turntable motor speed

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.

No. This is nothing more than a lab-grade highly regulated DC power supply. It's the motor itself that varies in speed, regardless of the accuracy or "purity" of the incoming voltage; you can put a $10K DC supply in front of the "best", most expensive DC motor and it will still drift in its speed stability - not a lot, but enough to be noticeable. Such is the nature of a dynamic load, like a turntable.

While the current-sensing option on the Acopian supply supply will work, my guess is that it will maintain the output voltage within a range of +/- 2 to 5%. Not bad for picking widgets off an assembly line, but insufficient for controlling a turntable's speed. You need something with an accuracy of around +/- 0.1%.

There aren't too many stand-alone DC controllers for turntables. The Origin Live box works OK, but once you pair the controller and the box together you're already looking at $600 or so. Oh, and Mark Kelly made a nice DIY kit controller as well...

So the problem is that it doesnt react fast enough to changes in load? I would be interested to see what actual fluctuations exist in load when turning a fairly heavy platter. I would think inertia alone on a 20lbs platter would smooth out any inconsitencies in load quicker and more thoroughly than any motor controller could. The motor itself wouldnt have enough torque to change a heavy platters speed that fast.

The above should have been 'once you pair the controller and the motor'.

No, the problem (for me) is long-term drift...such as the table running fast or slow up to 1 or 2% over the course of a side (we're talking simple voltage regulation-style control without a feedback loop). All those tiny errors add up. To me, the short-term, second to second errors of a DC motor are fairly benign, though certainly not insignificant when heard in the context of a good AC drive system, given its superiority of torque delivery, accuracy and so-called "PRaT" (and the spectral dividends that result from this approach). At least for me. YMMV and all that...

My experiences with various controller topologies for DC drive would imply a speed accuracy of around +/- 0.1% is achievable under ideal conditions.

Both systems (AC & DC) have inherent weaknesses and strengths; each listener has their own set of biases and expectations as well.