magnetic field, tv distortion center chan

My friend wanted a center channel to go with his L-300s. I suggested a pair of LE-111s on either side of an LE-85 over an 077. He built the cabinet with nice lacewood etc. We put a Toshiba 36 inch HDTV on top and the sound was perfect but the stray magnetic field from the LE-111s put a slight green haze on the bottom corners of the screen. I use the old alnico specifically to avoid this and have never had a problem before. I pulled the LE-111 and dangled a piece of iron near it and it seemed to have very little field. This set must be extremly sensitive. I'm thinking of a steel plate on top of the speaker or in the cabinet. Any thoughts gentlemen?

A sheet of mu metal?

Can you get any more physical seperation?? Just a couple of inches or try moving the set foward or backward an inch or 2?? You could try a bucking magnet. I have an 801 C coax driver as a center and through just plain dumb luck the magnets buck each other and I have almost 30Lbs of ferrite 12" for my TV with no distortion.

Rob:)

I've been looking into this, and here's what I've learned so far:

It's a common misconception that a sheet of steel or iron will block a magnetic field, perhaps because we all saw a demonstration of it in physical science class. It only works in the short term. In time it will become magnetized itself and may end up actually extending the field.

Some people also seem to think a sheet of lead will work. It will have no effect.

Mu metal is the only thing that works. It somehow is more permeable to a magnetic field than the surrounding air, and so short circuits the field. Mu metal will lose its salient quality if it is worked so you can't buy a sheet of it and make your thing. You must design the cage you need and have it made up. After it's made the manufacturer will retreat it to become mu metal again.

The saturation of the steel makes sense. I've heard of Mu metal but did not know where to find it. Would it work in a sheet or should a basket to cover the pot be necessary? I removed the LE-111 and lowered it a few inches and the green persisted. I believe the set automatically degausses when turned on and off. That makes the green go away. I also recall haveing the problem with another center channel and it went away in a few days.

So where does one find mu metal?

For detecting stray fields I think a dangling piece of iron will only show the strongest. I use a cheap compass. I set the driver face down on a table and pass the compass by it about three feet east or west and repeat the pass, gradually moving closer until I see deflection. JBL alnico drivers with the cast pots are by far the best in this regard, with detectable fields that are consistently only a few inches to a foot broad. I've tested Altec alnico and EV alnico and they both have much larger stray fields. Exposed large ferrite magnets can have fields out to three feet.

This is a very conservative method and detecting a field in this way does not mean the driver must be kept that distance from a CRT. I had a problem with an AR11 once and solved it be moving it less than two feet away. Also, as you suggest, TVs will differ in their sensitivity although this may be a function of electromagnetic shielding, which is different, and may not work long term with a DC field, the kind you get from a permanent magnet.

We use Mu metal where I work. After machining and forming it has to be anealed or it losses it's "magnetic" properties. It's also expensive. Do a search for it it should as it be readilly available. You can also look for a company called Amuneal in Pensylvania. They specialize in magnetic sheilds. That's where we have ours made. They may have a stock of pre-made shields one of which might work. They certainly have the raw material.

Rob:)

http://www.amuneal.com/pages/magshield.php

Arranging a greater distance between the speaker and the TV can be the simplest solution.

By the way, JBL has a technical note on this which I can't find right now, but I think it reads something like this: They suggest that you turn the TV on, hold the speaker in your arms about ten feet away laterally from the TV, walk toward the TV until you see some discoloration, back off a foot or two. That's the safe distance.

This seems like a very conservative approach. I wonder if the legal department collaborated on it.


Regards,

David

I think mu metal is a solid sheet. That becomes an issue if you make a little enclosure for the magnet. If it fits the magnet closely, the field will still come out the front through the basket and cone and the problems may persist. If it doesn't fit the magnet closely it may interfer with the back wave and reflect some frequencies back into the cone, creating coloration. If you make it a little larger and extend it to the baffle you have created a subenclosure. I would figure on making the mu metal enclosure fit snugly against the inside of the box.

Regards,
David