Automatic turntable/platter speed adjustment, is there such a thing?

[David Morris]

I was wondering if there is such a thing as an optical reader that counts the revolutions of the platter in a given minute and adjusts the speed automatically?

 

[handsome]

Yip, direct drive turntables use them. The sensor uses either a small magnets or ‘blobs’ of paint on the platter to calculate the platter speed and adjust the motor accordingly.

 

[David Morris]

Yip, direct drive turntables use them. The sensor uses either a small magnets or ‘blobs’ of paint on the platter to calculate the platter speed and adjust the motor accordingly.

But not belt drive? Which I would imagine would be subject to belt stretch

 

[handsome]

There could well be some belt drives that do use them. But you are right, motor speed changes would be ”absorbed” by the belts elasticity - I suppose you could integrate or average the platter readings but ultimately that would mean a gently varying platter speed. You’d want a fast control loop that corrects the speed much quicker than the general rotation, so the resultant speed variations would be a tiny fraction of overall speed and thus much harder to hear. Probably cannot do that with a floppy belt!

 

[Grumpybear]

Good belt drives do not need constant adjustment due to the heavy platter acting as a flywheel to keep the speed consistent. Lightweight platters on both direct and belt drive are subject to small fluctuations caused by friction, motor cogging and other issues.

 

[handsome]

Good belt drives do not need constant adjustment due to the heavy platter acting as a flywheel to keep the speed consistent. Lightweight platters on both direct and belt drive are subject to small fluctuations caused by friction, motor cogging and other issues.

Correct, most belt drives rely on momentum and inertia for speed stability - which is just a fancy way of saying they use heavy platters

 

[fdlsys]

Some better DC motor designs have/had a feedback coil inside the motor that served a purpose of keeping the voltage steady. Thorens was one of the early adopters of the idea and used Papst motors which turned out to be utter shyte.
I modified a 126 Mk3 to use a bigger and far better capstain motor from a pro video machine and speed stability of that one, even with its light (for a belt drive) platter, was the best of any belt drive I bothered to measure.

As long as the belt is in good condition, its "flex" during the operation is negligible because it's permanently driven and tensioned by the motor.

There is a potential artefact called "belt slap" but too is non existent on better designs. Thin soft belts like Thorens, hard belts like Linn, round belts, string drive like WTL have no belt slap.

Linn got absolutely everything right, IMHO.

 

[johns]

Linn Radikal (Machined) for LP12​

Key features​

• 
  External power supply for LP12 and Urika internal phono stages
• 
  Brushed DC motor
• 
  Speed management system checks and adjusts speed on every rotation

​