Turntable speed accuracy

Rockitman,

I have been doing almost the same. Card as test LP 1khz tone through my Prism Orpheus into my MAC Mini. I see it on the screen. You can see the stability of the signal and all. And I can hear the differences as well.

Here are some of my own observations and hypothesis:

1) Regarding how important platter mass is for speed stability. IMHO you cannot assume a higher platter mass is better independent of drive mechanism. In other words, the platter mass and drive mechanism need to be considered together. Picture a 100 lb platter driven by a wimpy motor. The motor needs to be able to keep the heavy platter spinning at speed. Assuming it can do that, any deviation would be difficult to compensate as the inertia of the platter would work against the wimpy motor.

2) Seems to me, resonance issues aside, the energy of the drive mechanism is much more important. That is why I think high torque motors and flywheels driving platters makes sense. The more powerful the mechanism the easier it is to keep a light or heavy platter moving at speed. Therefore, a TT should be designed with drive mechanism energy and platter weight in mind.

3) My understanding is that AC motors run at constant speed by default, whereas DC motors do not control speed per se but torque. That’s why most DC motor driven TT’s slow down with stylus drag. However, most AC motors, having sufficient drive energy fair better. I suspect that DC motor driven TT’s with large flywheels (e.g. Micro-seiki HS-80 and the Forsell) sound better because the flywheel adds drive energy and compensates for the DC motor instability on its own. Seems to me that a TT with AC motor with high torque or added energy through a flywheel has some advantages to DC motor, with higher motor torque, but without s flywheel.

Just my two cents

Andrew

I would like to qualify my statment above:

"That’s why most DC motor driven TT’s slow down with stylus drag. "

I'm referring to DC-motor drives TT's with an open-loop control system. This means that the actual speed is not compared to the set-point and adjusted for mismatches.

I would also add that TT's using multiple motors closely spaced toghether (e.g., TW Black Knight)help increase the tourqe of the drive system.

Andrew

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?

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