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?
Re: magnetic field, tv distortion center chan
Stray Magnetic Field (more)
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.
Stray Magnetic Field (still more)
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