Guru
Joined: 05/10/2019
Location: United KingdomPosts: 8955
| Posted: 06:26pm 08 Jan 2023 |
One of the main problems with turntables is vibration from the motor. People go to great lengths to minimise it, including mounting the motor completely separately and driving the platter using a long belt. That's not practical if the turntable is a mid-range one and already built. Most of these turntables use very similar motors, consuming about 1.8W.
The very cheap "suitcase" turntables, based on a Chinese copy of an old BSR combined platter and arm, almost all use the same motor. It's a little brushed DC motor with a chip mounted on it. It's speed stabilized by sensing the back emf. Unfortunately they aren't too good after a bit of wear.
Reducing the belt tension to the minimum helps, as does dropping the motor voltage. In both cases it's easy to go too far - the turntable will run fine once up to speed but it needs a manual push to get it started as there is insufficient torque. The system I'm playing with puts fiull voltage on the motor at startup and reduces it to a low level once it's running.
Stepper motors look ideal at first but most of them have too much vibration to be useful. (a 7.5 degree stepper can be made to run at 250 rpm from 50Hv AC with only a capacitor). Brushless motors are better but tend to need messy electronics.
There is a theory that DC motors can't run at the correct speed anyway. :) They depend on constant feedback in the loop to stat approximately correct and will always, to some extent, oscillate in speed. That's just something for the purists.