I've been wanting a PLC/SDS for some time to add to my (admittedly) low-end VPI HW-19 jr turntable (with Shure V15-VMR). After doing some Internet research, I concluded that the PLCs were too old to be reliable and that the circuitry of the SDS was far superior. So, I acquired a new SDS. There's two main reasons to purchase an SDS: 1) to get true 33-1/3 and 45 speed on your table, and 2) to overcome speed variations caused by noisy power. Other advantages are 3) no need to move the belt to get 45 RPM and 4) compensates for pitch variations as desired. In my case, I found that my table was running a bit slow off of the wall power. VPI ships a nice speed calibration disk to use with a florescent light.

A synchronous drive motor locks into the 60 hz cycle and uses that to regulate the speed. The SDS completely regenerates the current at a cycle output of your choosing (one for 33 1/3 and the other for 45). It remembers your choices, once set. You must have a synchronous motor to take advantage of the SDS.

I suspect, despite its price, that the SDS will do better speed regulation on low-end tables than a high-end table, assuming that the high-end tables already have better speed regulation (more platter mass, better motor assemblies with additional fly-wheels, etc.). That is, you'd expect folks who have already spent $$$ to also purchase an SDS (~$995), but I suspect the results will be more dramatic on tables with simpler/lighter drives/platters.

In any case, I've been re-spinning vinyl with the SDS in place. I haven't conducted a blind A/B test, but I'll say that the image stability and depth/width while listening to Michael Jackson's Thriller has markedly improved into the goose bump range. I'm hearing a more believable image and also hearing some micro details I'd never caught (especially on the title track).

The SDS is rather large, but easy to use, runs fairly cool (but should not be in an unventilated enclosure). The readouts are large/legible. Device is intuitive.

In summary, if you suspect you have noisy power (by that I mean unstable frequency power) and want fine-pitch control on your table, the SDS is a home-run. If you have a high-end rig with lots of platter mass and other speed stabilization features and don't want and/or need the pitch control, then I'd question the need for the SDS....of course, "need" is the operative word. You know you want want anyway, right?