As already mentioned by PRR, this is a classic example for system control via a servo feedback system. The motor platter system has a mechanical time constant determined by the moror torque, the transmission system and the platter weight (rotational inertia). If you know this step response (what is the time constant of the system for rpm if you switch the motor on - how long does it take to reach 63% of final speed), you can design a feedback controller that brings the platter to speed in the shortest possible time with 0% deviation for final rpm and having an optimally damped (Q=0.5) step response.
If you design the controller e.g with too much gain or faster than the mechanical time constant of the system allows for, the result will be an oscillator - pretty much the same as for the stability of any NFB amplifier.
If you design the controller e.g with too much gain or faster than the mechanical time constant of the system allows for, the result will be an oscillator - pretty much the same as for the stability of any NFB amplifier.
My opinion is that a feedback (servo) system is not good for audio.
Maybe not fully relevant here, but once I have made a copper weight for the turntable of my CD player. It did not workThe servo could not make it rotate, probably within the timeout.
Your opinion is naturally your own.
However, what you attempted with that copper weight is wrong, because the designers already done their job in the lab, and you disturbed the balance that they designed and thoroughly tested in.
Back to the subject. The key to using a heavy platter & DC motor, (with out feed back) is
a) the freeness of running of the bearing, there was a thread about this a while ago.
b) using a none stretchable belt like Mylar to drive the system.
Without the above the table will have to much speed variation.
Cheers
a) the freeness of running of the bearing, there was a thread about this a while ago.
b) using a none stretchable belt like Mylar to drive the system.
Without the above the table will have to much speed variation.
Cheers
Errr that's a given with any decent turntable
Disagree, a rubber belt allows a degree of disconnect to small error correction fluctuations from the motor
The flywheel (being a flywheel - especially with as you quote a "heavy" one) is the overriding force in the system, once up to speed it's a flywheel, it just needs topup energy from the motor
Quite possibly so.
However, in a well-designed system, the pulses of the driving coils are controlled in intensity so that any deviations to rotation are beyond trivial.
Quite possibly so.
However, in a well-designed system, the pulses of the driving coils are controlled in intensity so that any deviations to rotation are beyond trivial.
The phase angle between the rotating magnetic field in the stator coils and the rotor is how the motor produces torque; it is a necessary component if the motor is going to do any work.
Any change in load whether from friction, groove modulation or other outside forces will necessitate a change in torque requirement (higher current draw) from the motor which will manifest itself as a change in phase between rotor and stator fields if the platter is to remain at a constant speed.
Therefore Arch Stanton's analogy is correct: There is a compliant element in the drive train even with a direct drive motor (this effect occurs with belt drive and idler drive motors as well, but is reduced the platter/pulley ratio).
Even if the platter remains at constant speed under load, the temporary change in the relationship between rotor and magnetic field can create a temporary change in pitch of the recovered signal analogous to phase modulation vs frequency modulation in wireless communications.
What you're describing is of course the feedback/compensation designed into such circuitry, necessary for stability of platter rotation and loading inconsistancies.
And this can happen at extremely fast rates but depends on platter mass and other mechanical time constants.
With the system designed into my own turntable, it's not even a worry, in fact inaudible to any variables of platter loading.
The stylus at 1.5 grams is a trivial factor in such things.
I found while using PYRAMID's 3 phase motor and controller with my Lenco that with the stock platter the motor would keep close to perfect speed consistency.
But it sounded terrible.
When I would use all of my extra copper mats to add weight the sound was dramatically improved but it took some time to get the motor and the heavy platter to work well together.
With the PYRAMID controller where one can change the frequency the time it took for the platter to respond to changes was too long for some sort of servo to do anything more than compensate for gross changes from the motor heating up. The changes would be instantaeous wiht the stock platter and took many revolutions to settle down with the heavy platter.
I will probably add some kind of simple servo like the ARDUINO based one but only for that purpose. I could see that being fed new frequency instructions would only make for a tremendous amount of confusion.
Of course, this is idler drive but I would think the same thing would happen with a belt.
Getting all adjusted properly with the PYRAMID motor made for very good speed consistency but one does need to make adjustments to the frequency between start up and after the motor has been running for an half hour or so. One can attempt to assume that leaving the frequency where it was and not minding the incorrect speed until the motor settles is not a sure thing if you want to get as close to 33.333 rpm as possible. in my case the final frequency is always slightly different with each listening session.
Anyone working with turntables needs a ROAD RUNNER to really know what is happening with the platter speed. Luckily these are available from SOTA.
But it sounded terrible.
When I would use all of my extra copper mats to add weight the sound was dramatically improved but it took some time to get the motor and the heavy platter to work well together.
With the PYRAMID controller where one can change the frequency the time it took for the platter to respond to changes was too long for some sort of servo to do anything more than compensate for gross changes from the motor heating up. The changes would be instantaeous wiht the stock platter and took many revolutions to settle down with the heavy platter.
I will probably add some kind of simple servo like the ARDUINO based one but only for that purpose. I could see that being fed new frequency instructions would only make for a tremendous amount of confusion.
Of course, this is idler drive but I would think the same thing would happen with a belt.
Getting all adjusted properly with the PYRAMID motor made for very good speed consistency but one does need to make adjustments to the frequency between start up and after the motor has been running for an half hour or so. One can attempt to assume that leaving the frequency where it was and not minding the incorrect speed until the motor settles is not a sure thing if you want to get as close to 33.333 rpm as possible. in my case the final frequency is always slightly different with each listening session.
Anyone working with turntables needs a ROAD RUNNER to really know what is happening with the platter speed. Luckily these are available from SOTA.