VPI Analogue Drive System

melm is correct about the Sutherland.  It's a record weight with a strobe built into it.  The only commercially available add-on device that can really "control" the speed of a belt- or idler-drive turntable is/was the Phoenix Engineering Falcon or Eagle power supply combined with the RoadRunner tachometer.  When the RR is attached to the Falcon or Eagle, speed is held as close to constant as the workings of the turntable permit, to the speed 33.333 rpm, because the RR gives constant feedback to the Eagle or Falcon based on real time data streaming from the platter.  Sadly, PE is out of business, but Bill Carlin and some others on DIY Audio are making and selling parts that enable the DIY approach, and for much less money.  The SDS, the Walker Audio Precision Motor Controller, and many other similar devices allow you to set the speed precisely, but they cannot correct for variations in line voltage or current or stylus drag or bearing friction, all of which might cause at least momentary loss of speed stability, once you start listening to music.

lewm

... The SDS, the Walker Audio Precision Motor Controller, and many other similar devices allow you to set the speed precisely, but they cannot correct for variations in line voltage or current or stylus drag or bearing friction ...

This is not really accurate. The devices you describe work with AC synchronous motors; line voltage and current have no affect on speed, which is governed by the AC line frequency. (That's the "synchronous" in "synchronous motor.") AC line frequency is very tightly controlled by electric utilities, because deviations in frequency can unbalance power distribution systems. It's unlike voltage, which can vary widely without causing disruption in power delivery.

As for bearing friction and stylus drag - these are really only factors with lightweight turntables and platters.  Once the platter has sufficient mass, these variables have little to no influence on platter speed.

So it's kind of as I thought.  If the unit is left on all the time there should be little variation in speed from that point of view.  Stylus drag and bearing friction should also be constant.  Apparently the speed can be controlled with minimal turns of a dial. 
I was thinking along the lines of ramping the speed up or down in tiny increments to see how it might affect individual recordings which may too "bright" or too "dull".  I listen to classical music exclusively and many digital recordings sound "digital", just like CD's. 
Has anyone experimented along these lines, to make these recordings sound "better"?  Or am I way off kilter?

jerry95

... I was thinking along the lines of ramping the speed up or down in tiny increments to see how it might affect individual recordings which may too "bright" or too "dull". I listen to classical music exclusively and many digital recordings sound "digital", just like CD's.
Has anyone experimented along these lines, to make these recordings sound "better"?

I don't think adjusting turntable playback speed is a remedy to treat bright or dull sound.

cleeds,  Do you dispute what I actually wrote, in essence, that you can set platter speed with the SDS or the Walker?  I did neglect to mention that the motor should be of the AC synchronous type, but that wasn't really the question.  Most turntable motors fit this category, to one degree or another. Also, I don't know how you can say that current has no effect on platter speed, but frequency does.  "Current" has the property of frequency, which as you say does determine the speed of an AC synchronous motor.  I have read conflicting statements about the degree to which AC line frequency is tightly or not tightly controlled by power companies. Most sources say it's "pretty good" with small variations over short time that average out to 60 Hz over longer time periods.  Is that good enough for most audiophiles? I don't think so.

Jerry, Stylus drag is definitely NOT constant, by the way.  It's constantly varying across the surface of an LP.