I used to own a VPI TNT-V hot rod and a HRX.
EVERY time a pulled out the kab strobe the speed was different.
This had nothing to do with stylus drag. |
Downunder,
Yup, that will irk you. Alas, that's a fine example of a poorly designed drive system; there's a reason the SDS was developed and that goes back to the AC mains. Trying to accurately derive 33.3 RPM using a crude phase shifting capacitor and hoping that the frequency of the incoming AC is stable enough to do the job is bad engineering. While frequency accuracy (and adjustablity) is one issue, it doesn't begin to address waveform shape, harmonic distortion, phase amplitude and shift, and all the other interesting things that go into generating a waveform suitable for driving a synchronous AC motor.
DC motors have their own set of problems... |
Peter I forgot drive method. I have a Walker Ultimate motor controller plugged into an Exact Power 15 putting out a constant 120V, this combination keeps the speed as constant as I'm able to check. |
Don_c55: "Just buy a strobe disc and illuminate it with a light bulb. The 60 Hz AC frequency does not fluctuate enough to worry over." I wish that's true but using your method and comparing to a KAB strobe, there is a noticeable visual difference, even when your turntable is accurate the light bulb frequency will drift and you end up adjusting to the wrong speed. Not only the speed should be accurate but, more importantly, no wavering or no intermittent irregularity. To me cogging or analog jitter has the worst effect on the sound of playing a record. DD gets the speed accurate but often suffers from motor cogging. When done right then DD sounds great. Belt is good at filtering the jitters but the elasticity on the belt creates another set of problem. Sigh, what a circle jerk of a hobby! Don_c55: "If your platter is heavy, playing a record will not affect the speed." Didn't Halcro just said he tested the TimeLine on the TW Raven, which has a heavy platter? ---------------------------------------------------------------------------------------------- Below is an excerpt from the long winded and thought provoking piece that Lewm and Halcro was referring to in the below analogy. Good read, if you have time. :-) Peter Moncrieff in International Audio Review, issue # 80:
Consider the following analogy. Imagine first that you want to draw a music waveform, like the ones you've seen in previous IAR articles. Draw it on a square piece of graph paper. Note that you can freely move your hand in two dimensions on the graph paper, so you can simultaneously draw both the varying amplitude (height) and progressing time (horizontal axis) of the waveform on the graph paper. Next, imagine that you're doing the same thing, but you've turned the piece of graph paper sideways, so that your wrist moves from side to side (instead of up and down) as you're charting the waveform's amplitude variations.
Now, imagine that you can only move your wrist from side to side, and can't move your hand up and down at all. Your hand holding the drawing stylus has now become just like a phono cartridge holding a stylus that can only read the side to side variations in a record groove. Your hand holding the waveform drawing stylus is mounted on your arm, the same way that a cartridge holding the waveform reading stylus is mounted on the pickup arm of the record player.
If you were to try drawing a music waveform, while limiting your hand to only this side to side motion, you couldn't do it. There would have to be a further mechanism for moving the drawing stylus in your hand along the time axis of the graph paper where you want to draw the complete music waveform. You could for example rely on a strip chart recorder, which could dispense the graph paper in strip form at a fixed time rate (you've probably seen strip chart recorders in the form of earthquake recorders, where the side to side needle motion indicates earthquake amplitude, on a steadily unrolling strip of graph paper; if you're unacquainted with this, imagine a roll of toilet paper unrolling at a steady rate). The strip chart recorder makes the graph paper move along under your hand at a constant speed, thus creating a steady time axis for the waveform you wish to draw. And the strip chart literally creates this time axis. Your hand is limited to reproducing (accurately we hope) only the amplitude axis of the music waveform, since you are now limited to side to side motion.
That's exactly what a turntable does. It literally creates the time axis half of your music waveform, while the cartridge, which is restricted to side to side motion, reproduces (accurately we hope) the amplitude information that the grove contains. _______ |
Speed accuracy is important for me since I jam with guitar while record is playing and that's one of my ways to realize how precise the speed is. If it's faster or slower by small fraction of rpm I'll hear the difference in tunes. |
