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View Full Version : Stray magnet pull and storage question



robertbartsch
11-06-2009, 08:10 AM
Most vintage and pro JBL drivers I have known tend not to have much stray magnetic pull. I assume this relates, in part, to specific design features that tend to minimize this including metal covers over the motors.

Anyway, I have a pair of contemporary (newer) JBL tweeters, compression driver Model 2408 (i think) where the stray magnet pull is very strong. These appear to have a lot of cheaper (??) stamped, not machined pieces.

Anyway, are there any special storage guidlines for these or is it OK to store two together - next to each other - since the motors pull on each to the other is quite strong?

I think I read somewhere that back-to-back storage is bad but side-by-side storage is not too bad but not ideal either.

Is this correct?

Goodwill_HiFi
11-06-2009, 05:27 PM
Most vintage and pro JBL drivers I have known tend not to have much stray magnetic pull. I assume this relates, in part, to specific design features that tend to minimize this including metal covers over the motors.

If by vintage you're talking about drivers that use AlNiCo magnets, that's your answer right there....... They're esentially shielded, as the actual magnet is totally enclosed in steel in many cases. Even is not totally enclosed, the poles are covered.

robertbartsch
11-09-2009, 02:41 PM
Well these 2408 tweets have very small light ceramic motors.

From the pull on each other, however, I can safely say they are powereful.

So placing the drivers next to each other on the same plane is OK but placing them back-to-back is not OK?

ratitifb
11-10-2009, 06:47 AM
From the pull on each other, however, I can safely say they are powereful.
from my point of view a powerfull motor is a motor with low magnetic leak (no Stray magnet pull) , mainly in a crisis period ;)

robertbartsch
11-10-2009, 12:31 PM
Why did they surround alnico motors with steel cases? Why do they not do this with ceramic motors?

Robh3606
11-10-2009, 03:16 PM
Why do they not do this with ceramic motors?

It takes something like 2.5 times the amount of ferrite to make an equivalent strength magnet. The alnico slugs all fit inside the VC where the ceramics are all outside. Too big and too heavy to have them inside a closed pot like the older motors

Rob:)

Goodwill_HiFi
11-10-2009, 06:51 PM
Why did they surround alnico motors with steel cases? Why do they not do this with ceramic motors?

I'm not sure on the details, but the AlNiCo magnets need to be enclosed that way or they will loose their magnetism. If you disassemble a motor with an AlNiCo magnet, it will need to be recharged. Ferrite magnets don't do that.

sba2
12-28-2009, 10:59 AM
There's some good info on magnet storage here--

http://www.coolmagnetman.com/magcare.htm

"... Basically, store different classes of magnets separately. Keep your ferrites together in one place, your Alnico in another (away from the ferrites), your SmCo in still another location, and your NdFeB magnets by themselves. This is the biggest rule for storage - putting distance between the various classes of magnets.

Ferrite magnets should be stacked, North to South, with others their same size. .....
Alnico magnets should be kept in their own drawer or box... Other magnets can easily demagnetize them. "