Motional Feedback with 2nd voicecoil - diyAudio
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Old 24th September 2006, 10:23 AM   #1
durxter is offline durxter  Netherlands
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Default Motional Feedback with 2nd voicecoil

I want to build a motional feedback subwoofer with one or two Focal 7inch drivers with double voicecoil. I have been searching around on this forum, but could not find a circuit.

Does anyone have a copy of the article from the February 1997 issue of Electronics World (pages 104-109). The article was entitled 'Roaring Subwoofer' and the author was Russel Breden.

I know it is not the best way to make a feedback system, but I want to keep things simple. The sub will be filtered at 80-100Hz.

Is the circuit from the dutch Elektor article http://www.xs4all.nl/~loosen/elektronica/mfb/ usefull for voicecoil pickup?

D Siesling

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Old 24th September 2006, 07:42 PM   #2
Svante is offline Svante  Sweden
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Well, even if you want to keep things simple, you would want some improvement by using the technique, right? I think you'd be much better off using the second coil as it was intended, ie use it for driving the cone and skip the feedback.

At first it might seem as if the signal from the coil would be proportional to the cone velocity, and that it therefore would be a good signal to use for feedback. However, there are at least two problems with the approach. First, the two coils act as a transformer, and therefore there will be leakage from the driving electric signal to the sensing coil. You could probably balance this signal path away, but that is hardly simple. Second, what you want is an accelerometer, not a velocity meter for the feedback, since radiated sound is proportional to cone acceleration (not velocity). Yes you could differentiate the signal, but again, that is hardly simple.

So, my suggestion is, if you desperately want to build a system with feedback, and if you want to keep it simple (but still get an improvement), do it as it is done in the article you linked to, using an accelerometer.

...and no, that circuit will not work with coil feedback without modifications.

Sorry to sound pessimistic, but I think that this is what you will find.
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Old 25th September 2006, 01:59 PM   #3
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Hey that's my site :P

I think you should place a microphone close to the woofer and measure with the scope what signal you get from the microphone and from the second voice coil to see if it's about the same.
Also try tapping to cone to see what signal you get and see when sending a signal to the woofer and pushing against the cone if the signal decreases.

I think using a second voice coil has its benefits like better phase response at high frequencies due to the voicecoil working like a transformer and not picking up cone resonances.
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