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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
phoenixengr's avatar
phoenixengr
94 posts
 
"Cogging is a function of the number of poles; the rotor experiences a regularly irregular rotational force due to the naturally varying intensity of the magnetic fields produced by the stator. The rotor is therefore constantly inconstant in its speed. There is a regularity to it that is said to be audible to some, and that's "cogging". The tendency can be ameliorated by using a stator with a lot of poles, the more the better."



This is only partially correct.  Cogging is caused by the change in variable magnetic reluctance as the PM rotor passes the metal pole pieces of the stator.  Adding more poles does not decrease the frequency or amplitude of cogging.  If you turn the motor by hand you will feel the cogging and it "feels" finer with a 24 pole motor vs a 12 pole motor because the cogs are closer together.  A 24 pole motor turns at half the speed of 12 pole motor so the frequency of the cogs is identical in both (120Hz).  The magnitude of the vibration caused by cogging will be identical in motors with identical power ratings and the vibration is directly proportional to the power consumed by the motor.  In most cases, the 24 pole "upgrade" motor is higher power than the 12 pole motor it replaces, so the 24 pole motor will actually produce more cogging than the 12 pole motor it replaces.  This was investigated in the link below:


https://www.diyaudio.com/forums/analogue-source/309925-hurst-motors-300-rpm-vs-600-rpm-upgrade-myth....


Coreless motors have no metal pole pieces in the stator windings so they produce no cogging.
mijostyn's avatar
mijostyn
8,570 posts
 
Phoenix, Thank you for that explanation. Do DC motors cog at all?

Mike
mijostyn's avatar
mijostyn
8,570 posts
 
Mike, Yes you have to ask for it and it will probably cost you an additional fee. Gabon ebony has a specific gravity close to 1. 1 is the specific gravity of water. So Gabon ebony is very stiff and heavier than most wood. Once it is dry it is also very stable. Great arm for Koetsu's and Air Tights. Problem with it is that it is rare and very difficult to dry. Usually you wind up wasting 30 to 40% of the wood in the process. Good Gabon Ebony is jet black. Black piano keys use to be made out of it. I use it for details in cabinets like drawer pulls, handles, inlay and such. It turns very nicely. 
uberwaltz's avatar
uberwaltz
13,381 posts
 
Usefull information here.

https://www.rarewoodsusa.com/product/ebony-gabon/
phoenixengr's avatar
phoenixengr
94 posts
 
"Phoenix, Thank you for that explanation. Do DC motors cog at all?"


Any motor with iron pole pieces will exhibit cogging.  In a DC motor, the magnets are stationary but the rotor coils are wound on steel laminates with poles which also produces cogging.

Some of the better BLDC motors have stators with skewed poles (angled slots rather than vertical) so the rotor sees more or less a constant magnetic reluctance and produce much less cogging.

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