Elizabeth's reply is not accurate. Turntables that use DC motors also use a voltage regulator to hold the voltage seen by the motor rock steady. The line voltage may go up and down but the DC voltage seen by the motor will remain constant. Not only does the regulator help to isolate the motor from line voltage variations, but it is needed anyway to adjust the platter speed to get exactly 33rpm or 45rpm.
A DC motor can also be powered by a battery so it is totally immune from line voltage variations and contaminations. That is how my Galibier Gavia operates. It is powered by a big 12v battery designed for jump starting automobiles.
The bottom line is that it's possible to achieve very high performance with both DC and AC motors. The choice between the two is one for an experienced turntable engineer to make, not armchair designers.
