SDS or phono stage

Lewm, sorry about the typo. My iPad autocorrects Lewm to Lewn.

I have both the KAB strobe and PlatterSpeed app and Adjust+ disc. I found the platterSpeed to be more accurate and easier to use.

http://platterspeed.com/

Variac works quite well in my experience with my Gyrodec. It's a fairly serious bang for the buck as far as I'm concerned. No ability to control speed (as far as switching from 33 to 45 RPM) and it lacks the elegance and automation of supplies like the SDS but certainly no drawbacks in terms of holding actual speed.

Granted, the SDS probably does more (and should at the price) but HW is actually on record as recommending the use of a variac with certain AC Synchronous motors and the fact is that the variac does accomplish some of what power supplies like the SDS and Lingo do, namely reducing the voltage and, as a result considerably reducing motor noise/vibration resulting in a cleaned up presentation and lower noise floor.

The VPI SDS also reduces the voltage once the platter is up to speed. The end user can program the final run voltage.

02-26-14: Lewm
So an autoformer will not either block noise on the AC line or prevent tt motor noise from going back into the AC line. (I think this is true; perhaps Al will jump in and correct me, if I am wrong.)

02-26-14: Mcbuddah
All the variacs I have ever used or even seen have been of below par construction for audio use. I think most are used to control motors in certain kinds of factory production and OEM usaes in the same role, where critical adjustments are either not necessary or are supported with gauges or a servo mechanism. The speed precision needed to properly replay records far more important due to the sensitivity of our ears to even tiny speed inaccuracies. The KAB device used with VPI's SDS is capable of both setting and holding the speed steady, something else that the Variac likely does poorly in that it is set by mechanical means, whereas the SDS resolves in .01hz increments.

A Variac will provide some degree of filtering of high frequency noise, both ingoing and outgoing. The ability of noise currents to flow in response to noise voltages appearing between the AC "hot" line and either the neutral or safety ground lines will be constrained to some degree by the bandwidth limitations of the Variac, that limitation resulting mainly from the various inductive reactances that are present in an autoformer (or a transformer). The likelihood of that noise reduction being audibly beneficial in a given turntable application is anyone's guess, but personally I wouldn't bet on it.

Variacs are widely used in or in conjunction with test equipment for many electrical and electronic products, where testing requires precise setting and/or variability of the applied AC voltage. As everyone seems to realize, a Variac cannot change the AC frequency, as an SDS or other regenerator can, it can just vary the voltage. But, consistent with the posts by Bpoletti and Hdm, I would by no means categorically rule out the possibility of their being useful in some audio system applications. The leading exception to that being that I suspect their bandwidth limitations would make them unsuitable for use with power amplifiers or other components whose current draw fluctuates widely and rapidly.

Regards,
-- Al

Thanks everyone for the useful information. I got a bit of an education there. Mcbuddah, thanks for the tip about the ECPA. I had not previously heard of it.