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http://members.fortunecity.com/pirimoglu/FaradayRingsVoiceCoilImpedance.zip
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A little bit of history. Up through the 70s most woofer magnets were made of Alnico. In the mid 70s the Cobalt mines of Zaire went through unrest and stoped producing. Everybody scrambled to find a replacement and Ferrite motor structures became popular.
The only problem was that midrange distortion was considerably higher. JBL was one of the first to address the problem and then later McIntosh, both with similar patented magnet structures.
A flux modulation ring creates a "shorted turn" that resists the flux modulation. Basically the signal in the voice coil drives the magnet structure and pushes the operating point of the magnet around. Since the magnet operating point is on a nonlinear curve then this leads to 2nd harmonic midrange distortion. Alnico was fairly immune to this but ferrite is not. The flux rings on ferrite structures got the distortion performance as good as it had been on Alnico structures (and without the easy demagnetization that Alnico suffered from).
This is a seperate phenomonon from the inductance effects that copper pole caps or copper and silver plated core poles address. I'm not sure which of these effects the Faraday term applies to?
David S.
The only problem was that midrange distortion was considerably higher. JBL was one of the first to address the problem and then later McIntosh, both with similar patented magnet structures.
A flux modulation ring creates a "shorted turn" that resists the flux modulation. Basically the signal in the voice coil drives the magnet structure and pushes the operating point of the magnet around. Since the magnet operating point is on a nonlinear curve then this leads to 2nd harmonic midrange distortion. Alnico was fairly immune to this but ferrite is not. The flux rings on ferrite structures got the distortion performance as good as it had been on Alnico structures (and without the easy demagnetization that Alnico suffered from).
This is a seperate phenomonon from the inductance effects that copper pole caps or copper and silver plated core poles address. I'm not sure which of these effects the Faraday term applies to?
David S.
And even before this, a few premium drivers used copper and even silver shorting rings in them. The vaunted JBL LE8T (I have a pair, and yes, they're THAT good) was one of them, a big underhung alnico motor with a silver shorting ring, presumably in the middle of the gap. Some of the phillips fullrangers in the 70s used full sleeves.
Yes, If I remember the LE8T was a silver plated core pole that kept coil indutance really low and contributed to the wide bandwidth.
Note that rings are usually mounted above or below the gap. A ring of any thickness in the gap will knock down gap strength considerably, since it isn't a ferrous material it would be like opening up the gap. Thin copper caps have been used along with plated poles. SEAS (I think) had a version with an extended pole with a ring above and another below for symmetry.
These are all inductance reducing measures. The flux rings seem to work well at the bottom of the pole against the backplate.
David S.
Note that rings are usually mounted above or below the gap. A ring of any thickness in the gap will knock down gap strength considerably, since it isn't a ferrous material it would be like opening up the gap. Thin copper caps have been used along with plated poles. SEAS (I think) had a version with an extended pole with a ring above and another below for symmetry.
These are all inductance reducing measures. The flux rings seem to work well at the bottom of the pole against the backplate.
David S.
Here's a scanned copy of one of the old JBL whitepapers about that (another blast from the past):
They work especially well there partially because with the undercut pole (necessary to put the ring there) makes the inward stroke act more like the outward, since it eliminates the inductance effect of this pole section, just as the outward stroke approximates an aircore to some extent. Not that I'm telling you anything you woudn't know but useful for others perhaps. A T-shaped extended pole would probably improve the symmetry further in most cases.
You are right, the inductance varies between up and down and the T undercut helps to balance that out. This is also a benefit agains DC offset problems, where a big inductance increase for the downstroke would create solenoidal forces.
Important little details!
Wayne, thanks for the SFG diagram, my recollection wasn't too bad on the subject. SFG was just released when I went to JBL in 1980
David S.
While not the main thrust of a shorting ring, you can use a similar device to wick heat out of the motor core. Magnetic eddy currents really heat the center pole, often hot enough to boil water. Not a good thing for the voice coil to have local temperature this high. Pressing or threading a cooling plug into the center pole greatly reduces core temperatures.
Maybe the info about the Lambda motor and also the "Apollo upgrade" can shead some additional ligth on the subject?
AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.
AE Speakers --- Superb Quality, Unforgettable Performance, Definitely.
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A bit off topic, but I built a pair of minimonitors using a very small double-walled enclosure that was stuffed with fiberglass, and one day I had one of the enclosures apart and noticed that the fiberglass stuffing had melted where it was touching the back of the Dynaudio's 6.5" voice coil. Yikes! I had no idea that drivers could put out that much heat, and with such a small enclosure the heat from the voice coil had nowhere to go, and as a result it got a little toasty in there.
I thought that the drivers would be ruined from the heat, but they're still working just fine.
Pole saturation with B field is a somewhat mythical thing since it's only true when there is little or no current flowing through the voice coil, or at high frequencies when eddy currents are induced more efficiently in shorting rings and in steel motor parts.
Even in the strongest motors, it doesn't take very much low-frequency current in the voice coil to push the faces of the pole piece and top plate way out of B-field saturation.
And of course, since shorting rings' effectiveness is directly proportional to the efficiency of their induction, they drop out of the picture more and more as the signal goes down in frequency. At 100Hz, they might as well not be there at all.
Here's another sim set up the same as the previous one, only at 100Hz instead of 1kHz.
Interesting plots. Can you explain the meaning of the reduced "field"? How do we interpret them?
David
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