Turntable speed accuracy

Halcro, as I posted on 11/16 the SP-10 Mk2 manual states that table will maintain correct speed if up to 500 arms could be lowered simultaneously while tracking at 2 g. Even with an error range of 10%, it should be correct with up to 450 arms! From that, your three arm test was not much of a challenge. ;-)

However I suppose the problem with the Technics statement is the table could "maintain correct speed" with up to 1K gram weight applied. That could be different from not maintaining speed at the moment the weight was applied. In other words, should one allow say one revolution to correct the speed with this weight? That would be a big difference in sonic terms.

So if I'm understanding this, your laser mark should be measured precisely at the moment each stylus is lowered onto the record with no time interval to allow your table to correct for the added drag.

I found an article online about the Nak. Very interesting tt. The author had an excerpt from an interview with the designer. He substantiates my point exactly. He says vinyl record runout is the elephant in the room that tt designers ignore. As for inertia: Torque= J*omega, which is the angular term for F=ma. The tt motor provides the torque and the platter bearing and stylus apply a counter torque. The inertia of the platter determines the rate of change in speed (deceleration). Say for example the motor is uncoupled from the platter. The platter is spinning at 33.33 rpm. (ignore bearing friction for a moment) Now drop the stylus onto the record. A 20 kg platter is going to decelerate at a lower rate, for example, than a 2 kg platter.
Now let's hook the motor back up to our platter. The motor is either clocked to the 60Hz line frequency or is feedback servo controlled. So it holds the platter at 33.33 rpm. Any perturbation in the platter speed causes the torque output of the motor to change in order to restore 33.33 rpm. The motor could do it's job regardless of the amount of inertia in the platter. The stability of the platter speed is based on the control loop and torque of the motor combined with the system inertia. That means the designer has to couple a motor and platter as a system. The platter is designed to be a mass damper. We use mass dampers in dynamic systems. We use mass to tune System Natural Frequencies and keep them out of certain operating ranges. A bigger platter requires a higher torque motor in order to be stable. Perhaps the youtube example is a tt design with an undersized motor. I would say as a rule of thumb, the motor in a tt should be able to accelerate the platter up to speed within one rotation. To me that would indicate that the motor has sufficient torque to maintain a stable speed. btw- I just checked my tt and it is up to speed within one rotation.

Correct speed with 500 arms? That is not a helpful statement since we don't know what the definition of "correct speed" is. What matters is to reduce the effect of a single stylus to below audibility. I don't think that anyone has been able to quantify what that specification is. But there is ample evidence that extremely small speed perturbations are audible.

It is is misnomer to assume that a massive platter fixes the stylus drag issue. A heavy platter results in a smaller perturbation spread over a longer period of time. A light platter will slow more but will recover more quickly. A heavy platter changes the problem but does nothing to eliminate it. I think that most people prefer the shallow but longer perturbation from a heavy platter, but ultimately it's a matter of taste, not superior stability.

Dear Lewm,
ok didn't jump so far. Maybe I have a different / not typical understanding of footprint or better let's say character or personality of a turntable.
I know some audio gurus carry the philosophy of neutrality around the world, which should mean, the more neutral a turntable is the better for the reproduction. In their sense it is absolutely necessary that a turntable should not become musical - but neutral. A neutral turnable cannot be musical, warm or dynamical, or lively. It is and has to be just neutral. Everything else is coloration or distortion. Am I right? Okay. In this understanding I am happy to live with coloration & distortion - and I don't need to jump.

best @ fun only

"Mosin, as inertia is directly related to mass and friction.....are you saying that heavy platters have inherently greater inertia than light ones?"

What I'm saying is that sheer mass is less effective when it is capriciously used than when it is purposefully assigned to achieve an optimum moment of inertia. As far as mass on a platter, I feel that its benefit as an isolation component is another key reason to use it.

Of course, there are more ways than one to skin the proverbial cat, so lighter platters may also work with some designs. I work solely in the arena of idler drives, so that is where I spend my time looking at such things. Recipes vary, but the performance depends on the entity as a whole, and not just a single part because every part is critical to the outcome. Any shortcut is a compromise. At least, that's what I have come to believe.