All DC motors for TTs will have by necessity some form of feedback to control their speed. If a TT has a DC motor it will come with a dedicated speed controller. AC motors "synchronise" with the frequency as has been dicussed. That is not the only reason for a controller. In an ideal world all loads would be purely resistive and the AC waveform would not be distorted. With compact flouresce lights and AC motors and other reactive loads adding distortion, throw on top of that the typical PSs that draw most of their current at the AC peaks which tends to flatten the peaks, the AC waveform coming from the wall is hardly a pure sine wave. This will effect the sound of a AC driven TT. Even if your TT ran dead accurate, there is still a case for an outboard controller.