turntable speed control

To me a a constant stable speed is always better than a changing one.

Agreed.  But constant motor speed does not equal constant platter speed, it's basic physics.  Belt creep does exist and it is measurable.  With an elastic drive belt, it cannot be cured, but the effects can be mitigated by holding the platter speed constant as we do.

By your own assertion, a constantly changing speed could affect the audio.  A table driven by the SDS will have constantly changing platter speed for the first 30-45 minutes;  it will either be off-speed (slow) initially then correct after 40 mins, or correct speed when cold, then off-speed (fast) after that. 

With  the Eagle PSU and  tachometer feed back, the platter speed will always be constant, therefore, it will sound correct both when started and after the table warms up.

Thanks for helping make my point.

phoenixengr   "Agreed. But constant motor speed does not equal constant platter speed, it's basic physics. Belt creep does exist and it is measurable. "

That really depends on the turntable.

" A table driven by the SDS will have constantly changing platter speed for the first 30-45 minutes; it will either be off-speed (slow) initially then correct after 40 mins, or correct speed when cold, then off-speed (fast) after that."

Again, that depends on the turntable. It certainly is not an issue with my VPI and, yes, I've measured it.

It certainly is not an issue with my VPI and, yes, I’ve measured it.

May I ask what you used to measured it? My VPI table changes speed considerably from cold to warm and so does every belt drive table I’ve measured. If you have the first belt drive table that doesn’t change speed over time, then congratulations.

If a particular turntable platter truly doesn’t change speed over time, then the argument is moot: The Eagle would not apply correction so there would be no perceived issue either from "constant correction" or constant drift. End of problem.

As far as the rest of the tables that do drift over time, would you agree that constant platter speed is better than a slow drift upwards in speed?

phoenixengr " May I ask what you used to measured it? My VPI table changes speed considerably from cold to warm and so does every belt drive table I’ve measured. If you have the first belt drive table that doesn’t change speed over time, then congratulations"

I use a DIGIstrobo to measure turntable speed. It's very precise, although I find you can't use it hand-held if you want a really exact measurement.

This notion of off-speed turntables until after warmup is rather odd. Not only doesn't my VPI have this issue, but neither did my previous turntable, which was an Oracle Delphi Mk. III. (To be clear, I didn't have the DIGIstrobo when I owned the Oracle. Back then, I relied on a strobe disc to ascertain speed.)

You've claimed that " belt creep does exist and it is measurable." That's another rather odd claim. How have you measured this phenomenon? How can you be certain that whatever speed issue you may have detected is attributable to "belt creep?"

You’ve claimed that " belt creep does exist and it is measurable." That’s another rather odd claim. How have you measured this phenomenon?

Are you denying that belt creep exists? That’s not only an odd claim, it’s counterfactual. Without belt creep, elastic belt drives could not transmit torque to the platter; it is undesirable, but necessary for these systems to work.

AC synch motors operate at 2 different speeds: 100.00% of rated speed determined solely by the drive frequency or 0% when the cogging torque is exceeded and the motor stalls. There is nothing in between. If you even lightly touch the platter while it is moving, it will drop in speed, yet the motor maintains its speed at 100%. How do you account for that if there is no belt creep, a phenomenon that is well documented in physics?

It’s trivial to measure the difference in platter speed caused by stylus drag (if you have the right equipment i.e. a tach with 3 decimals of resolution; a hand-held tach with 1 decimal is a poor indicator, strobe discs are even worse). The difference in drag at the beginning of a record is even different than when the stylus is closer to the spindle. Anything that creates drag on the platter, including changes in bearing friction, will increase the torque demand on the drive system, increase belt creep and reduce platter speed.

If I let the platter speed stabilize at 33 RPM for several hours where little or no correction is applied, switch to 45 RPM for even one side of an LP, then switch back to 33 RPM, the speed will be 0.15-0.2RPM higher while using the exact same frequency as when I left 33 RPM. The difference is due to changes in the belt and bearing.

I’d suggest you do a little more research before you decide to debate how these systems work. You seem to have little understanding of the physics involved, yet you "know" so much.