VPI Classic upgrade what any had the SDS, it is a

I have the SDS on a VPI TNT jr. The voltage and hertz from line voltage (wall socket) fluctuates. The SDS produces a non-fluctuating signal of constant voltage. This allows the motor to spin at a constant speed. The sonic difference is music with constant pitch. This impacts the entire frequency range and is very noticeable with smoother high frequencies and truer sound. It was well worth the investment at the used price.

Also, one of the biggest upgrades was the addition of the outer ring to couple the LP to the platter. See the other threads on TTWeights periphery ring.