speed stability

Does anyone know if using a device like a PS Audio power regenerator accomplishes the same speed stability goal as using the VPI SDS controller? I have a VPI Classic turntable and run it, along with the rest of my audio components (except the power amp), through the P5 Power Plant. Since one of the goals of the P5 is to deliver a nearly perfect 120 volts, is an SDS superfluous?

The reason for using PSA, SDS or any other regenerator isn't voltage regulation but frequency control. Frequency control allows you to control the speed on tts without that capability, like your Classic. You're going to need a strobe and a disc to measure the speed of your Classic. I'm not familiar with the PSA units, don't know how flexible they are with frequency regulation.
The PS Audio devices are limited in their capacity to alter AC frequency. They were designed to supply regenerated power for any audio gear. However, if your tt is running at correct speed direct off the PS Audio, there may be little to gain by inserting an SDS. Not nothing, but "little" to gain. Unless the PSA is dedicated to the tt, you probably don't want to alter the frequency of its AC output above or below 60Hz, in any case. That might be bad for the other gear.
The spped contol of most TTs is based on teh FREQUENCY ofthe incoming AC. (as others have mentioned)
If you want the exact 60 cycle then yes a PS audio type regenerator may improve the AC frequency control.
But with no testing done it is hard to say if it really does better than the line itself? or better or worse than the average TT speed control.
The most accurate TT speed control are built in ones with a feedback servo.
Like Denon and Technics.
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Benjie, SDS & PSA are exactly the same thing, both are digital regenerators! The only difference might be that the SDS will allow finer tuning of frequency adjustment to control speed of synchronous motors. I'm not familiar with the current ones but earlier PS Audion units did have a certain amount of frequency control too.

"By reducing the voltage to the tt motor, the motor produces less vibration and makes a significant improvement in sound reproduction. Hey, we all know that less vibration to a turntable means better sound. The SDS is a truly amazing piece of electronics." This is completely false and is in the same moronic category of using expensive power cords on tt motors and claiming it improves sound quality. There's nothing amazing or revolutionary here, the SDS is just another AC regenerator and at the 2k price there are plenty of more featured industrial units around. Only the interface is audio specific.

Elizabeth, a steady 60hz cycle doesn't guarantee the correct speed of your tt. A frequency controller is a pitch/speed control for synchronous motors and allows one to set the correct frequency to achieve the right pitch for 33/45/78 rpm if your table is setup for it, not every table will give you all 3 speeds. Servo controlled motors, direct or belt driven aren't the most accurate and further servo control has negative impact on sound quality while continuously tweaking the speed.

Lewm, tweaking the frequency slightly above or below 60hz has no effect on electronics, just motors. In general you really shouldn't use a regenerator with your electronics because of the negative impact they have on sound quality. Some manufacturers do advise against using any regenerator with switching power supplies, irrespective of frequency control.
I used a PS Audio P300 to power the motor of a VPI Scout in the past. My experience is exactly summarized by Dkarmeli's post above. While it didn't provide the finer increments of frequency tuning the SDS did, it was good enough in my case. I heard no deleterious effects on preamp and cd player (also powered by the P300) by altering frequency 1-2 hz.
Thanks to everyone fir their input.

Dkarmeli: I had question about you comment regarding using electronics through a power regenerator. I originally purchased the PS Audio piece to protect the preamp, cd player, turntable, tuner, etc.

I knew it didn't have the ooomph to protect my Audio Research VT200, but I assumed the other components would be more stable as well as protected by the regenerator. Watching the scope on the PS Audio, I can see how much better the THD is as well as the voltage. But I had never heard that using such a device can be deleterious to the sound.

What is the reason for this? Thanks in advance.
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+1 Benjie. The SDS definitely does make a difference. I recently had to remove it from my system to have it serviced in the middle of a listening session, and although the speed of my Classic was still incredibly stable and accurate without the SDS, the sound lacked the transparency and the background quietness (transparency most likely affected by the decreased blackness of the background) I had with the SDS powering the turntable. My brain adjusted to the sound after a few days, but the decreased voltage allowed by the use of the SDS and reduction in vibration does result in a quieter table and ultimately better sound. .
Wynnosu, I'm not sure where you live that you would need protection for your equipment. The PS Audio regenerators are sold for cleaning your ac and in theory making your system sounding better. In my experience with the PSA along with quite a few other regenerators the opposite was true. Hence my comment regarding the PSA. Also all regenerators dump a lot of noise back in the lines and you have your amp plugged in that line. Of course depending on the equipment and location your mileage might vary. Interestingly I am testing a new regenerator that is showing potential and might be the exception to the rule.
Hi Benjie, I never said that SDS or other speed controllers don't affect sound, of course they do. A stable speed with the correct pitch will have a positive effect on sound. And I agree that motor induced vibrations can have negative sonic effects. My comment was about reducing motor vibrations by dropping voltage. What are they dropping the voltage to? Does the SDS ramp up to over voltage and then drop down to the correct voltage or does it drop down to some nominal voltage like 40 volts?

Don't know about your TNT but none of my tt motors will work properly with significantly lowered voltage.

The SDS will drop the voltage to as low as possible to maintain the necessary speed and frequency (user controlled). It does have a minimum specification is always reached: 72volt, if the motor is working properly.

You can see the unit change upon start up from 115V to the 72 volts at 33 rpm. At 45 rpm the voltage is 83V as I recall.

The user has no control over voltage, just frequency - which is set to a strobe such as KAB's.
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Rodrigal and Benjie, thanks for the update.