Turntable speed accuracy

Sarcher, What is the evidence that the hum is (or was) due to the motor/cartridge interaction?

Banquo, I have some knowledge but not much skill when it comes to the electronics of either a motor controller (for BD) or a drive system for a DD tt. If I have a problem, I rely upon Bill Thalmann to deal with it, at my expense. But I do have the good fortune to live within 30 min of Bill's shop.

Halcro, I'm not sure I agree with Peter on this one. Our model 208 has the motor quite close to the platter (opposite the tone arm; there are no hum problems even with Grados). However the 208 uses a motor that would easily qualify as a flywheel in its own right. Keep in mind that in a belt drive, cogging is almost a non-issue due to the rotational speed of the motor, and then really only applies to DC motors for the most part.

AC motors are not particularly prone to cogging effects. The best tape drives all have direct-drive capstan motors, usually AC, and they often have rock-solid speed stability that turntables struggle to match.

The Empire that our machine is based on was one of the most speed stable made for quite a long time. Julian Hirsch complained in one of his reviews of it that the speed variation was "buried in the noise" of his instruments, making an exact number difficult to gauge (although Empire did publish specs at the time). Sounds to me like he could have used a Timeline...

I've played with a variety of belts on the machine. Interestingly, even mylar variants sound exactly the same. Its my opinion that if the belt alters the sound then there is a problem in the drive system somewhere.

So I don't buy the idea that drag in the bearing results in speed stability at all although it would not surprise me to find that some designers may have created this story for reasons of their own (obviously the 208 flies right in the face of Peter's comments). OTOH I don't doubt that the belt-drive Thorens had some problems! I think that is why the earlier Thorens are the ones people are looking for.

Halcro,

In my statement about AC belt driven TT's vs DC motor driven TT, I was meaning belt driven DC TT's, not a DC driven DD.

I have no experience with DD's yet. If I get a timeline will try to post some results.

Atmasphere,

Which Empire TT are you referring to? AC or DC drive ,belt type?

(quote)Sarcher, What is the evidence that the hum is (or was) due to the motor/cartridge interaction? (/quote)

The hum occured when the tonearm was swung over the platter in both cases. With the motor off on the Classic there was no hum with the tonearm over the table. The LP12 was likely the power supply because I had to unplug it to make the hum go away with the arm over the platter. I have solved the hum issue on the Classic by using an outboard Teres motor.

Hiho, The motor was grounded. I am not the only one with a hum problem on their Classic. There is a thread about it on this forum.