I'll probably stick to an open-loop voltage-mode control of a dc motor. And heres why: There is a big issue with the speed controlled type - feedback. You get a servo circuit that is constantly chasing its tail.
This is the classic problem with the direct motor tables from the 70's. The feedback loop was in action, more specifically a PLL, that had minor instabilities in the audio band. Think of it this way, you have a control loop bandwidth dead nuts in the middle of your vocals. Speed up, slow down, speed up, slow down. The angular velocity is always hunting. You can play games and slow response down. You also have a horrible time closing the loop on an unkown platter inertia. Not to mention an elastic belt.
Making the platter super heavy will help, but the problem is always there. Get rid of the feedback, and you will get a more constant torque. Only modulation now is from stylus drag. Yeah, it changes too. But if there is one thing I've learned in audio, it's that feedback is not the best way to cure sonic ills. Pure open-loop linearity is.
A combination of the two may work. The PLL (integral) correction could be made to be subsonic. That way you would be guaranteed a long-term speed stability over time. A non-feedback system will drift over time as the bearings and motor warms up. The temperature of the copper in the coils will change the current draw and hence torque (hotter is weaker).
What we have learned, is that a super-stable dc voltage to the motor sounds the best. It just happens to drift a bit over stime and temperature. Which is the lesser of evils? The side benefit is that you have infinite control of speed. You want 33.6rpm? No problem. I guess this is analogous to tube electronics. You have to let them warm up to speed before they sound their best.
So we have our choice. Do we want the platter to rotate exactly 2000 times in one hour? Or have it sound good? It's like THD disctortion figures in transistor amplifiers. We audiophiles are a breed apart, and I think the answer for us becomes pretty clear.
jh
