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.
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.