Turntable speed accuracy

Peteayer: "Well, I found that the Fieldpiece readings were not repeatable and by just moving the device closer to or further away from the platter, the reading changed, though I know the platter speed remained constant."

Can't you hold the the tachometer steady or clamp it to something so it would not move?

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Yes, and I did. I guess I was not clear enough. I placed the Fieldpiece on a stand and took the reading. I then took a second reading from the exact same spot and they were not the same. I then tried about three different locations each at a slightly different distance from the reflective tape on the platter, and at different angles and these readings also did not match. They varied +/- 1 RPM, ie 33.845, 34.352, 33.908 etc. Each time the device was fixed and never moved. I had the KAB strobe on the platter and it did not waver, so I assume the platter was not changing speed. The stylus was not on a record and I did not adjust the speed on the motor controller.

For such a device to be useful, I would think that any reading from any distance within say 2-5 feet from the platter should be extremely close if not exactly the same. The point is that without getting consistent readings, there is no way of telling exactly how fast the platter is spinning. My conclusion is that for me, this device failed the test.

If anyone would like to buy this mint condition device for perhaps a different use, please contact me.

Dear Peter, I hate to be a pain in the arse, but you wrote, "I found that the Fieldpiece readings were not repeatable and by just moving the device closer to or further away from the platter, the reading changed, though I know the platter speed remained constant." How in fact do you know the platter speed remained constant? I think you are going to say that you know it by virtue of the KAB strobe, but I have been told ad nauseam by others who use both that the Timeline is more sensitive to very small aberrations (like the ones you report) than is the KAB. (I must admit that when I compared a borrowed Timeline to the KAB in my home, using four turntables, I got the same impression.) This says nothing up or down about the Fieldpiece, however. Does the Fieldpiece strobe plug into the wall socket? If so, there could be its Achilles heel. There is a slight variation in AC line frequency, which is why KAB use a battery-powered strobe.

HI Lewm, the Fieldpiece is powered by three AAA batteries. I will accept your proposition that I don't know for sure that the platter remained at a constant speed during the trial. I do know that within the resolving capabilities of my eyes, the numbers on the KAB did not move relative to the leading edge of my headshell which was placed over the numbers when I did that test for a period of 60 seconds.

I also know that I could not repeat a reading using the Fieldpiece when I took separate measurements within 10 seconds of each other. The unit was not hand held, but placed on a platform. The location remained constant (perhaps not precisely the same, but one that seemed to me to be the same). If one reading is 33.845 RPM and the next reading is 34.520 RPM taken from the same location, that is a variation of about 2%, if my math is close. I have confidence that if the platter speed varied by as much 2% within a few rotations, that my eyes would notice that by looking at the KAB. According to the KAB instructions (6.5 drifts per minute = 0.2% too fast or slow), I was easily able to notice a speed variation of 0.2%, 1/10th of the variation indicated by the Fieldpiece. I did not see this variation with the KAB.

I also presume that my ears could detect a speed variation of 2% as indicated by the Fieldpiece. I am able to hear a speed variation of 0.2% (based on the KAB) which was the case with my old turntable. So I agree with you that I can not know that the platter speed is constant with the KAB. But based on my observations of both the KAB and the Fieldpiece and assuming that my math calculations are correct, then I must conclude that my particular Fieldpiece unit is not very accurate, at least relative to the KAB.

I do know someone who has a Timeline. Perhaps I can ask him to bring it the next time he comes by for a listen. We could then do a direct comparison between the Timeline and the Fieldpiece on my turntable. That will tell me something about the Fieldpiece and my turntable. Or, I think he has a DD turntable and I should take the Fieldpiece to him for testing.

After reading the posts, I thought it would be helpful to add my experience with a turntable design that is very unique (at least as far as I know).

I have been working for 4+ years on a turntable design that came to fruition recently. The turntable is a rim drive turntable with a super massive bearing (the largest built, as far as I know) and a massive platter. The theory behind this design was to have very high inertia to overcome stylus drag derived speed variations.

I can tell you with great confidence that the design/theory does work as intended. The high inertia platter/bearing combination brings forth drive and rhythm like no other turntable that I have heard. It is quiet amazing.

That said, the turntable is not perfect, as the rim drive motor speed variations need to be tamed. Speed accuracy is not a problem, but minute variations in speed is an issue for now. I have been working extensively with different rim motor solutions, including rim wheel materials, sizes and durometers. Each has there own sonic signature and pros/cons.

IF the motor speed variations can be tamed, then I believe the current design is unmatched. BUT, I am still seeking a better motor solution that I currently have.

Here is another finding that I came across: if the table has a stable the motor speed and has the ability to rip thru transients and not be adversely impacted by stylus drag, then you will discover that your next culprit is eccentric record centers. Wow/flutter derived from eccentricity in the LP will become more apparent/audible.

This is just an FYI and thought it might help in your thinking.