Most dc motor come with a controller which varies the voltage output to achieve the desired speed. The better the quality transformer and controller the better the speed stability (see Origin Live DC motors). A few of the higher end tables with a dc motor (Linn LP12 Radical) have a sensor that is installed under the platter and monitors the platter speed and feeds back results to the controller to either increase voltage or decrease voltage based on the RPM.
turntable motor speed
I understand that an AC turntable motor uses the incoming line frequency of 60hz to set the motor speed, but how does a DC motor determine speed and how do you vary the speed to adjust for changes in platter or pulley size. Is the voltage changed and therefore the speed is controlled by the voltage or is there some other method at work. For instance, if I am running a 24vdc motor, would increasing the voltage to 25vdc cause the motor to run faster? Is it important to ensure that the voltage is exactly 24vdc, or is anything close acceptable?