Speed accuracy- how accurate is accurate?


Certainly, running at 33.333333333333 . . .  with a stylus in the groove is ideal. Running an iPhone app I just discovered (RPM), I'm running about 33.55 or 33.56. Table is an LP12 with an 18 year old Lingo. Everything sounds OK- is it ok? 
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Showing 2 responses by lewm

I think you and I have gone back and forth on this subject in the past. The issue with the voltage going to the turntable motor could be of two sorts. In one case the motor can directly radiate EMI which can affect the cartridge. This would mostly be a problem with direct drive turntables where the motor is installed right under the platter. In the second case, the AC interference from the motor goes back on the AC line and goes to your other components via The AC line. This latter problem can easily be avoided by isolating the AC supply to the turntable from the AC supply to your other components, particularly your phono stage, line stage, and amplifier. Reducing the AC voltage supply to the turntable by as much as you suggest, going from 120 V to 75 V, can have a deLeterious effect on the performance of some motors. Because at some point the torque will drop dramatically.

I also kind of disagree with the idea that the speed accuracy of the turntable needs only to be as good as that of the cutting lathe. Because if the turntable speed constancy is poor, then you are adding an additional source of error to the speed at which the record runs. This error in the turntable could be additive with the error of the cutting lathe, in which case you are losing some fidelity that would otherwise be obtainable.
By most accounts, the iPhone method is not the most accurate way to measure platter speed, but it puts you in the ballpark. So I would not conclude that the speed error of your turntable amounts to the difference between what you measured and 33.33 RPM. It might be more, or it might be less.