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Old 17th February 2016, 09:19 AM   #1
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Default Damping factor explained - or not?

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. In moving, the speaker generates EMF that has to be absorbed by the amp. 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.

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!

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. So the amp damping and speaker EMF generation is a continuous process and not just with impulsive signals.

Am I correct?

Jan
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Old 17th February 2016, 09:28 AM   #2
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Quote:
Originally Posted by jan.didden View Post
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. In moving, the speaker generates EMF that has to be absorbed by the amp. 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.
The standard model

Quote:
Originally Posted by jan.didden View Post
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!
Of course it does. It is also a microphone with a pretty impressive current delivery capability. Stereo set up... you have in effect two 'microphones' feeding back uncorrelated energy into the amp output nodes.

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Originally Posted by jan.didden View Post
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. So the amp damping and speaker EMF generation is a continuous process and not just with impulsive signals.

Am I correct?

Jan
I think you are correct

Now ask the question about how we test amplifiers into 8 ohm resistive loads and leading on from the results of that, why amplifiers all seem to sound 'different' in subtle (and sometimes not so subtle) ways.

Conclusion. 8 ohm resistive loads don't answer back
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Old 17th February 2016, 10:02 AM   #3
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I find it easier to view DF as an impedance in series with the speaker. But that doesn't help explain to the layman...
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Old 17th February 2016, 10:12 AM   #4
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OK, I get all that. Then how about this: the amp sends a current trough the voicecoil and that gets the speaker moving. As a result of that, the moving voice coil generates current as it moves through the mag field. This current is opposite to the driving current, or not?

If it is opposite, the amp 'sees' a higher impedance (lower net current drawn) than with a nominal resistance. If the generated current has the same direction, the amp sees a lower impedance (more current) than in the case of a resistive load.

Or is it the other way around?

Jan
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Old 17th February 2016, 10:24 AM   #5
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But if you apply an impulse, the impulse pushes the cone out, but as it returns to zero does it pull the cone back, or is the restoring force supplied by the spider? It's a complex electro-mechanical system that behaves differently over frequency so I fear too hard to explain simply!
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Old 17th February 2016, 10:28 AM   #6
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Originally Posted by billshurv View Post
But if you apply an impulse, the impulse pushes the cone out, but as it returns to zero does it pull the cone back, or is the restoring force supplied by the spider? It's a complex electro-mechanical system that behaves differently over frequency so I fear too hard to explain simply!
I think it is partly spider, and partly the amp 'pulling' it back.
What I would like to get clear is the basics without worrying about the spider and such, without worrying about frequency dependence etc.

Assume a very slow varying DC ;-)

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Old 17th February 2016, 10:35 AM   #7
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hmm Well stepping back to F=BIL, with no spider and surround other than to stop the voice coil firing across the room application of F will move the cone, but removal of F will not pull it back. You need F in the other direction. Either mechanical, or from negative current.
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Old 17th February 2016, 10:38 AM   #8
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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

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Old 17th February 2016, 10:55 AM   #9
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Originally Posted by wintermute View Post
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.
I think thats why some prefer to use low global feedback.


How I see it:
the current from the amp moves the coil
the moving coil generates a back-emf voltage (with opposite direction), corresponding to the speed with which the coil moves through the magnetic field
soundwaves (reflections, other speaker,...) generate a voltage in the coil, as it acts like a microphone
the mechanical system (spring-damper) adds is contributions to all this
the output-resistance of the amp (in series with the the crossover parts) dampens the current generated in the speaker coil
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Old 17th February 2016, 11:45 AM   #10
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newbie alert !
Can we simulate and exaggerate this back EMF artificially, so we can measure and audition its effect.
Regards.
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