turntable speed control

SDS is better as once you set a speed it is locked to that. Phoenix corrects speed so it would be constantly changing which could be audible.

This is a common misconception and is actual the opposite of reality. The SDS does not change its output frequency once it is manually set so the speed of the motor will remain the same, however the platter speed will constantly increase 0.2-0.4 RPM as the belt and bearing warm up over the first 30-45 minutes of play and it is audible to many listeners.

The Falcon/Eagle PSU does not correct "constantly"; there is a window of error that the PSU responds to (only if connected to the tachometer). When the platter speed drifts outside the window, the PSU applies extremely small corrections** evenly over the next revolution to keep the platter on speed. The correction is not audible. Once the table speed is stable, it is not uncommon to see 20-50 revs without correction, but if the PSU did not correct, the speed error would build up and it would be audible.  Of course, if feedback is something you are opposed to, then don't connect the cable between the tach and PSU and adjust the speed manually.

** The SDS uses a PLL circuit and the output is constantly wobbling a minute amount. The amount of correction the Falcon/Eagle PSU applies is smaller than the constant wobbling of the SDS:
https://www.youtube.com/watch?v=QOs7oB8N9Zs&nohtml5=False

The Phoenix does no AC regeneration, or lower the motor voltage after startup like the SDS. More noise.

This of course is also incorrect. The Falcon/Eagle are AC regenerative power supplies and they use DDS technology to create AC regeneration which has much lower distortion and noise as well as being hundreds of times more accurate than the PLL circuit the SDS uses (see link above).

The Eagle/Falcon also has reduced voltage output which is front panel programmable.

The Eagle is capable of 230VAC output so it works with tables worldwide. It will also work with 50Hz motors where the SDS does not. The Eagle/Falcon works with the RoadRunner tachometer to automatically correct for long term drift which the SDS cannot.

The Eagle also delivers more output power than the SDS:
https://www.youtube.com/watch?v=tt9AuTg3QXA

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?

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.

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.

When phoenixengr asked how I made my measurements, I responded and the answer is above. But when asked how he substantiates his claim that " Belt creep does exist and it is measurable," he becomes argumentative. Of course, he’s trying to sell something here, and I’m not. Perhaps that accounts for the difference in our attitudes.

Wow, you managed to get just about everything wrong on that one; from your straw man argument about my sales motive to the lack of understanding of even basic physics. You deny that belt creep exists, yet you can blame speed variations on "some other" mysterious force? If you reread my post above, you will see that I did answer your question. Belt creep manifests itself as reduced speed, and I measured it with a digital tachometer. But even simpler than that, you can do as melm has indicated and just listen when you touch the moving platter while a record is playing.

Not incidentally, the question isn’t only, "Is it measurable?" but, "Does it create an audible effect?"

Yes. See above.

A light touch to a 22-pound platter doesn’t effect speed - the force of
the touch is insignificant compared to the moving mass of the platter.
But phoenixengr already knew that.

You are wrong on both accounts. Inertia will only affect the rate at which it slows, but it will slow down. And, I know the opposite of what you said to be true. Apparently you not only know what you know, but now you claim to know what I know.

phoenixengr, your products are highly regarded by many in the audiophile community. I’m sorry to point it out to you, but you’re really not helping yourself with the illogic you’re using here.

There is nothing illogical about my posts. Everything I've posted about the operation of belt drives is correct and easily verified not only by application of simple physics, but also by empirical testing (listening) and simple measurements.

I guess what's at issue here regarding speed deviation is what defines a "light touch" and, of course, variations between different turntable platters. I can certainly slow down my VPI platter by touching it - I can make it stop, for that matter.

Semantic arguments aside, if you put a digital tach on your 22 lb platter you will see that the speed slows down even when the stylus makes contact with the record (I'd consider 1.75 grams to be a light touch).

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?

You still have never answered this question.

+1 stringreen!

I would add that another problem is some people can’t debate issues using facts and data, so they try to impugn someone’s character with innuendo:

Others have other goals.

and try to make their point with straw-man arguments:

You’re funny, phoenixengr. I certainly never stated that belt creep didn’t exist. To refresh your memory, here’s what I wrote:

" How have you measured this phenomenon? How can you be certain that whatever speed issue you may have detected is attributable to ’belt creep?’"

What you actually preceded that statement with was this:

You’ve claimed that " belt creep does exist and it is measurable." That’s another rather odd claim.

I took that to mean you questioned the existence of belt creep or that it couldn’t be detected or measured. When I asked you if that was the case, you didn’t respond, so I proceeded with that understanding (you still haven't denied it).

I see no other purpose to any of your posts other than to start an argument, a rather simple and tiring one at that, and one that I’m done with.