SDS or phono stage

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.

At least now I know that I can blame autocorrect for converting my moniker to "Lewn". No one should think I am at all angry about it. It's been happening ever since I started posting under "LewM" on Audiogon. I find it amusing, in fact.

Arsh, from all I have ever read about it (and that's all I know about it) the Herron phono stage you have purchased will likely make you very happy for years to come. I think you've made a good choice.

Thanks, Al.

Thanks again. I hooked up my sds and set speed using the kAB. Question: do you leave the sds on all the time and turn the turntable motor on and off, OR do you leave the tt motor on all the time and use the sds to turn it on and off? Thanks again.

Motor always on, and use the SDS to control the on/off. If you do it the other way, the SDS will not start at full power and then ramp down.