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2 Coil Speaker Confusion

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currentkills91

Wed Jan 30 2013, 06:33AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
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The video pretty much explains it..

Josh Campbell

Wed Jan 30 2013, 05:45PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

Maybe it's a push pull setup with the top coil 180 degrees out of phase with the bottom one. Or the top coil could be going through the high pass cap and acting as a tweeter using the rigid grill as it's medium.

Patric

Wed Jan 30 2013, 09:49PM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

Long time ago Philips had a feed-back system with a second coil on the speaker: motional feedback...

testtest

Wed Jan 30 2013, 10:58PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

Would be nice to have the schematic diagram of the crossover network. If I "read" the video correctly the extra coil is connected to it, not to outside electronics. So it could use the domed dust cap as a tweeter or it could passband and control some resonnances. The coil does seem substantial with many turns in a strong magnet. Considering the second magnet is fixed to a mostly flat surface there would be nasty effects at the lower frequencies from the force that would be exerted.

Over the years I have seen several speakers with an extra voicecoil. They were mostly co-wounded on the same former in the primary gap however. Some were pick up coils for either motion feedback or introduced in the amplifier feedback loop to react to box resonnances or extend low frequency response. Mostly the extra coil was just a sensing coil of a few turns. One unit from the late 50's had identical dual coils to accept both sides of a stereo and act as a central bass unit. It was a 12" job and well suited to the LPs of the era that did not have large variation in the bass signal from each channel. They sounded horrible with later material or would fry if wired out of phase as the two windings were fighting each others!!

currentkills91

Thu Jan 31 2013, 05:35AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
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Okay so the motional feedback idea makes sense, and I was kinda thinking that's what it was for. My amp had a motional feedback system in it, but I didn't pay any attention to the design of the amp when I stripped it down..

EDIT:
Here is a schematic I quickly drew, its a bit harsh, but easy enough to follow I think. Anybody know of any motional feedback circuits that use tubes I can look over, and get an idea of how it worked?

currentkills91

Thu Jan 31 2013, 06:07AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
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Okay sorry for the double post.. I don't know how to add a photo to the reply above..

So here is the schematic, wwhere it says "2X" it means there are 2 in parallel:

1359612433 3831 FT149985 Crossover Schematic

Dr. Slack

Thu Jan 31 2013, 08:42AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

currentkills91 wrote ...

I don't know how to add a photo to the reply above..

Post the photo in the attachments thread. Close and edit that post and copy the link. Edit your post above and paste the link.

testtest

Thu Jan 31 2013, 06:29PM

Registered Member #3271 Joined: Mon Oct 04 2010, 02:29AM
Location: Canada
Posts: 159

currentkills91 wrote ...

Okay sorry for the double post.. I don't know how to add a photo to the reply above..

So here is the schematic, wwhere it says "2X" it means there are 2 in parallel:

Thanks for the schematic. I think you a right about its role.
I was under the (false) impression that the second coil was also somehow wired in the cross-over circuit too, not an external amplifier.
The crossover network looks pretty standard for a three way speaker. The signal generated by the second coil should then be the first derivative of the position profile (velocity and direction) and used for either motion feedback or passband for box/speaker resonnance by affecting the amplifier response.
Since the second coil is acting as a sensor I would assume there is little force appiled to it by currents and it is just threre for the "ride". Most implementations I have seen used a coil that was wound on the same former as the true speaker driving coil.

I'll have a look for a vacuum tube motion feedback amp.
Edit; I thought I had some old vacuum tube schematics with motion control but all I found was one where the usual feedback from the last stage was using the secondary coil of the speaker for frequency response levelling. Not motional feedback at all. There are several motional feedback in the "Panning for Loudspeaker gold" anthology from the Audio Engineering Society though.

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