That's exactly what I'm working on. I started this thread just to make sure I was on the right track, and that there wasn't something inherently bad about direct drive that would make the whole thing pointless. Apparently, there isn't.
The only thing I can't control is the quality of the original bearing. It may be possible to retrofit the motor onto a quality bearing if that becomes an issue.
The only thing I can't control is the quality of the original bearing. It may be possible to retrofit the motor onto a quality bearing if that becomes an issue.
That was my idea, I have access to a machine shop with grinding equipment as well. I will probably buy a high polished shaft from McMaster Carr and fit that into an oil-lite (brass impregnated with lubrucant) bushing that I will grind to fit the shaft with minimal clearance. Then press in a polished carbide steel ball riding on a thin piece of teflon in an oil bath. I know it would be rediculous overkill, but that would be what I am after.
DaveM
DaveM
Teflon does make a stable trust plate ...
.. i must know it since my homebrewn TT's platter bearing has a ruby ball of 3mm dia and a teflon trust plate. And one trust plate lasts for more than 5 years. Platter weight is about 4kg .
The trick is that the ball is a calibration ball with extremely low form deviations and ultra-low surface roughness. Any steel ball would eat itself thru the teflon within weeks.
With the ruby ball, there will be a stable indentation in the teflon surface.
And you have heard that thing, too, EC8010, as we met at the ETF in Langenargen 2004. My TT was the one with the fancy modified Rabco tonearm on it. You remember?
.. i must know it since my homebrewn TT's platter bearing has a ruby ball of 3mm dia and a teflon trust plate. And one trust plate lasts for more than 5 years. Platter weight is about 4kg .
The trick is that the ball is a calibration ball with extremely low form deviations and ultra-low surface roughness. Any steel ball would eat itself thru the teflon within weeks.
With the ruby ball, there will be a stable indentation in the teflon surface.
And you have heard that thing, too, EC8010, as we met at the ETF in Langenargen 2004. My TT was the one with the fancy modified Rabco tonearm on it. You remember?
davidsrsb said:
An aluminum platter makes a great diamagnetic shield.
Although, I have read that Grado carts don't work well with some tables for lack of sheilding. My dad had one on his direct drive KD-500 and had no issues. Ironically, if I recall, the problem is with Regas (belt drive) and Grado cartridges. Hmm...
Max
--> I suspect that the hardest part of building a DD is keeping that magnetic field out of the cartridge. <--
Why... what do you think will happen? What would it sound like?
--> Why not use the alu plate as a DD Lenz motor ? Only a bearing and a plate. No magnets, only two C-core electro magnets and some control electronics <--
What is a Lenz motor? Is it the kind of motor that drives a power meter, with the spinning alumminum disk?
--> I have a spare SP10 Mkii with what appears to be a worn bearing (the platter rocks) <--
Yes, at least take it apart and find out why it rocks. If the bearing really is bad, then it will be interesting to see if the motor is constructed in such a way that you can put a different bearing into it, assuming some mechanical skills.
Neil
Why... what do you think will happen? What would it sound like?
--> Why not use the alu plate as a DD Lenz motor ? Only a bearing and a plate. No magnets, only two C-core electro magnets and some control electronics <--
What is a Lenz motor? Is it the kind of motor that drives a power meter, with the spinning alumminum disk?
--> I have a spare SP10 Mkii with what appears to be a worn bearing (the platter rocks) <--
Yes, at least take it apart and find out why it rocks. If the bearing really is bad, then it will be interesting to see if the motor is constructed in such a way that you can put a different bearing into it, assuming some mechanical skills.
Neil
The usual belt drive arrangement keeps the motor a long way from the cartridge and arm tube.
A typical DD motor either under the platter or at the rim is going to have 360 degree coil windings, so the cartridge will pass straight over them at some point, a couple of cm. The coils create a rotating field, which from a fixed point looks like an ac field. The cartridge body is plastic or non ferrous metal so the magnetic field will act directly on the coils. An aluminium platter is poor at blocking magnetic fields.
One way out is to put permanent magnets around the platter edge in a NSNSNS.. sequence and arrange coils to attract and repel at the correct time at only a small part of the edge, well away from the arm. I believe one Japanese manufacturer used to do this. The problem will be that a fancy coil waveform would be needed to avoid cogging, but a microcontroller and DAC could do this.
A typical DD motor either under the platter or at the rim is going to have 360 degree coil windings, so the cartridge will pass straight over them at some point, a couple of cm. The coils create a rotating field, which from a fixed point looks like an ac field. The cartridge body is plastic or non ferrous metal so the magnetic field will act directly on the coils. An aluminium platter is poor at blocking magnetic fields.
One way out is to put permanent magnets around the platter edge in a NSNSNS.. sequence and arrange coils to attract and repel at the correct time at only a small part of the edge, well away from the arm. I believe one Japanese manufacturer used to do this. The problem will be that a fancy coil waveform would be needed to avoid cogging, but a microcontroller and DAC could do this.
It's true that aluminum would make a poor shield, but in the DD motors I've seen, as well as in VCR motors, the magnet is encased in a very effective ferromagnetic shield. You can put a peice of metal very close to it, and not be able to sense the presence of the magnet.
How strong would the disturbance to the cartridge be? I guess it would appear as some very low frequency pulses at some multiple of the platter rotational frequency. E.g. if there were 6 poles, at 33.3 rpm you might expect to see pulses at 3.33 Hz. But I don't know how much energy they would carry.
It's different for a belt drive motor perhaps, because the frequency is much higher.
How strong would the disturbance to the cartridge be? I guess it would appear as some very low frequency pulses at some multiple of the platter rotational frequency. E.g. if there were 6 poles, at 33.3 rpm you might expect to see pulses at 3.33 Hz. But I don't know how much energy they would carry.
It's different for a belt drive motor perhaps, because the frequency is much higher.
I guess that means you would want to make the platter nice and thick to get the cartrige away from the motor field
Just another rationalization for a big platter.
If you made a layer of the platter of steel and then grounded through the bearing you could in effect make a very effective shield. That in combination with mu-metal around the motor would completely eleminate any noise or field effect created by the motor.
This didn't seem to be a problem with the Technics or Denon tables. They were fairly well reguarded in their day, so I have to assume that there was not a ton of noise from the motor to begin with.
DaveM
Just another rationalization for a big platter.If you made a layer of the platter of steel and then grounded through the bearing you could in effect make a very effective shield. That in combination with mu-metal around the motor would completely eleminate any noise or field effect created by the motor.
This didn't seem to be a problem with the Technics or Denon tables. They were fairly well reguarded in their day, so I have to assume that there was not a ton of noise from the motor to begin with.
DaveM
With all due respect magnetic field coupling is much less of a problem in DD tables. This is because the frequency of rotation of the motor is only 0.5 Hz. The change in field at the cartridge is very slow, its inaudible. A belt drive has the motor running at 60Hz, for example, thus the change in field is 60hz, or multiples thereof causing that much loved HUMMMMMM particularely in non-shielded MM or MI cartridges.
You have to consider the difference in motor speed both for vibrational and magnetic coupling. The motor in a DD table runs about 100 times slower, which is a big offset to the inherent disadvantage to mounting it directly on the spindle.
-R
PS There was some mention of BLDC motors, but aren't most DD tables powered by AC servo motors? I know mine is. I confess I dont know much about the difference, though.
Yes, of course you need to control both the frequency and strength of the field. It would be insanity to put a strong unshielded magnet close to a cartridge. But it if was close enough to affect the signal, I think you would be having other more serious effects, such as tracking problems due to the force field of the magnet.
It doesn't matter how many coils there were. They would have wired them as three phases, so the commutation frequency doesn't multiply as you suggest. However, if there were a large number of magnets at the rim, these would generate an alternating mag field as they swing past the cartridge. They would need to be shielded. Such a design is asking for trouble because of the difficulty of matching a large number of magnets.
I opened up my Technics SP10mkII motor to see what was inside. It's an outer rotor motor with a 15 pole stator (remember it's driven by three phases) inside a multipole (can't be 15) annular ferrite magnet rotor with a pressed steel screen around it. In the middle is another rotating annular ferrite magnet and three small fixed coils, presumably to provide the tacho signal.
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