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Old 2nd May 2010, 12:15 AM   #31
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I've been reading a lot of the literature past few days. For every possible permutation of positive, zero, and negative output impedance and in all degrees, there is some learned person who advocates it is THE mathematically necessary solution.The only thing I have found across all the papers I've looked at is that nobody seems to care about copper temperature coefficient. Most don't care about coil inductive values.

At some point, infinite introspection, as in this thread, will become self-parodying, like the bird that chases its tail until....

Time to get out the soldering iron and experiment. The performance is MF is too challenging to parse in a dark room with your computer instead of a little bit of testing.
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Old 2nd May 2010, 01:49 AM   #32
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Quote:
Originally Posted by steph_tsf View Post
hello,

however very interesting to read, this post is slightly drifting out of subject. Aren't we supposed to talk about Servo-Sound here ? This is a system NOT involving a dedicated sensor. This is a system aiming at recovering the motional voltage (back EMF if you want to call it so), sensing the currents and sensing the voltages in different locations of the circuit.

Don't you think that the unity damping concept can deliver something usefull ? Not directly, but indirectly, for being able to extract (or synthesize should I write) the back EMF with great precision, even in the presence of factory dispersion, thermal drift and aging effects ? See post #6 and post #12.

And, can somebody help me determine what's inside the Servo-Sound "black box" ?

Cheers,
Steph
As I recall Practical Electronics had an active speaker box design based on this idea a number of years ago.

Motional feed back amplifier

There rational was that above the box resonant frequency the mass of the cone dominates the motional impedance and thus the current in the voice coil is a representation of the force on the cone. Because force equals mass x acceleration the force and hence the current in the voice coil is analogous to the cone acceleration.

They sense the current through the voice coil and feed it back as a voltage developed across a sensing resistor. But this is no different than negative current feedback used to increase the amplifier output impedance.

The main advantage of this scheme is driving a speaker with constant current instead of constant voltage and avoiding non linearities with voice coil inductance which could be fairly bad for a small speaker.

The main disadvantage would be raising the Q of the system response at resonance and thus causing a large peak in the response at the box resonant frequency. As well this type of feedback does nothing to flatten the response below the resonant frequency.

regards
Trevor

Last edited by Trevor White; 2nd May 2010 at 02:11 AM.
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Old 2nd May 2010, 09:05 AM   #33
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I have just come full circle having followed Forr's link from the "forgotten techniques" thread on the Audax forum.

I surfed the Audax forum and found Bruno & Jean-Claude (aka JCB) who have created a Sub-woofer based on the JBL W15GTI. JCB to the British is an “earth mover” (mechanical excavator) and that sub must be the same!

The sub uses some ideas on negative impedance output amplifiers which I used on my Consort design. Ian Hegglun pointed out in an article in Wireless World that it was a form of motional feedback. Jean Claude has spent 40 years developing the idea having been impressed by the Servo-sounds

My blog:
Consort3′s Blog
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Old 2nd May 2010, 04:17 PM   #34
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Trevor's Practical Electronic circuit shows a very interesting feature: if the amp output phase is right, ANYBODY can experiment with MF with ANY amp by just adding a resistor to their speaker "reference" output terminal and maybe fuss a tiny bit inside with the pre-existing neg. feedback. Roughly what I've been pointing out about the Kenwood Basic M1 "Sigma Drive" amps.

This concept has been showing uyp in commercial hifi gear forever - or at least as far back an obscure but truly beautiful purple Fisher mono tube amp that had variable MF in late '50s, I'd guess.

But to help Trevor a bit, I find it easiest to think in terms of speaker impedance. If the impedance of the speaker rises due to back-EMF (for example, at resonance or for other reasons correlated with cone motion) it "grabs" less power from the amp when the driver impedance is being mirrored by internal impedance of the amp.

That controls resonance and lets you feed the speaker whatever power is needed to keep it on an even keel despite the speaker system's wishes... which is exactly what you want.
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Old 2nd May 2010, 04:27 PM   #35
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Quote:
The main disadvantage would be raising the Q of the system response at resonance and thus causing a large peak in the response at the box resonant frequency.
Maybe this is an amplifier for dipoles with low Qts drivers?
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Old 2nd May 2010, 04:29 PM   #36
latala is offline latala  United Kingdom
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I can remember coming across something like this resistor monitoring spk current in an 70-80,s H+H amplifier circuit
I think it was 1 of their 100 watt pa models
I intended to experiment but did not get around to it
regards Trev
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Old 3rd May 2010, 04:32 AM   #37
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Quote:
Originally Posted by bentoronto View Post
Trevor's Practical Electronic circuit shows a very interesting feature: if the amp output phase is right, ANYBODY can experiment with MF with ANY amp by just adding a resistor to their speaker "reference" output terminal and maybe fuss a tiny bit inside with the pre-existing neg. feedback. Roughly what I've been pointing out about the Kenwood Basic M1 "Sigma Drive" amps.

This concept has been showing uyp in commercial hifi gear forever - or at least as far back an obscure but truly beautiful purple Fisher mono tube amp that had variable MF in late '50s, I'd guess.

But to help Trevor a bit, I find it easiest to think in terms of speaker impedance. If the impedance of the speaker rises due to back-EMF (for example, at resonance or for other reasons correlated with cone motion) it "grabs" less power from the amp when the driver impedance is being mirrored by internal impedance of the amp.

That controls resonance and lets you feed the speaker whatever power is needed to keep it on an even keel despite the speaker system's wishes... which is exactly what you want.
Yes you can do all of this to flatten frequency response but it doesn't compensate for BL and inductance non linearities. Only a true mfb system with a sensor to detect cone acceleration or velocity can reduce distortion in a loudspeaker.

regards
Trev
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Old 3rd May 2010, 11:08 AM   #38
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Sony sub with 12" driver, 180 watt amp and motional feedback
at 130 $...

Sony SA-W3000 12" 180W Amplified Subwoofer (SA W3000) at Vanns.com
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Old 3rd May 2010, 04:19 PM   #39
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Attached you will find the continuation of post #30 about modelling a loudspeaker in fee air, using LTspiceIV.

See the attached pictures, and enjoy !

Steph
Attached Images
File Type: jpg Loudspeaker in free air.jpg (169.0 KB, 255 views)
File Type: jpg Loudspeaker in free air (Bode plot).jpg (80.2 KB, 237 views)
Attached Files
File Type: zip Loudspeaker in free air.zip (158.6 KB, 49 views)

Last edited by steph_tsf; 3rd May 2010 at 04:23 PM.
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Old 3rd May 2010, 07:41 PM   #40
Elvee is offline Elvee  Belgium
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Congratulations steph, this looks excellent. And useful too.
You render a great service to all the community.
Thanks!

Ps:

You should also post this material on the Yahoo LTspice group, I'm sure Helmut would be pleased.

Last edited by Elvee; 3rd May 2010 at 07:46 PM.
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