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Old 20th February 2007, 05:41 AM   #1
YNWOAN is offline YNWOAN  United Kingdom
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Default Magnetic turntable bearing

I have just built a magnetically repulsing bearing to retro fit to my turntable. The turntable itself is a home built and designed affair but the main bearing and platter are from a Linn LP12. The magnetic bearing consists of two opposing rings of small (6mm diameter) Neodymium magnets pres fitted into CNC machined acrylic discs. The intention of using many small magnets (40 in each half) was to provide an even magnetic field with no cogging effect. The magnets are placed very close to each other and surround the main bearing. A bit of ‘fancy’ machining was required but I am lucky enough to have easy access to a CNC router. I am yet to do final measurements but I think the magnets suspend the platter by as much as 5mm but I will investigate further tomorrow. Note, the main bearing shaft is still used for horizontal alignment.

To say that I am delighted with this modification is a huge understatement. I have read elsewhere on the forum that the edge of the platter can be seen to rise and fall with some magnetic bearings; this is not the case in this instance. Friction levels are substantially reduced and background noise is hugely reduced. Bass instruments are significantly easier to follow and the whole sound is substantially more open and natural. Literally every aspect of reproduction has been enhanced. In the past I have experimented extensively with turntable design and have investigated many materials and construction methods. My current design uses quite an elaborate, dual layer, suspension, for example, to reduce the effects of feedback. However, this bearing modification is the largest single improvement I have so far achieved.

I strongly urge anyone planning on building (or currently doing so) a turntable to investigate this type of bearing as they may also be as pleased with the results as I am.
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Old 20th February 2007, 10:30 AM   #2
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could you not build a wall around the platter and have similar magnets opposing themselves around the edge? Then you dont need the center sleeve bearing.
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Old 20th February 2007, 10:38 AM   #3
YNWOAN is offline YNWOAN  United Kingdom
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Hello Graeme, I too live in Sheffield S11.

It may be possible to do as you suggest though it would require a significant re-working of my turntable and stability could well be an issue. In fact, thinking about, it as I type, I think the side force exertd by the belt would be a real problem. One of the magor advantages of my mod is that it is entirely retro fitable and can easily be removed if so desired.

I now believe the magnetic gap to be in the order of 3mm or so and as such there is very little 'bounce' from the magnets.
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Old 20th February 2007, 10:42 AM   #4
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Interesting stuff.

Does this cause problems with the cartridge? I have heard that a magnetic bearing can pull the cartridge down, damaging the cantilever.

How thick is the platter? If thick enough it may be that this is not a problem.

James
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Old 20th February 2007, 12:18 PM   #5
YNWOAN is offline YNWOAN  United Kingdom
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The cartridge seems to be completely un affected (a re-built Linn Troika) - still experimenting though. The platter is not particularly thick - about 12mm in the area where the magnets are placed. As I said before, the platter and inner platter/main bearing are from a Linn LP12. The platter is cast (post machined on all surfaces) from Mazak which is a lead, Zink alloy.
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Old 20th February 2007, 12:37 PM   #6
SY is offline SY  United States
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Slightly OT: Who did the Troika rebuild for you?
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Old 20th February 2007, 12:43 PM   #7
YNWOAN is offline YNWOAN  United Kingdom
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I had it done through Avondale Audio. However I know that Les doesn't actualy do the work.
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Old 20th February 2007, 12:44 PM   #8
AuroraB is offline AuroraB  Norway
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A few pictures would be nice...

BTW Mazak- or it's more common trade name Zamac, is a zink/aluminium alloy.......


The use of a spindle bearing handling the side load only, and magneitc repulsion in the vertical, I beleive was first seen in the "Platine Verdier" late 80s....
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Old 20th February 2007, 12:49 PM   #9
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Nice mod!

Quote:
could you not build a wall around the platter and have similar magnets opposing themselves around the edge? Then you dont need the center sleeve bearing.
Earnshaw's theorem says stability is impossible to implement this way.

It could be done with some distance-sensing and feedback to electromagnets though...
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Old 20th February 2007, 01:30 PM   #10
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another sheffield poster here. S7.


any chance you could upload a few pics.
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