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Old 22nd January 2007, 09:37 AM   #1
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Default Motional Feedback Subwoofer

Many moons ago I had a go at building the motional feedback subwoofer designed by Russel Breden in electronics world magazine, feb 97. It didn't work
I was told that this may be because the circuit diagram in the article is wrong, and not being an electronics wizz I have no idea how to make it work.

Does anyone have plans for this subwoofer which they know work? I've bought the expensive DVC woofers and it would be a shame to waste them!
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Old 22nd January 2007, 02:37 PM   #2
forr is offline forr  France
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This was a very interesting project. As far as I know, there was no correction or update. Have you any more details about what could be wrong in, the schematics ? Does youy speaker work open loop ?
In AudioXpress, Dan Ferguson published a project with a similar idea without knowing previous Breden's work and using a Dayton dual voice coil driver. This may help.
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Old 22nd January 2007, 03:39 PM   #3
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Elektor had long time ago a motional feedback project.

Send me message if you want the article.
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Old 23rd January 2007, 02:29 PM   #4
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Check this post from me: http://www.diyaudio.com/forums/showt...378#post987378

It's a working schematic for feedback with a microphone.

For use with a secondary voicecoil you should leave R11 and R12 out.
Connect the secondary to the two outer legs of a 10 potentiometer, connect the middle leg at the point where R11 and R12 are connected to C2.

Now one of the outer legs should be connected to ground so that the signal is in phase with the input signal.

Now you have to tune it, turn R4 all the way to the left or the right so that pins 1 and 2 of R4 are basically connected together.

Turn the circuit on and play a signal through the woofer.
Now turn the potentiometer connected to the secondary voicecoil so that the signal at the middel leg has about the same amplitude as the input signal.

Now you can start turning R4 until you start to get oscillation, then turn it back so the oscillation stops, now it ready for use.
If you have a very powerfull amplifier, connect a 60 watt lightbulb in series to protect the woofer when setting the feedback level.

This should work, assuming the signal from the secondary voicecoil represents the cone's acceleration.

I you don't understand something about it, just ask, or you might be building a one time smoke-machine.

Pherhaps you could send me the electronics world article?
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Old 23rd January 2007, 03:04 PM   #5
Puggie is offline Puggie  United Kingdom
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someone on here did a motion feedback system using a conductive foil inside the VC former and a metal ring ontop of the polepiece to make a basic air cored capacitor that changed value as the VC former moved, anyone remember and have a link to the project? I've looked for it a couple of times but not managed to find it.
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Old 23rd January 2007, 03:27 PM   #6
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http://www.servospeaker.com/index.html

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Old 23rd January 2007, 03:40 PM   #7
Puggie is offline Puggie  United Kingdom
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Thanks (you don't want to know how many searches I did without finding that)
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Old 23rd January 2007, 11:07 PM   #8
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If you look at the schematic in the pdf, you can see they used a 4th order lowpass in the feedback path, that's not a very good thing to do imho, because the phase shift of such a filter is more than 90 degrees.
A first order lowpass should be used so that a higher level of feedback can be achieved.
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Old 24th January 2007, 04:54 PM   #9
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Quote:
Originally posted by forr
This was a very interesting project. As far as I know, there was no correction or update. Have you any more details about what could be wrong in, the schematics ? Does youy speaker work open loop ?
In AudioXpress, Dan Ferguson published a project with a similar idea without knowing previous Breden's work and using a Dayton dual voice coil driver. This may help.

Which Issue of Audioxpress?
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Old 24th January 2007, 05:55 PM   #10
johnnyx is offline johnnyx  United Kingdom
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Default Acoustic1

If you want to use your DVC woofers, have a look at this thread for ideas.
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