In addition to Jimbo3's points there's this crucial one: all AC motors have cogs. At each cog the motor gives the spindle a little push. Between cogs the spindle just coasts along, losing speed to friction of course, until it reaches the next cog. IOW, no AC motor ever spins at a true steady speed. They all proceed in an endless series of pushes and coasts. Clearly this is less than ideal.
Cogging effects can be reduced by having more cogs, by using a lower torque motor and by isolating platter from motor with a stetchy drive belt. But these are bandaids. In the end no AC motor can match the inherently stable velocity of a DC motor, and a stretchy drive belt opens the door to stylus drag that a dimensionally stable drive belt or DD mechanism would play through much better.
DC motors have their own set of problems of course, but they are easier to overcome without audible compromises. Read the motor information on the Teres website for a description of the best solution I know of for TT's under $10K.
Cogging effects can be reduced by having more cogs, by using a lower torque motor and by isolating platter from motor with a stetchy drive belt. But these are bandaids. In the end no AC motor can match the inherently stable velocity of a DC motor, and a stretchy drive belt opens the door to stylus drag that a dimensionally stable drive belt or DD mechanism would play through much better.
DC motors have their own set of problems of course, but they are easier to overcome without audible compromises. Read the motor information on the Teres website for a description of the best solution I know of for TT's under $10K.