Direct drive vs belt vs rim vs idler arm

...unlike my clunky grammar which required zero thought. :D

Tony,
The issue I see with test record frequency measurements is that on the test record, groove anomalies and the good possibility that the test record center hole is not exactly center can throw off the measurements. Prior to using the laser reader I used a 1000 hz test tone on a record and used a Fluke digital multi-meter to measure the output frequency. It would oscillate + or - 1 hz measuring the frequency at 1000 hz...With the laser, my table is stable at 33.00x rpm, where x fluctuates up to .008 every few rotations. I think that is quite good for a belt drive. I am going to try the platterspeed app for iPod to see how it compares and I am especially interested in the time plot.

Richardkrebs -
Interesting post. Could you please elaborate on a couple of points.

Contrary to some opinions expressed in these forums, the servo does not intervene unless an external load causes a speed change. ( this assumes that the TT has been put together as the designer intended).

the motor does slip back in phase due to stylus drag...This can be seen by scoping its power supply. And watching the current draw. It was actually possible to recognize the music being played at the time on the scope. The speed sensor was measuring these tiny errors and correcting. This could be observed by watching the motor draw current in sync with the music. An amazing observation.

The conclusion is that some form of speed correction is required if we are to seek good dynamic speed stability.

So as I read it
Para 1 - The servo only cuts in when speed changes
Para 2 - Motor slip is occurring as a result of varying stylus drag to a degree that you can follow the music by scoping the current draw
Para 3 - Your conclusion is speed correction is required for dynamic speed stability based on your observations.

These seem contradictory. Are you saying that most direct drives dont speed correct all the time, but that they should be correcting more frequently for variable stylus drag ?
Isn't speed correction for stylus drag shutting the door after the horse has bolted ? If the current draw is up AND down with the music, when do you propose to correct the speed UP and DOWN before, during or after the event ?
Doesn't this just suggest that for whatever drive system/platter/arm/cartridge you have it must have enough energy storage or overkill in terms of motor drive & inertia to ensure the variable stylus drag does not impact speed at a micro level.
Are you not presenting a strong argument that all TT's should have a very high mass platter ? ( unless of course you can scan the grooves and preprogram the required changes to the motor controller to preserve micro dynamics ).
The other question I have for you is torque ripple, or cogging.
As I understand it the torque ripple or cogging torque will vary because a Direct Drive motor is running much slower than a small pulley/platter.
For example
DD - 33.33rpm x 20 poles = 20 poles per revolution of record.
AC/belt - 1800rpm x 4 poles = 216 poles per revolution of record.
Disclaimer here - I know you also have to factor in the number of slots, which could increase or reduce the differential in this example, but doesn't eliminate it.
Cogging torque results in torque as well as speed ripple; however, at high speed the motor moment of inertia filters out the effect of cogging torque.
Presumably a high mass platter, provided it is direct coupled to the pulley with an inelastic belt, can assist in smoothing out any torque ripple if it exists in both instances.
Can you explain why I can hear such a big difference in speed stability between the Technics SP10mk2 and SP10mk3 ? Do you think it is the higher torque, superior motor controller, higher mass platter of the mk3 or all of the above ?

Cheers,

Dover

Dover, with my Salvation, correct speed is normally reached within 3 seconds, as opposed to the 10 seconds plus my belt drive did. And additionally, it takes a LOT of manual pressure to slow the platter down in the Salvation, a lot more than my belt Orbe. I'll take that as a high torque system. Certainly higher than any of the more expensive belt drives I auditioned. But I'm sure it's not quite in SP10 Mk2/3 territory.

Dover
Yes we are measuring an error and correcting it before it gets worse.
This is the old argument about feedback correcting an error that has already passed. The nature of this feedback and the torque/ platter moment are critical here. The motor must totally dominate the platters rotation. We are only asking the motor to correct a tiny change in speed. With sufficient torque and the correct feedback curve this is not an impossible ask.
I agree it has to go some way wrong for it to sense and thus correct. What we are preventing here is it getting any worse. Further, as I said the motor self corrects as well. This action being independent of the servo.
You said in one of your posts that you needed to correct for stylus drag when you tested with the timeline. Obviously the drag is not constant so we are ( in an open loop system) relying on inertia to save the day. The question is. Is this high inertia enough? (I wonder how quickly you observed the speed change after lowering the stylus)
Lets say in an extreme example we had a sustained high amplitude low frequency organ note lasting say 20 seconds followed by a flute playing very softly.
The platters inertia even a massive one would not be sufficient to maintain the correct speed during the organ note. Thus the motor would slip back in phase slightly to compensate for the higher torque demand. When the flute comes along the torque requirement would reduce. If the motor has high torque capability there is a risk of temporary over speed.

So the question of always needing a high inertia platter is better answered that the motor torque, platter inertia need to be matched to optimize speed stability. A platter of say 20kg has 8 times the moment of a 2.5 kg platter as fitted to the Goldmund I tested, assuming they have the same radius of gyration. The correction current I observed would indicate that a 8 fold increase in moment would not be sufficient to push through the drag modulation. Note high frequency correction was evident

BTW I was not arguing the superiority of DD over other drives. I was stating that stylus drag is real and significant and to improve the resultant speed errors, closed loop speed control is indicated irrespective of the drive employed or for that matter the platters inertia. It just happened to be a DD where I observed stylus drag in action. It is everyone's personal choice coupled with their biases when choosing a drive method and if open or closed loop. As I said earlier there are many paths to enlightenment and this should be celebrated. My opinion is simply that. My opinion no more valid than anyone else.

Clogging of course this is present in all drives employed
It is up to the listener to decide which is less intrusive

Clogging at approximately 11 hertz with the DD example you gave or clogging at approximately 120 hertz with the BD example,while acknowledging the lower amplitude of the BD.

As to the perceived superiority of the MK3 over the MK2
I can only assume that the engineers learnt something in the intervening period between the two designs. So probably all of the above. Further the general engineering in the 3 is superior. This has to have some bearing on the performance.