Voice coil *former* generates motional feedback voltage?

I posted a related thing to a subwoofer thread. This expands on it a little.

Been thinking a bit about MFB for woofers lately, and one of
the ways it can be done is via a separate coil next to the voice
coil that produces a voltage proportional to actual cone movement. Nothing new there, but it is a bit hard for most of us to experiment with if we don't have a suitably equipped speaker. Then I had an idea. The voice coil former on higher powered speakers is often a strip of aluminium shim wrapped into a tubular shape with the ends not quite meeting so they don't form a shorted turn. This is in fact a ready made 1 turn coil that if you were very careful you could attach a pair of flexible wires to and *voila* you now have a feedback signal from it's own dedicated coil.

Since our "coil" extends beyond each end of the airgap it would
always have a constant amount of flux passing through it no matter what it's position. That's just what we need for accurate feedback voltage. Has anyone out there ever tried this before. Is anyone bold enough to? I haven't yet because I don't have a suitable speaker at the moment. One final thought. To make it work even better I think it would be a good idea to have a copper shorting ring on the polepiece so that when the voice coil is nearly stationary it doesn't induce unwanted emf into the coil former simply by transformer action. The shorting ring would prevent this.

Hi circlotron

You seem to be an unlimited source for crazy ideas !! ;) :cool:

I think your idea might work in principle though I'm not sure whether the feedback "sensor" would be better than the "motor" in terms of linearity.

The German manufacturer Backes & Müller uses electro-magnetic acceleration sensors as well for their active speakers (ALL drivers within their speakers use MFB !!).
They are a fairly simple construction consisting of a small PCB carrying the coil. It is glued to the coil former just below the dustcap. Two rare-earth magnets are glued to the pole-piece.

You can see a (not very clear) photography of such an assembly under http://www.jessen-highend.de/sensor.htm

For a skilled DIYer this would as well be a feasible solution I think.


Hi mrfeedback. You can get special solder from Farnell for instance that enables you to solder onto aluminum. I have never tried it though. A copper voice coil former would be better thermally and easy to solder too, so that would be a good choice if you got a driver made up special.

Hi phase_accurate. Crazy ideas? Yeah I know. :D My family knows even better. :rolleyes: As for linearity, provided the former *always* extended beyond the area of the fringing field then it would always have the same *total* flux passing through it, linearity would be perfect. Just gotta find a suitable speaker now! Also gotta discipline myself to finish previous projects!!

ShiFtY said:
What about using a dual voice coil woofer similar to car ones? Some of those are pretty high quality and have insane power ratings...

Just an idea.
That's true. I had in fact thought about that but then you could not use both voice coils to drive it full tilt. :) Also the feedback coil not being as long as the former would be more subject to magnetic nonlinearities. The basic idea was to try and use it on existing speakers. Thanks for your interest ShiFtY.

This sounds like an interesting idea to try out. I was talking about motional feedback with a friend of mine the other day.

I'm intrigued as to exactly how you'll use the feedback voltage. Are you just going to use it in some kind of feedback correction circuit between an active crossover and the woofer amp, or is it something cleverer than that? I've always thought that the group delay would pose too much of a problem to enable this to work effectively, but I'd love to be proved wrong.

Good luck with it anyway, let us know the results......

I have tried this, 20 years ago. It does't work :bawling:

The VC former acts as a VC with all turns in parallel, and the turns that are not within the magnetic field exhibit very low impedance, quasi short-circuit, thus the motional voltage is very low. Furthermore, direct induction from the VC causes some trouble.

Regards, Pierre Lacombe.
"the thread of crazy ideas"[tm].. sweet.

Any other methods to monitor cone movement in woofers?

Crazy idea 101

1. What if you make a circuit to "look" at the cone
movement, ie laser and detector?

2. Radar like the ones the cops use - lol
aim at center of dutcap - heheh

What would grandpa from the Munsters TV show do?
He'd prolly attach a string and use pulleys - heheh
P.Lacombe is 100% right!

P.Lacombe said:

I have tried this, 20 years ago. It does't work :bawling:

The VC former acts as a VC with all turns in parallel, and the turns that are not within the magnetic field exhibit very low impedance, quasi short-circuit, thus the motional voltage is very low. Furthermore, direct induction from the VC causes some trouble.

Regards, Pierre Lacombe.

Doh!:xeye: Of course! The strip of coil former that is hanging outside the field still has voltage along it's length because it is in parallel with the section that does have emf induced in it. So the ouside-the-field part of the former just short-circuits the inside-the-field part.

Oh well, at least I was put right by someone who knows what they were talking about, not just by someone who didn't and just fluked the right answer for the wrong reason and therefore everyone listens to him from then on. Thanks P.Lacombe, you saved me a lot of messing around. :)

I have the Sept. 87 German edition of Elektor with the Piezo sensor. Could scan it if anybody wants it.

Actually, an electret microphone used sideways should be more linear, and it should also have negligible delay. So Charles, if you have any idea about the year of the Funkschau article, I could copy it on Friday when I visit the Deutsche Bibliothek.

The bit about the extra rare earth magnets attached to pole piece and coil to dust cap looks also interesting. Any idea where one could get a magnet assembly that is small and has a very linear field over say 20 mm?

Hi Eric and circlotron

Unfortunately I do have the EW&WW article BUT NOT a scanner. I will try to find somebody who can scan it for me, otherwise I might fax it as well.

I was going through the majority of Funkschau magazines I found (and I am sure that I own more of them than I found yesterday) but I couldn't find the article anymore:bawling:



I think the invention was patented even before any prototype was built. One thing I can remember is that the microphone in the Funkschau article was mounted like the one on the Manger subwoofer (i.e. sideways, glued to the joint between cone and dustcap, within the patent there were more versions mentioned). There was a photograph of a commercially available speaker. And it was indeed a B&M speaker. Finding the patent was not that easy since inventor and applicant were both Friedrich Mueller (without Mr. Backes). Now go and search for Patents by Mr Friedrich Mueller .......... :xeye:


Hi Circlotron

Are you still interested in the EW+WW article ?

I didn't have time to scan the article so far (I was on holiday down under for the whole length of November, and the days just before holiday are always very stressed).

I intended to take the whole magazine with me to send you a photocopy by mail, but then I forgot to pack the magazine unfortunately.

Are you still interested to get it ?



P.S. Melbourne is indeed a good place to be