larryi
... there is not much you can do about the frequency and any variation would have little effect--it would really only affect a turntable with an AC synchronous motor ...
Line frequency is pretty tightly regulated by the NERC, in part because the stability of the electrical grid requires it. Although the line frequency is accurate when averaged over time, there can be momentary deviations. Turntable speed boxes such as the VPI SDS regenerate their own current with voltage and frequency tightly controlled.