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In Reply to: Yeah, I looked at that but ... posted by MaxwellP on April 29, 2007 at 09:25:27:
On the scale of things (how much money you've spent on your table, the OL motor, etc.), why hesitate to spend $100 to get it (the strobe) done right?I own this little gadget (the KAB strobe), which I bought at the same time I bought a VPI SDS power supply for my AC-driven turntable. See my review of the SDS, if you're interested in detailed comments.
Frankly, I've never heard of anyone successfully using anything but a neon light for a strobe, certainly not a normal incandescent or even a flourescent light.
With the KAB Strobe, I was able to see the effect of and calibrate the powersupply for the drag of the stylus.
I kind of doubt that anybody's been able to do that using a GE lightbulb or a flourescent desk lamp.
Follow Ups:
/nt
Glad to see ya' got your priorities straight!next time the money's burning a hole in yer pocket, consider the strobe.
I dunno, maybe you can get a plain ol' incandescent to work for you.
For $100 though that seems like a dealer tool. Why can't a bunch of forum members get together and buy one to share?
Tom
/nt
Hmmm, a platter speed changes with the drag of a stylus? Must not be a very robust motor.An incandescent light WILL work. It's just not as easy to see as with a neon bulb because the filament in the bulb doesn't immediately extinguish with the reversal of the current whereas the neon bulb will react much quicker.
A flourescent will NOT work.
I don't see the need to spend 100 bucks on a strobe either.
Have you ever checked this yourself? Or are you just talking through your . . .?Actually, the required compensation (to achieve an exact 33 1/3 rpm) seems to be 60.04 Hz on the SDS, with the stylus at the outer grooves of the record. Since the angular velocity decreases as the stylus nears the center, I assume the drag decreases as well.
60 Hz on the SDS yields an exactly correct speed, BTW, according the KAB strobe.
So, if the motor loads down such that it is off-speed by .0167% I guess it's not "robust."
The synchronous motor will not change speed at all due to stylus drag, it is locked to synchrony with the supply frequency. At 60.00 Hz it's doing 300.0 rpm, at 60.04 Hz it's doing 300.2 rpm.The measured speed change is due to belt creep. For the belt to transmit torque it must stretch and the stretch must recover as it goes around the pulley. The stretch recovery means the belt moves slower than the pulley - this is called belt creep. The amount of stretch varies with the load on the belt so belt creep increases with stylus drag.
Your figure of 0.067% (not 0.0167%) or 667 ppm corresponds neatly to my theoretical calculation of 750 ppm, within the limited precision of the SDS (one step = 167 ppm)
Origin Live even recommends checking the turntable speed with the tonearm and stylus playing in the middle of the record. Makes sense to me.
Tom
If the drive incorporates I x R compensation the torque constant for the drive / motor combination approaches an infinite value so this becomes unnecessary. The limit I have found is that the resistance of the motor coils rises with temperature, changing the compensation factor.I am still working on correcting that, my first two attempts failed but I learnt a lot.
if one manages to get an incandescent light to work so you can read the strobe, it is running off the same AC power as your turntable. since you are running off a DC motor, it may not affect you, but folks using an AC synchronous motor could benefit from a strobe that runs off an independent, calibrated 60Hz source.The KAB battery-powered strobe does this.
If one's strobe is "off" (essentially blinking at the wrong frequency -- not exactly 60Hz), then it will be impossible to make accurate adjustments with a strobe disc calibrated for exactly 60Hz.
Also, some strobe discs have a tendency to get warped. Slight warps will affect what one sees, too.
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