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Old 22nd February 2007, 07:20 AM   #31
YNWOAN is offline YNWOAN  United Kingdom
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Graeme , yes a meet does seem in order. I have e-mailed you so...I look forward to hearing from you :-)

With regard to using my bearing to suspend a direct drive platter I think it would still be possible. I'm not that familiar with direct drive so will give it an investigate.

With regard to idler wheels, it may also be possible. However, I feel it very much depends on the specific implementation of the drive system.
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Old 22nd February 2007, 10:41 AM   #32
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re and idler driver set up,if it's a rim drive then the contact would be in the same axis as the bearing sleeve, not such a bad thing.

if it was a platter drive, Garrard, most Lencos then the contact would be in the verticle axis which would introduce contact along the floating axis,maybe not so good as the primary load of the wheel to the platter would interrupt the magnetic bearings.
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Old 22nd February 2007, 10:43 AM   #33
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thats what im thinking. Im not worried about my bearing anyway, but if the magnetic bearing blows me away i might have to either work out a way to implement it, or swap drive systems

YNWOAN, ill call you later this evening as i assume you'r at work in the day.
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Old 22nd February 2007, 06:41 PM   #34
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Hmmm, interesting stuff.

I expect a solid ring magnet would be better for this but they can get pretty darn expensive and are, as has already been said, tricky to find.

I don't think you'd want to do away with the radial bearing and replace it with a magnetic one. Firstly you have the pull from the belt, which will mean that your platter inevitably ends up sitting off centre in the magnetic field in a form of 'dip' where the magnetic repulsion and belts pull are in equilibrium. Secondly, you're asking for problems with the platter moving on the horizontal plane, which would make it harder for the arm to track and introduce distortion. Some of the horizontal component of the field ends up being absorbed by the squishyness of the horizontal magnetic suspension - the same as it would in the play of a poor radial bearing, an ideal bearing has 0 tolerance of coarse.

I've spent a lot of time wondering about what the ultimate way to suspend a platter might look like. And I've seen something pretty close on the net.

The platter used regular thrust and radial bearings (I think, and by regular I mean, expensive ceramic bearings... but normal in the contact sense; ceramic obviously less of a problem with the magnets involved) and magnets to offset the weight of the platter. That's important if your platter weighs 30kg, because the weight will mean accelerated wear and increased noise generation / transmission. Using magnets, the design took most of the platter weight off the bearings, reducing those problems.

So then you wonder why bother with the regular bearings at all. Back to the first bit. They'll provide much, much higher rigidity than the magnetic fields, allowing for better tracking. If the platter is completely floating in the vertical direction, some of the force produced as the stylus goes over a vertical component in the groove will be bypassed by the squishyness of the vertical support provided by the magnetics. Hence, how a contact thrust bearing backing up the magnetic bearing might help.

More thoughts....

You could build a magnetic bearing with copper coils or something probably. Might be worth some investigation. A big, flat, multiturn coil should be able to levitate a heavy platter with not a lot of current. Stealing from Valvitude's CD idea for his tesla turbine, you could use two CD's separated by their moulded spacers as a former for a flat coil.

You only need a fraction of a mm for clearance, as the platter doesn't have much vertical force on it. Magnetic fields decay exponentially with distance, and that means the force they produce does as well. So separation distance is expensive, and large values not needed in such an application, making large separations even more of a money burn when magnets aren't cheap

Although the magnetic field might not visibly pull the cartridge down, it does worry me that a magnetic bearing is potentially bathing it in a DC field. The only AC noise with a big, two solid ring magnet layout will be due to grain errors in the magnets. With individual magnets, there will be some AC component as the individual magnets spin past the cartridge.

You could shield the cartridge from the field with strategically placed iron / cobalt / mumetal sheet. I'd recommend waving a cheapo hall sensor over the platter, above the magnetic ring, as it spins, and that way you can see what's actually going on magnetically. Failing a hall sensor, an ultra nasty guitar pickup. Failing the guitar pickup, a coil of wire with a steel bolt through it.

Any change in the DC magnetic field over the cartridge will equate to a magnetic bias. Obviously, it might be so tiny you can't even hear it, but being the people audiophiles are, it's always interesting to consider if something could be improved even further still.

The alternative to a magnetic bearing is an air bearing, but then you're going down the pumps road obviously. Valvitude has an interesting TT thread with mention of a tesla turbine in it. That might a nice, quiet way of levitating a platter.

To test for floating errors on the platter, you'd want to get hold of test record with a flat groove and watch the cartridge output for changes. Changes will, of coarse, include errors in the test track depth and errors in the angle the record makes with the platter surface. You could use a cheap cartridge dragging over the platter surface, with the tonearm locked in place, to test for errors but you'll get a whole load of nosie if the surface isn't smooth, and the cartridge will probably die soon after. Maybe you could put some kind of bearing between the stylus and surface such that the stylus doesn't make direct contact with something moving; effectively turning the tonearm into the arm of a balance, upside down. The only other idea I can think of immediately is to electrically contact the platter with a bent bit of wire than use a second contact above the platter, very close to the surface, and watch for contact / breaks. The second contact could be attatch to a micrometre to allow for very fine adjustment.

The only reasons for the platter to have vertical errors would be;
the vertical 'errors' in the groove (music) pushing it up and down
vertical errors in the turning of the platter
the cogging effect of the magnets

Maybe you could go insane and add an extra ring around the top edge of the disc, pushing it downwards, for even more vertical rigidity.

I'm obviously not trying to say any of this matters and it could all be completely uncalled for.... i'm just thinking aloud in case you find anything interesting to try!

Short of doing all this stuff, I think I'd go for some AAA taper bearing from someone like National Precision. But getting hold of one cheaply would be a mission

You UK guys aware of the UK DIY Audio meet?
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Old 22nd February 2007, 07:21 PM   #35
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Bismuth has been mentioned as a good shielding material. Here is a source in the US for ring magnets.

http://www.unitednuclear.com/magnets.htm
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Old 22nd February 2007, 07:29 PM   #36
YNWOAN is offline YNWOAN  United Kingdom
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The problem with using a ring magnet is that the magnetic force cannot be modified. By using a large number of smaller magnets it is relatively easy to adjust the forces by increasing or decreasing the number of magnets.

I agree that one does not want to do away with the radial bearing - partly for the reasons you give - but also because I doubt that it is possible to do so, as posted earlier.

I am not convinced that rigidity is essential, or even necessarily, desirable, in the vertical plane. The magnetic field is 'squeezed' very tight in my system already.

I am happy that there is no cogging, even at speeds considerably lower than 33rpm if a large number of small magnets are used.

The magnetic bearing sits within the circumference of the label area of the platter so the cartridge does not pass over the magnets at any time.

An electro magnet would have to have a very even and continuously stable field generated.

Air bearings can also suffer from uneven pressure fluctuations and must be a continual loss system (in my opinion).

The whole stability - in terms of flutter - issue is a 'red herring' in my view and is easily avoided with accurate manufacture. Absolutely no 'wavering' in the platter can be seen.
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Old 22nd February 2007, 09:27 PM   #37
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Quote:
Originally posted by YNWOAN

I am not convinced that rigidity is essential, or even necessarily, desirable, in the vertical plane. The magnetic field is 'squeezed' very tight in my system already.

The whole stability - in terms of flutter - issue is a 'red herring' in my view and is easily avoided with accurate manufacture. Absolutely no 'wavering' in the platter can be seen.
Yes, but it has been calculated that the smallest groove variation on an LP is the same size as the wavelength of light- are you THAT sure that there is no movement? Can you even see the vibrations of your stylus? Just a thought.

I was thinking about this today, and I agree with eeka chu that a rigid bearing providing the location for the platter, with (permanent) magnets taking 99% of the weight is the way to go. Although with the electromagnet the thrust plate would take a hell of a beating when the magnet is off, especially if there was a power cut while listening to your 78s.

This could even let you use a Rega planar or Linn/Thorens bearing on a table with a 50kg platter- they are good, readily available bearings, but can only really take 1 or 2 kg without damage.

I can see that a floating platter seems nice from a vibrations point, but a suspended plinth should take care of those. Also, the radial bearing will transmit vibrations anyway.

I did read somewhere (the huge 'Let's make a DIYAudio TT' thread?) that having a suspended platter (e.g. magnetic or air bearing) on a suspended plinth would cause all sorts of horrible vibrations all over the place, with each element vibrating at it's own resonant frequency.

James
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Old 22nd February 2007, 09:42 PM   #38
AuroraB is offline AuroraB  Norway
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Your easiest source of ring magnets, are blown ( or new ) bass or PA loudspeakers. The description of the "Platine Verdier" published in l'Audiophile in the late 80's, called for large loudspeaker mag's encased in turned polepieces to increase the field strength.
I started this project many years ago, but it came to a halt when I could not find suitable magnets without buying new and expensive speaker units - as I live in rather rural areas, my sources of "interesting scrap and junk" are quite limited.
If your platter is of reasonable weight, this could be fairly easy. .the Verdier platter I started was over 10 kg's....

EDIT: the bearing for the Verdier, was essentially a combination of the magnet cushion and an inverted bearing, as the platter was also resting on a steel ball on the top of the steel shaft, to take a minor part of the load. Personnally, I think the idea of no true radial bearing is totally futile, given our demands for platter stability.
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Old 22nd February 2007, 10:05 PM   #39
YNWOAN is offline YNWOAN  United Kingdom
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Quote:
Originally posted by jrevillug


Yes, but it has been calculated that the smallest groove variation on an LP is the same size as the wavelength of light- are you THAT sure that there is no movement? Can you even see the vibrations of your stylus? Just a thought.

I was thinking about this today, and I agree with eeka chu that a rigid bearing providing the location for the platter, with (permanent) magnets taking 99% of the weight is the way to go. Although with the electromagnet the thrust plate would take a hell of a beating when the magnet is off, especially if there was a power cut while listening to your 78s.

This could even let you use a Rega planar or Linn/Thorens bearing on a table with a 50kg platter- they are good, readily available bearings, but can only really take 1 or 2 kg without damage.

I can see that a floating platter seems nice from a vibrations point, but a suspended plinth should take care of those. Also, the radial bearing will transmit vibrations anyway.

I did read somewhere (the huge 'Let's make a DIYAudio TT' thread?) that having a suspended platter (e.g. magnetic or air bearing) on a suspended plinth would cause all sorts of horrible vibrations all over the place, with each element vibrating at it's own resonant frequency.

James

James,

Obviously, I am unable to see the specific movement of the stylus but luckily it is attached to the rest of my Hi-Fi and my ears tell me that a significant improvement in all areas has been achieved. I wonder what specific mechanism you think is likely to cause erroneous signal to enter the cartridge. A suddenly applied force in the nature of pounds would be required to compress the bearing further and this assumes the decks suspension system does not deflect first (which obviously it will). Also, although information is contained in the vertical plane the majority is in the horizontal - the left and right walls of the groove.

I do not agree that taking most of the weight off the platter is the ideal solution - two surfaces scraping together are still scraping (even under a lesser load), though I do agree that it is much better - I may well have a bit more of an experiment along this line.

I do not see the bearing as being in any way a replacement for suspension as it is in fact very stiff and offers very little 'bounce'. I should add that I am not a great fan of very high mass platters or of suspension less decks.

As far as the 'horrible vibrations' issue, I believe this to largely be nonsense. If an element of the structure is going to 'ring' or worse vibrate at its natural resonant frequency it will do whether you are using a point bearing or not. The mechanical damping afforded by a ball and thrust pad is minimal at best. To make 'each element' vibrate at its own resonant frequency would require an energy input level significantly beyond that existing in a turntable in my opinion. Obviously vibration exists and must be channelled or damped but this in no way means that the natural resonant frequency of anything will necessarily be excited.
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Old 22nd February 2007, 10:26 PM   #40
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Quote:
Originally posted by jrevillug

I was thinking about this today, and I agree with eeka chu that a rigid bearing providing the location for the platter, with (permanent) magnets taking 99% of the weight is the way to go. Although with the electromagnet the thrust plate would take a hell of a beating when the magnet is off, especially if there was a power cut while listening to your 78s.
I'm still debating trying out the 99% method myself. I hadn't really thought about completely suspending the bearing because I was concerned about controlling vertical motion during playback. I see now that YNWOAN seems to have achieved this and congrats to him for accomplishing that.
My only concern with his design is the possibility of cogging with multiple magnets eventhough they are equally spaced and defined on a precise circular pattern. I think YNWOAN brought up a good way around possible cogging by using an unequal number of magnets on one surface. BTW YNWOAN, I am not implying that your design suffers from cogging, just concerned that I may not be so "lucky".
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