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Old 22nd February 2007, 10:33 PM   #41
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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.
It's certainly something that I've been thinking of when I've been scheming my own magnet designs.

It might be cartridge dependent too, and (obviously) a non-magnetic cartridge body would have to be preferred here. Of course there will always be a magnet inside the cart. and we might find that MM's show a higher sensitivity to being affected than Moving Coil's (?)
(Presuming there is any affectation at all)
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Old 22nd February 2007, 11:15 PM   #42
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having seen and heard Ynoans deck i'm pretty sure theres no cogging, the deck is rock solid stability wise with no audible flutter or pitch distortion.

i think the trick is the fact that the float is so small, only a few mm.

and it's definately quieter from a bearing point noise point of view than any ball, thrust plate or inverted bearing i've sat next to...

the side load from the drive belt pails into insiginificance compared with the weight of the platter on the bearing shaft.
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Old 23rd February 2007, 07:22 AM   #43
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BE ADVISED, ALL INFORMATION IN THIS POST IS AUDIO TECHNICAL WANKERY, AND IN NO WAY BASED ON SOLID OR EVEN TESTED FACT. INTENDED SOLEY FOR 'Hmmmm' CONSIDERATION

Quote:
Originally posted by Vinyl-Addict
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
Yep, that's what I was looking at as well. I like the gigantic spherical magnet, man they look fun! I'm sure I could invent a use for that, necessitating it's purchase. The rings magnets I saw were a bit small though. Ideally you'd want one that was the same size, or bigger, than the platter's diameter to offer the highest stability. Because obviously if it's smaller, it's more and more like balancing a plate on your finger.

Quote:
Originally posted by YNWOAN
The problem with using a ring magnet is that the magnetic force cannot be modified.

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).
Good point re: modifying the force, you'd want to be careful selecting if you just went with two ring magnets. It should be possible to calculate the required magnet strength with some calculator time.

In a perfect system, everything but the stylus would be rigid. Anything less than that means increasing the degree of signal aborbed in the compliance of the moving parts. The only reason for the tonearm not being rigid is so that it can move. Obviously, we need to be realistic here....

Cogging is likely low even with separate magnets because you have quite a lot of them and the platter's mass will require quite a bit of force to accelerate up and down. With the platter spinning, the up and down cogs of the individual magnets will be averaged by the platter mass, in the same way a capacitor averages the bumps on rectified AC.

You're right about an electromagnet system. If you have one on the underside of the platter, you also need some way of power it. Depending on the guage of wire used to wrap it and the way it's wrapped, neither is it a perfectly homogenous field. E.g. imagine wrapped a one layer deep coil with copper pipe. Now one with tens or hundreds of depth layers from ultra fine coiling wire. The second will produce a much more even field. I started doing the work to check the currents and wire sizes needed for a CD sized magnet to lift a platter but went to bed instead, I've just had a weeks worth of maths and physics work every night.

You're also correct on air bearings, they're not fundamentally better, just a comparable I'd guess.

Good point about the positioning of the magnetics under the label, that should help. And also entirely contradicts my stability point about wider magnetics. So that's one contradiction in design terms.

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.
Yep. I'd do away with trying to watch for changes in height and just see if you can measure them using the cartridge. One way to do that would be to excite some kind of vertical motion on the platter. So you'd play a groove with vertical modulation without the magnetics and look at the skew rate and amplitude on a scope. Then put the magnetics in place and look again. If any of the signal is being absorbed in the magnetic field, the skew rate and amplitude of the signal on the scope will decrease.

Quote:
Originally posted by AuroraB
Your easiest source of ring magnets, are blown ( or new ) bass or PA loudspeakers.
That's a great idea! I got super excited when I read this. Although, from what I understand, one of the few magnetic materials that can survive long term with it's poles in close opposition is neodymium. I think a lot of speakers use ceramics or something like that right? So I'd want to check about life time of the bearing if I used old speaker magnets.

Some of the newest, high power speakers might have neodymium in them I guess.

Which is one of the plus points for an electromagnet, that it doesn't wear down. Another consideration could be a composite. Pricey ring magnet on the platter underside, electromagnet coil on the plinth.

Re: weird vibrations... I've been thinking that my design might have the motor and plinth arrangement hanging on springs and critically damped. So I'd have the motor damped at the frequency it produces most noise for example. I'm just wondering if lead will be strong enough to make the platter from if it's an inch or two thick.... I don't want it to start sagging or warping.
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Old 23rd February 2007, 07:32 AM   #44
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An inch of lead will not 'sag or warp' I guarantee. However, it may not be the ideal material to make a platter from - you will have to do some experimenting. Also, it will not be easy to machine to a high tolerance.
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Old 23rd February 2007, 07:54 AM   #45
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Before I built the prototype I have posted photographs of I did in fact build a more simple version to check that the principle would work. The earlier experiment used the same magnets but half as many were used and they were spaced apart by over double the distance (a gap of 7mm existed between each magnet). This first bearing was made so that both halves would rotate on a shaft. No cogging effect was noted even when the bearing was spinning at very low speeds. If a cogging problem exists it will manifest itself as the bearing comes to a halt and when no flywheel is used; the first prototpe weighed only a few ounces.
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Old 23rd February 2007, 06:20 PM   #46
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Quote:
Originally posted by YNWOAN
This first bearing was made so that both halves would rotate on a shaft. No cogging effect was noted even when the bearing was spinning at very low speeds. If a cogging problem exists it will manifest itself as the bearing comes to a halt and when no flywheel is used; the first prototpe weighed only a few ounces.
How about using one ring magnet on the stationary surface and multiple smaller magnets (as you chose) on the moving part of the bearing? Wouldn't this approach improve or eliminate cogging?
FWIW, My current platter weighs 36 pounds so I'd like to find a magnet structure to elevate/repell this weight.

Here is a calculator for disc (not ring) magnets.
http://www.magnetsales.com/Design/Ca...iscmagnets.asp
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Old 23rd February 2007, 07:32 PM   #47
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http://www.engconcepts.net/List_Of_Ring_Magnets.asp

another source of ring magnets. the R1410 ring near the bottom of the page seems ideal, 4.5" diameter.
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Old 24th February 2007, 10:24 AM   #48
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Quote:
Originally posted by YNWOAN
An inch of lead will not 'sag or warp' I guarantee. However, it may not be the ideal material to make a platter from - you will have to do some experimenting. Also, it will not be easy to machine to a high tolerance.
At first, I'd been planning to find some wide OD pipe from somewhere, weld some plate over a 1" length of it and then pour lead into that, then machine it down to size. The pipe and plate would then form something easier to machine to tolerance and less likely to get scratched and so forth. However, finding someone who can give me an off cut of pipe in those dimensions would be difficult. If I wanted stainless steel, it'd only get harder (and probably less likely to get it for free from the off cuts bin). Maybe I could use a section from a suitably sized gas cylinder... lots of work though.

In terms of playback, lead should theoretically be one of the best choices I would have thought. The harder and lighter something is, the more prone it is to exictation. The softer and heavier, the less.

I was thinking of lead mainly because I literally have a bin full of it and a crucible spare.

Quote:
Originally posted by Vinyl-Addict


How about using one ring magnet on the stationary surface and multiple smaller magnets (as you chose) on the moving part of the bearing? Wouldn't this approach improve or eliminate cogging?
FWIW, My current platter weighs 36 pounds so I'd like to find a magnet structure to elevate/repell this weight.

Here is a calculator for disc (not ring) magnets.
http://www.magnetsales.com/Design/Ca...iscmagnets.asp
That approach would eliminate it and allow for changing the field strength. Myself, I'm not planning to fully float the platter. I'll just put one ring magnet in the platter, one in the base and then adjust the base one closer and closer until most of the platters weight is off the taper bearing. That way, you can use AAA precision bearings, which should be absolutely smoking.... very hard to find high precision bearings on eBay, so that'd have to be a list price purchase. The lower the load on the bearing, the cheaper it is.

Not fully floating the platter also has the advantage that any errors in aligning the magnets parallel to each other won't be directly represented as vertical errors in the platter position as it spins (bobbing up and down), it will only change the coupling of the platter to the bearing and possibly noise production as the loading on the bearing changes throughout the rotation.

Cool link... I've been trying to calculate how strong a magnet is required. Unfortunately, a lot of the hobbyist magnet sites only grade magnets as 'strong, stronger, ultra strong'. Email a lot of these sites asking for ampere / m^2 ratings and you'll probably get nothing back.

The repulsion force at contact should be reasonably close to the force needed to separate the magnets, which is easier for most people to measure. The field decays 8 times for every one unit of distance (from memory).

One problem you would still have with this arrangement is that you still have individual magnetic elements in the platter, spinning under the pickup coil in the cartridge (depending on where you put them on its radius of coarse). In effect, that's a ripple generator. Having both as solid rings would remove that. Whether or not it's an issue is another thing. It should be fairly easy to measure if it's having an effect by simply watching the cartridges output on a scope for an AC signal at the frequency the magnets in the platter are spinning past the cartridge.

If you use a taper bearing on the platter, you could also use a disc magnet dropped into the centre of the bearing.

Quote:
Originally posted by Tuukka
http://www.engconcepts.net/List_Of_Ring_Magnets.asp

another source of ring magnets. the R1410 ring near the bottom of the page seems ideal, 4.5" diameter.
Yep, I saw this site too, they do have quite a lot on there. That big one would win on stability, but as YNWOAN mentioned earlier, having a small OD also means that the magnetic field it produces is kept further away from the cartridge.

I was trying to guestimate the required magnet strength I might need to lift most of the platter weight off the bearing and remembered seeing ring magnets an inch or so in diameter on eBay that could lift 145kg for about 25 I think. So two together should stick with ~290kg of force, which means they should repell with roughly that much on contact with each other. Since a platter weighs 10 - 20kg, I'd guess magnets like those would repell the platter by quite a considerable distance. If you're only trying to lift some of the weight off the bearings, you'd either need to separate them by quite a lot (good because they're even less likely to depolarise over time) or you could just go with weaker magnets.
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Old 24th February 2007, 01:51 PM   #49
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Why do you feel that a taper bearing is the way to go? I would have tought that it is way too noisy for this aplication. The purpose of the magnetic bearing (for me) is to remove the grinding effect of the ball on the thrust pad in a more conventional turntable bearing. The sleeve and shaft arrangement used by almost all turntables is already hugely quieter (and much lower friction) than a taper bearing and I still feel that it is too noisy. I can't help but feel that you need to do some real prototypes.
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Old 24th February 2007, 04:49 PM   #50
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Quote:
Originally posted by YNWOAN
Why do you feel that a taper bearing is the way to go? I would have tought that it is way too noisy for this aplication. The purpose of the magnetic bearing (for me) is to remove the grinding effect of the ball on the thrust pad in a more conventional turntable bearing. The sleeve and shaft arrangement used by almost all turntables is already hugely quieter (and much lower friction) than a taper bearing and I still feel that it is too noisy. I can't help but feel that you need to do some real prototypes.
I would be worried about noise and you're right that hands on doing is required!

There are just so many things to make mistakes on and wish you could go back on that I want to consider as much as possible before I start buying anything... I'm a student as well, which means s is a premium. The lead idea is purely so I don't have to buy plates of steel or aluminium. I also have a more than unhealthy urge to cast some of it.

Bearing wise, I'm planning to use something like one of Gamet's precision taper bearings. These beauties are so well made you have to warm them up to cup of tea temperatures to slide them into place without damaging them. So I'm hoping they should also proove to be virtually silent at ~1 RPS. Although, I need to check the price of them because it could be frighteningly expensive.

http://www.gmnbt.com/gamet_bearings.htm

The radial bearing in your own table will be generating noise, but barely any because there is next to no radial load on it. The only load on your radial bearing will be from platter balancing imperfections and the pull from the drive mechanism. By magnetically lifting 99% of the weight off a taper bearing, a similar effect would be produced.

Why a taper bearing? The design I saw used a thrust plate and radial bearing. A taper bearing does both.

The thrust / radial bearing would be better in terms of stability because the thrust plate can be made to have a massive diameter, so the platter absolutely cannot tilt. But the tolerances on bearings goes down as size goes up (the manufacturers even publish specifications for it and it makes sense, it's not audiophile estimation), so bearing noise goes up as size does. You also need to be sure you have the thrust bearing precisely centred under the platter to avoid weird dragging effects due to speed differences about it's circumfrence (think about how the inside tyre of a car goes slower than the outer one when you go round a corner, because the two aren't exactly the same distance from the centre of the turning circle) and now have two sources for noise, the thrust and radial bearing.

I just chose a taper bearing because it seemed slightly cheaper and less noisey by comparison with the thrust and radial bearing design I'd seen, with the only drawback being that it's more like the plate on a finger example.

The bearing in my design would be next to useless in terms of load carrying. I'd set the platter to be almost floating, like yours, only just about resting on the bearing with a few N of force. That means that any forces developed by the modulation on the groove only has the play of the bearing to push down through (the play in its tolerance).

I'm trying to get access to our university's machine shop at the moment. I'm an electronic engineering student, but even still... they're not really used to having people in there I think, it's for the techicians to make stuff for the department. I keep walking past and drooling at all the stuff in there. It's about the size of a two semi detatched houses and packed with lathes, mills, saws etc... If they'll let me in there and it's open during the holidays, I might stay here over some of my holidays and work on it in the quiet.
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