Faraday ring in louspeaker driver, what is it?

[Manolo47]

I have a pair of Reference 3a MM Decapo loudspeakers and I have been reading about the latest modifications by Reference 3a and one includes the aluded item in the 8" woofer/midrange and was wondering what would that be. If someone could explain I would appreciate it. Thanks.

[pjp]

You're supposed to use the search function before asking a question.

Effect of a Faraday ring...

http://members.fortunecity.com/pirim...lImpedance.zip

[Manolo47]

I did the search but did not find an explanation per se. Anyway thank you for the link, but it does not seem to work though it is blank once clicked.

[tinitus]

1500AL

[badman]

The link works for me, and this has been discussed pretty extensively. Faraday rings work in a couple ways, reducing and often linearizing inductance with respect to excursion is one of the major functions.

[speaker dave]

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.

[badman]

Originally Posted by speaker dave

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.

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.

[speaker dave]

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.

[Wayne Parham]

Here's a scanned copy of one of the old JBL whitepapers about that (another blast from the past):

• Magnet Structures

[badman]

Originally Posted by speaker dave

These are all inductance reducing measures. The flux rings seem to work well at the bottom of the pole against the backplate.

David S.

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.