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Turntable speed under load
Turntable speed under load
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Old 3rd April 2008, 06:16 PM   #1
YNWOAN is offline YNWOAN  United Kingdom
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Default Turntable speed under load

Has anybody experimented with measuring how much their turntable speed is affected (if it is) by the load presented by playing a record? I am lucky enough to have a very accurate strobe (battery powered) and matching disk and have always set the speed to be accurate when only the strobe disk is on the platter. However, a recent re-reading of a review of the SME 20, and a post on another forum, has made me wonder if the real speed of the platter when playing a record was not slightly too slow. Both the review and the forum post suggested that the stylus drag was sufficient to slightly slow the record. As a result I re-jigged my measurement procedure so that a record could be played at the same time as using the strobe (this sounds easy but is a bit trickier as the strobe disk is full platter size - a 10" record placed on top was used) and I can't detect any speed difference with or without load. Has anybody found any different?
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Old 3rd April 2008, 07:42 PM   #2
sek is offline sek  Germany
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Hi,

stylus drag is the effect of the diamond scratching in the groove.
The force this drag applies depends on the stylus position (largest drag on the outside of the platter), the groove content (larger drag with deeper cut/louder bass) and stylus shape (i.e. lower drag with spherical styli).

I've experienced the effect of stylus drag on a Technics direct drive turntable, the SL-2000. This one has a "platter update servo" controlled low-torque motor and a medium weight platter.

I've then - after first experiencing this effect - tried it on a Technics SL-12x0 MK2. Those are direct driven ones with PLL-servo controlled high-torque motors and relatively light platters. No stylus drag effect at all!

So it entirely depends on the regulation of your turntable. As presented by the better of the above, stylus drag induced slow-down of the platter can and should be avoided in good record players.

Cheers,
Sebastian.
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Old 3rd April 2008, 07:57 PM   #3
YNWOAN is offline YNWOAN  United Kingdom
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Quote:
Originally posted by sek
stylus drag is the effect of the diamond scratching in the groove.
Umm yes, I am aware of that :-)

Interesting though. Indeed, DD decks are often cited as being better in this regard (not sure how true this is or not). My deck is belt driven from an AC motor (8lb or so platter) - I've just checked it again and really can't see even the slightest slowing (stylus force is 1.7g approx).
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Old 3rd April 2008, 08:26 PM   #4
sek is offline sek  Germany
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Quote:
Umm yes, I am aware of that :-)
This was meant more as a general reply with a little explanation to get everyone going.


Quote:
Interesting though. Indeed, DD decks are often cited as being better in this regard (not sure how true this is or not).
I'd say, theory would dictate that the amount of this effect only depends on the quality of regulation, within the limits of the motor/platter system's capabilites. One of my direct driven drives fails the test, the other one passes marvelously. The same would happen to different kinds of belt-driven turntables.

A strong enough motor and a suitable platter are a prerequisite for good regulation.
A well engineered PID controller can then easily compensate for any drag-induced slow-down.
A quick (tachometer like) speed feedback also helps a lot.
A (quartz referenced) PLL then tops it off with long-term stable absolute speed.

Good drives usually feature at least the first three points, the worst feature non of the above at all. For example, the SL-1xx0 MK2 series features all of them, my SL-2000 literally fails in every single point.
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Old 3rd April 2008, 08:27 PM   #5
harwoodspark is offline harwoodspark  England
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I fitted my Linn Sondek with an OL dc motor kit. The motor speed control board has load correction built into it and I've always been aware of some speed fluctuation when playing certain records. When I built my own tt which uses the motor from the Linn, I decided to run the motor straight from a stabilised power supply. No load correction is used and I don't experience any speed fluctuation with this setup. The platter is a lot heavier than the Linn platter.
The more I think about it the less I am convinced that load correction can work properly as it is applied after the event. I think a heavy platter is way to maintain steady speed.

Si.
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Old 3rd April 2008, 09:02 PM   #6
sek is offline sek  Germany
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Hardwoodspark,

this essentially is the old debate about feedback versus non-feedback in audio.

Cheers.
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Old 5th April 2008, 11:01 AM   #7
YNWOAN is offline YNWOAN  United Kingdom
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Well perhaps it's about feedback or non-feedback and perhaps it's about DC versus AC or even platter mass.

I am interested by what Si has to say though. I wonder how OL built load correction into their power supply - does the motor allow a feedback loop of some kind? As you state Si, I can't really see how this approach could work either. If the purpose of the load correction circuit is to overcome the drag of the record you are inherently doomed from the start. If the torque from the motor is so low that stylus drag slows the motor then it seems logical that bass transients will put even more load on the system and slow it even more. By the time this transient has happened it will almost certainly be too late for the circuit to compensate particularly given that it has the flywheel mass of the platter to accelerate.

I can't help but wonder though Si if it is not the use of an AC motor rather than a heavier platter that results in the improvements you note.
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