Hi Mark. I'm just guessing here, but it seems to me that theoretically the power cycle tweak is to make up for slight deviations in equipment geometry from ideal and the voltage tweak is to provide enough power for a steady speed. I.e., if the voltage is too low, the platter won't get enough power to spin at a constant rotation. In the limit, the difference in the amount of needle drag between loud passages and quiet passages would be enough to cause wow distortion, despite the smoothing effects of a weighty platter. My guess is that the VPI factory settings imply that 72 volts is sufficient for no record and no needle, and 84 volts is enough to overcome essentially all effects of varying needle drag. (Of course, there is always the secondary effect that needle drag may cause some change in geometry -- belt stretch for instance -- that would require a power cycle adjustment also.) In any case, unless there is a strong reason to cut the record playing voltage to 72 volts, maybe keeping it at 84 volts would be prudent.
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