Closed-Loop Amplification with Driver Cone Position Fedback - diyAudio
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Old 30th June 2013, 04:34 PM   #1
xrk971 is offline xrk971  United States
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Default Closed-Loop Amplification with Driver Cone Position Fedback

I am not sure if this is the right forum to post this in, but have built servo amps using the Burr Brown OPA512 op amp chip which can deliver 10 amps from a 100 V supply into 8 ohms.
This IC is perfect for an closed loop speaker experiment.

If one can build into a driver, a position feedback device, and then uses this to provide closed- loop feedback into the op amp for error correction. This should really improve response linearity and reduce distortion. Has anyone ever seen this done? I am not having any luck doing a search.

It would be interesting to put an optical position sensitive feedback device with a fiber optic coupled LED and photodiode to get absolute cone position and use that to actively correct drive current. With sufficiently high power motors (low Qts, high BL drivers) the driver can overcome any distortion imposed externally by having the amp swing a higher correction current.

Theoretically, the distortion can be made to go to zero within the amplifier/driver actuation band width.
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Old 1st July 2013, 03:27 AM   #2
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moved to planars/exotics.
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Old 1st July 2013, 08:21 AM   #3
Calvin is offline Calvin  Germany
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Hi,

wouldn´t it be better moved to subwoofers? Where basically all other discussion about Motional Feedback take place?

jauu
Calvin

ps. "Theoretically, the distortion can be made to go to zero within the amplifier/driver actuation band width."
No, not even theoretically will the distortion become zero. ;-)
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Old 1st July 2013, 09:21 AM   #4
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Yes and in fact I saw a thread just a short while ago --> Low distortion, DSP based high gain servo controlled woofer controller.

XRK971 would you like it moved to subwoofers instead?

Tony.
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Old 1st July 2013, 01:46 PM   #5
oshifis is offline oshifis  Hungary
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The problem is in the linearity of the position sensing device.
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Old 1st July 2013, 02:12 PM   #6
DUG is offline DUG  Canada
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Quote:
Originally Posted by oshifis View Post
The problem is in the linearity of the position sensing device.
Probably still better than just the woofer but it is something to address.

The response time of the positional sensor is also something to look at.

Slow response = power oscillator?

A few years ago I had a circuit for this and plotted the distance vs output.

I was trying for X of 1.5"

And found out that my dmm was giving me the voltage but when I put a scope on the output I found the cct was oscillating and the output was giving me more pulses vs distance. The poor dmm was just showing me what it could.

Got distracted with building something else and abandoned the project.

It is on the list...

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Old 1st July 2013, 03:13 PM   #7
SY is offline SY  United States
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Actually, you can make a superbly accurate linear positioning device. It's something I've thought about but never had time to work on, but it's used in some spectrometers with great success- use a laser with a beamsplitter, then recombine the beams. You get fringes which are perfectly periodic. You need a separate path to generate a quadrature signal so that you can tell if the cone is moving forward or backward.
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Old 1st July 2013, 03:32 PM   #8
DUG is offline DUG  Canada
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Originally Posted by SY View Post
Actually, you can make a superbly accurate linear positioning device. It's something I've thought about but never had time to work on, but it's used in some spectrometers with great success- use a laser with a beamsplitter, then recombine the beams. You get fringes which are perfectly periodic. You need a separate path to generate a quadrature signal so that you can tell if the cone is moving forward or backward.
Accurate but difficult to get a linear signal for full cone excursion.

Drop the frequency to about 1GHz and it can be done.

Direction I plan to go with.

It is on the list...

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Old 1st July 2013, 03:49 PM   #9
jcx is offline jcx  United States
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additional difficulty comes from the cone usually being operated over the range of bending modes, "breakup" - it is a distributed system so the exact location of the motion sensor limits the knowledge of the cone's whole response, sound output

the mechanical phase shift, modes limits the amount of feedback that can be applied:

Quote:
...one thing Lurie does really well is show that the “conservation” relation for the total amount of feedback - the “Bode Integral” is exactly such a practical "good theory" - and has been the underpinning fundamental argument behind my posts in this thread

http://trs-new.jpl.nasa.gov/dspace/b.../1/98-0905.pdf
...
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Old 1st July 2013, 04:57 PM   #10
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Scanning interferometry. Or for woofer how about just a microphone. Let the air do the averaging.

Last edited by Andrew Eckhardt; 1st July 2013 at 05:04 PM.
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