a tradeoff between dissipation within the inductor and dissipation outside the inductor in the resistor. That tradeoff includes the speed at which the energy will be removed. High resistance dissipates faster..but that is for step functions...edit: just ask CERN about high resistance and dissipation....I have pics of the damage..) :eek::eek:

As to damping factor of an amplifier that is driving a reactive load, the question is not a simple one. Yes, the standard definition is simple, but reality is far more complex.

If you setup a scope to look at the voltage as y axis (up is positive voltage), and current as x axis (right is positive current), you examine the output space of the amp. Quadrant 1 is upper right, 2 is upper left, 3 is lower left, 4 is lower right.

A pure resistance will produce a line, traversing Q1 and Q3. A pure reactance will produce an ellipse which traverses all quadrants.

Operation in Q2 and Q4 is very important in that it is the area where the opposing output devices are being forced to sink current despite the output voltage being closer to the opposite rail. For example, in Q2, the voltage is positive but the negative pass devices are being required to support current..Q4 is the reverse condition. Both Q2 and 4 are operation into a reactive load.

It gets even more nefarious when one considers two frequencies, where the woofer is creating a huge operational ellipse (in the VI space), and a second higher frequency is demanding additional power riding on that elliptical VI line.

When I am asked about damping factor, the first thing that pops into my mind is..what quadrant are we talking about? All? 1 and 3? 2 and 4?


When I consider amplifier design and think of the power rails and their effect on feedback ground loop coupling, I have to ask... When the amp is mushing back and forth between two quadrants where the currents actually switch between supply rails even when the output voltage has not switched polarity, what is the response of the circuit?

As for kick drums, that's why I like horns. Not reflex, not any speaker that requires the buildup of stored energy for it's full spl. Nuttin but horns for me.

John
 
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btw, I see mention of the vc inductance as part of the reactance, but nobody's mentioned the resonance of the cabinet. To me, that's a big part of the picture, as the speaker is an electromechanical device. Relying on the spider to return the VC only works for DC (in essence), damping factor must really consider the air pressure and energy storage there as well..

John
 
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