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Old 29th March 2008, 02:05 PM   #1
allears is offline allears  United Kingdom
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Default Magnets in tonearm effecting cartridge?

Hi , I'm looking at ideas for making my own tonearm , and am interested in the use of magnets as in the Schroeder and the Graham Phantom and Dynavector , all of which use strong magnets in various ways . However , on buying a few strong magnets I find that they can effect each other over quite long distances ,I can make a little button magnet twitch and spin more than a foot away by turning another magnet through a small arc . If this is happening , why doesn't the magnetic field from the damping magnet in each of these these tonearms effect the very sensitive magnetic field in the cartridge , which is only 9 inches away ? Particularly with the moving magnet cartridge , I would have thought. Any ideas?
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Old 29th March 2008, 05:01 PM   #2
bulgin is offline bulgin  South Africa
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Default Magnets in tonearms...

Hi allears

Good question. What type of magnets have you been experimenting? I know neodymium magnets are very powerful. As far as I know, button magnets of say, 5X5mm is often used in mc carts and I've just tried moving two of these at the 12" distance you mentioned and there's no reaction at all.

Can't say for sure. Perhaps some more opinions from wiser members?

bulgin
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Old 29th March 2008, 05:30 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

The magnetic field in a cartridge is very insensitive to external
influence by design, otherwise hum pick up would be terrible.

/sreten.
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Old 29th March 2008, 10:36 PM   #4
marekst is offline marekst  Poland
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There are individuals who use very powerful neodymium magnets to elevate thin turntable platters (with unshielded magnets just a couple of inches from the cart) and they do not see any problems.
I tried similar magnets, shielded and under thicker platter – when the cart was moving closer to the magnet the sound was getting bad. I didn’t want to damage the cart so I took platter magnets out. I do not know much about the subject but something was degrading the sound and I couldn’t stand the thought that my cart was being magnetized, demagnetized, saturated, pushed, pulled or twisted or whatever.
I have two 9” Schroeder clones with additional magnetic anti skate on my TT. I used magnetic compass to check the field and at 9” the field seemed to be not stronger than the field of the earth. But one of my carts is only 4” from the magnets of second arm so I put a magnetic shield between them, better safe than sorry.
Marek
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Old 29th March 2008, 11:34 PM   #5
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A magnet just suspended in space will have a huge field, easily detected a foot away. Most commercial applications provide a path for the field, and focus it where it's needed. If you've ever taken apart an old disk drive, the heads are usually driven by a voice coil motor. The very powerful magnets are mounted inside a little steel frame, so the coil can move in the gap between them. The magnetic platter and heads are just an inch or so away, and don't have any problem at all. That design would be suited for damping. Same thing in a speaker- the magnet is usually a donut, and the steel structure consists of a plate and cylinder to create an external gap for the voice coil to run in. That design, however tends to have a pretty big external field. I'd be nervous about strong magnetic fields near any phono pickups or even wires because the slightest vibration between them will generate a signal.
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Old 30th March 2008, 12:01 PM   #6
allears is offline allears  United Kingdom
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Thanks to everybody for your replies . I'll be a bit more specific - the magnets I'm using are 10 mm by 5mm neodymium , 42N so not quite as strong as the strongest Schroeder ones , and I just hung one one a thread to give the clearest response and it will oscillate at twelve inches and move quite dramatically at 9 inches (as it will if it's just stood on its edge on a flat surface). Big speaker ring magnets as you say have a huge external field . It still puzzles me a little , the wires from the cartridge pass a lot closer to the magnets and you would think there would be some response , but obviously this has been shielded in some way or simply doesn't effect the sound in the designs that use them. I will keep on playing with them (endless fun! I'm looking at ways to replace the counterweight with a repulsive set at the moment ) and post again if anything interesting crops up.
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