Turntable speed accuracy

Hiho, There is much ignorance among us as to how the servo system of this or that direct-drive turntable actually works. From my casual observation, they are not all the same, by any means. But from a Newtonian point of view, max torque is only ever attained at the moment of start-up, when the platter is completely at rest and then must undergo a change of inertia to being in motion at 33.333... rpm. So, the huge torque of an SP10 Mk3 is what gets its 22-lb platter up to speed in 0.25 seconds (or something like that, according to the Mk3 owner's manual). Once the platter (any platter) is in motion at its set speed, it does not take much torque to keep it there, except that which is needed to counter-act stylus drag. Here is where the design and implementation of the servo mechanism is different for different vintage types. For example, I am not sure I understand exactly how the L07D system works, but it appears that the full torque of the motor is only invoked when or if there is a major loss of speed for whatever reason. (I think the service manual says more than a +/-3% speed deficit.) Otherwise, the drive system doles out torque in small increments, and I think this is done to minimize the audibility of tiny corrections that need to be made to maintain exact speed. The engineers of the 1970s and 80s were well aware of all of these issues that we are now still obsessing over. The L07D may turn out to be my all-time fave, and it's not the highest torque in town.

And, lest we forget, among designers of both belt- and direct-drive turntables there seems to be a divide around weak motor/huge platter vs strong motor/light(er) platter. There are logical arguments either way. So, I would not be so bold as to make any declarative statements.

And, lest we forget, among designers of both belt- and direct-drive turntables there seems to be a divide around weak motor/huge platter vs strong motor/light(er) platter. There are logical arguments either way. So, I would not be so bold as to make any declarative statements.

Well said- every design is a compromise, and barring a "paradigm shift" or revolutionary advance in technology or theory, implementation and qa/qc are the make or break points. Says the guy who can't carry a tune;-)

Dear Swampwalker, 'Our' Lew is a scientist so no wonder he
use what Wittgenstein called 'scientific grammar'. I would prefer 'indicative statements' above 'declarative' but in any case the 'truth valued ' statments are presupposed. I believe that thinking about different drive systems involves a 'paradigm shift' but not a 'revolutionary' advance in technology. If there were a real revolutionary advances in the technology all TT's designs would use this technology. Otherwise the producers of the 'old technology' TT's would obviously speculate about possible 'sentimental buyers'.

Regards,

Lewm: "The engineers of the 1970s and 80s were well aware of all of these issues that we are now still obsessing over. The L07D may turn out to be my all-time fave, and it's not the highest torque in town."

Good point and good write up. At one point I thought the higher the torque the better and forgot about the increase in cogging. Torque for torque sake does not a good turntable make. I agree that Kenwood knew what they were doing back in the days. Recently I recommended a Kenwood KD-770D, one with a rather low torque coreless motor by DD standard, to an acquaintance and he is so happy that even his wife thinks the sound is smoother and that's with a cheap cartridge. I really think the reduction of cogging by using a good motor is a worthy effort in DD designs. And a precise but "gentle" servo is less harmful to the sound than some brute force detecting system. eg., someone in a DIY forum did the below to his modified Technics SL-Q3:

"I modified the negative feedback loop network to make the whole thing underdamped. As it is from the factory, it is overdamped and after doing some A/B test by switching instantly between the factory network and the new one, it is obvious the change in sound. The modded version is much more relaxed and clear and all the distortion (similar to jitter in digital) in mids and highs is gone."

Some of that sterile DD sound may be caused by the "overdamped" servo, a kind of "analog jitter" Mosin and others mentioned before.

It all comes down to execution, I guess...

_______

Lewm,I struggle with this statement -
"among designers of both belt- and direct-drive turntables there seems to be a divide around weak motor/huge platter vs strong motor/light(er) platter"
I have never associated high mass platters with low torque motors.
If you had said the 2 schools of thought were light platters with sophisticated speed correction or control versus high mass platters with more inertia and less speed correction that would be a more accurate synopsis in my view.