VPI SDS....?

The SDS provides higher voltage on turn-on, then reduces the voltage to 72 volts. The turntable motor needs more power on start up, to accelerate from zero up to 33 rpm. After achieving 33 rpm, less power is needed to maintain speed (i.e., to overcome friction, etc.). The voltage is managed so as to meet these power requirements. VPI apparently believes there are benefits (either sonic or better motor longevity) to using lower voltage when possible.

The voltage, by itself doesn't determine the correct record speed. Rather, it is the frequency of the voltage, 60hz for 33.33, 81hz for 45 rpm in a perfect world. What the SDS allows you to do in combination with a strobe disk and neon light, is make minute adjustments in the frequency of the voltage to get the platter speed exact, usually requiring a small variation from 60 and 81hz respectively.

The SDS drops the voltage after startup in order to reduce motor noise. The higher voltage is used at startup to get the platter up to speed quickly. Hope this helps.