In this series of articles I will be building a vinyl record player mostly from scratch. I will be using an off-the-shelf motor and a headshell, but otherwise build all audio and power amplifiers myself. I think this is a great way to demonstrate amplifier design and implementation.

Fortunately, the record only needs to rotate at a constant angular velocity, since the groove density is varied with distance from the center during manufacturing. If that were not the case, record players would require variable-speed motors whose speed would need to be controlled by the position of the tonearm on the record.

We can build a relatively simple speed controller using a DC motor with hall sensors and some feedback loop. Take hall sensor signal, pass it through low-pass filter. Take the difference between the filtered feedback signal and a set point, invert it and amplify it. Finally, use amplified error as control signal to drive the motor. This is a standard proportional control loop that can be implemented with discreet components.

Geared DC Motor with Magnetic Encoder Outputs - 7 VDC 1:20 Ratio.