Turntable speed accuracy

I have just tried the Fieldpiece SRPM2. It is a laser digital tachometer. It works by pointing a laser at a piece of tape on side of the platter, or any other rotating object in other fields. It purports to have a high level of accuracy and it can be found on Ebay for about $75. I thought this would be an inexpensive device and more accurate than a strobe disk as it goes to three decimal points. The other advantage is that one can play an entire side of an LP while doing the test.

Well, I found it quite disappointing. The reading seemed accurate at first: 33.436. My table speed is finely adjustable so I thought I could just get it to read 33.333 by adjusting the motor controller. 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.

So I am going back to the KAB strobe disk which indicates that my table is operating at the correct speed. Perhaps this device is accurate enough for HVAC technicians and people working with motors who need to know RPMs or the total number of rotations from some equipment, but I plan to stick with dedicated tools for turntables. I just can't convince myself to spend the $400 for a Timeline.

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?

_______

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.