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Direct drive/rim drive/idler drive vs. belt drive?

O.K. here is one for all the physics majors and engineers.

Does a high mass platter being belt driven offer the same steady inertia/speed as a direct drive or idler drive?
Is the lack of torque in the belt drive motor compensated for by the high mass platter. Object in motion stays in motion etc. Or are there other factors to take into consideration?
I am considering building up a Garrard 301 or Technics SP10, but is it all nonsense about the advantage of torque.
I am aware that the plinths on these tables can make a huge difference, I've got that covered.
My other options would be SME20 or Basis 2500 of Kuzma Stogi Reference etc.
If I have misstated some technical word, please avert your eyes. I don't want a lecture on semantics, I think everyone knows what I mean.
Thanks in advance.
mrmatt
mosin
410 posts
 
The primary hurdle to jump with belt driven turntables is their inherent belt creep.

It is best described here...
http://db.audioasylum.com/cgi/m.mpl?forum=vinyl&n=694178&highlight=belt+creep&r=
quiddity
119 posts
 

The first thing to deal with is the torque issue. The thing about high torque idler motors versus low torque belt motors is largely a furphy.

The output torque at the shaft of the Garrard motor is a little under 10 mNm. The output torque at the shaft of the 3W Hurst motor used by VPI is 26 mNm.

The reflected torque at the platter is the shaft torque times the gearing ratio. For the Garrard the gearing ratio is about 48 to 1 so the final torque at the platter is about 440 mNm. For the Hurst motor the gearing ratio is 18 to 1 so the final torque at the platter is 470 mNm.

The "low torque" Hurst thus has more torque than the "high torque" Garrard.

To add insult to injury, the Garrard motor slows by 10% when delivering that torque where the Hurst motor does not slow at all.

Mark Kelly
quiddity
119 posts
 
Oops

I looked at the wrong motor on the Hurst table, the actual motor used by VPI is the lower torque model so the figures are 16mNm at the shaft and 290mNm at the platter respectively.

This would seem to favour the Garrard but only if you find a 10% speed reduction acceptable. If you want the Garrard to slow by a lesser amount (say 1%) the useable torque drops almost proportionally - at 1% the torque is about 1mNm at the shaft, say 50 mNm at the platter.

Mark Kelly
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