turntable motor speed

All DC motors used for such a purpose have a tachometer and feedback regulation circuit that controls the motor voltage. It's most commonly built right into the motor itself (just like a cassette-deck capstan motor) but occasionally you'll find it as a circuit external to the motor.

Simply regulating the value of the input voltage will not keep a DC motor at an exact speed.

Ok, so what does the feedback regulation do to the motor to change its speed. Does it change the voltage, and if so, how? more for faster? I mean, its fine to say that a circuit regulates the motor, but my question is how does it do that. The motor only has two wires that feed it, so something in those wires is telling it to speed up or slow down. What is it? voltage?

Yes, the change in speed in a DC motor is directly proportional to its voltage input: more volts = more speed; less volts = less speed. A 24VDC motor typically exhibits its nominal operating characteristics (speed, torque) when operating at its rated voltage. Too much voltage and the motor will be destroyed; too little and the motor will stall.

The regulation can take on many different approaches: since most brushed DC motors have mediocre speed stability, constant compensation (closed-loop feedback) is a must. This compensation can take the form of a tachometer which measures some point of reference as it moves by (either on the motor itself, or the thing (like a platter or pulley) the motor is driving) and adjusts speed according to how far off the from the reference mark the resulting measurement is - more points of reference equates to higher levels of accuracy. Another form of feedback measures the average current draw of the motor, and compensates for any deviation from this ideal. There are several other types of feedback mechanisms as well.

Brushless DC motors behave much like multi-phase AC motors, and require substantially more complex drive and feedback circuitry to accurately 'time' each phase.

Given the complexities of DC motor control, one can understand why synchronous AC motors remain a mainstay of turntable manufacturers despite some of their drawbacks.

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.

there are a number of fancy dc voltage converters that contain an input to run a wire back from the motor to the converter to ensure that the rated voltage is maintained. Would these converters be useful as a power supply for a turntable motor. I have used one as a Sota power supply, but have not attached the wire back to the supply. this is what I am talking about (Acopian B24G210 Power Supply). Some of these supplies are more than $500 new.