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Stylus-Drag..Fact or Fiction?

Most audiophiles can't seem to believe that a tiny stylus tracking the record groove on a heavy platter could possibly 'slow-down' the rotating speed of a turntable.
I must admit that proving this 'visually' or scientifically has been somewhat difficult until Sutherland brought out the Timeline.
The Timeline sits over the spindle of the rotating disc and flashes a laser signal at precisely the correct timing for either 33.33rpm or 45rpm.
By projecting these 'flashes' onto a nearby wall (with a marker attached)....one can visualise in real-time, whether the platter is 'speed-perfect' (hitting the mark at every revolution), losing speed (moving to the left of the mark) or gaining speed (moving to the right of the mark).

RAVEN BELT-DRIVE TT vs TIMELINE 
Watch here how the laser hits the mark each revolution until the stylus hits the groove and it instantly starts losing speed (moving to the left).
You can track its movement once it leaves the wall by seeing it on the Copperhead Tonearm.
Watch how it then speeds up when the tonearms are removed one by one....and then again, loses speed as the arms are dropped.

RAVEN BELT-DRIVE TT vs TIMELINE
Watch here how the laser is 'spot-on' each revolution with a single stylus in the groove and then loses speed as each additional stylus is added.
Then observe how....with NO styli in the groove.....the speed increases with each revolution (laser moves to the right) until it 'hits' the mark and then continues moving to the right until it has passed the mark.

Here is the 35 year-old Direct Drive Victor TT-81 turntable (with Bi-Directional Servo Control) undergoing the same examination:-
VICTOR TT-81 DD TT vs TIMELINE 
128x128halcro
uberwaltz's avatar
uberwaltz
13,381 posts
 
Thx Phoenix
Great post which actually makes sense to myself. I must admit some prior posts have shot right over my head but now I am trying to think in industrial terms which is something I am at least vaguely familiar with.
We rotate, amongst other things, 900 to 1200lb steel rollers with a required very high degree of speed accuracy precision.
Think in terms of  3 phase 5 to 25hp motors for this task.
Current feedback has proven very effective at precise control AND eliminating the pesky noise problem sometimes seen on 0-10vdc and 4-20ma control feedback circuits.
richardkrebs
422 posts
 
Hi phoenixengr. why do you think  VPI use trapezoidal commutation?
Surely this is not optimum if they want to minimize torque ripple. 


Cheers

phoenixengr's avatar
phoenixengr
92 posts
 
"....why do you think VPI use trapezoidal commutation?
Surely this is not optimum if they want to minimize torque ripple."



The controller is made by Elmo Motion Control (Gold Twitter Solo), and while Elmo's PC control software is fairly comprehensive, I'm not sure if the controller is capable of sinewave drive;  it is not capable of sensorless operation (FOC) which would use sinewaves.  Sinewave drive is considerably more complicated and would result in smoother operation, but for whatever reason, VPI chose to operate the motor in the simplest mode available, as a DC motor which uses block commutation and Hall sensors. 


They do some other rather strange things:  The motor is designed for high power (500W) so the windings are 0.3R;  not the easiest load to drive especially from a class AB output amp.  The Elmo controller uses PWM output at 10kHz, which IMHO is a bad choice for use around a sensitive audio circuit like a cartridge pickup (especially MM).
testpilot
916 posts
 
I am getting a sneaky sensation that phoenixengr just might know a thing or two about TT motors.
mijostyn's avatar
mijostyn
8,518 posts
 
No kidding. Just for the record phoenix what turntable manufacturers drives do you like the best and why?
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