VPI Analogue Drive System

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.

I moved my motor drive belt up to the top slot on the motor drive pulley which effectively lowered my 3150 test tone from 3160 to 3149 on my PlatterSpeed app.  There was a significant improvement in sound quality with all my MC cartridges (I have 4 of them).  This obviously didn't cost me anything and I should probably have done it awhile ago, but I've just started paying attention to this tweak.  So far I'm not running out to buy anything yet.