I agree ,sorry, I shouldn,t have made the joke in the first place but
then adverts belong elsewhere also .
My apologies Bill.
then adverts belong elsewhere also .
My apologies Bill.
Many DD (direct drive) turntables have the magnets coupled to the platter which may affect the sound. Also many aluminum platters have too much ringing like a gong which messes up the music image. Luxman PD-121, Technics SP-10 and some high end Kenwood have the much less ringing platter which also separated from the magnet drive mechanism. These platters just rest on the DD motor. The mass of the platter and the motor torque are matched set. This way the motor just applies just enough torque to turn the platter smoothly. One can replace the aluminum platter with an acrylic or a wooden leadloaded platter on a DD turntable as long as the replacement platter has the same weight as the original platter.
nghiep said:
I think you mean match the moment of inertia. I don't think you actually need to match it though for any direct drive I've seen. Although, some platters (JVC, Denon) have a magnetic ring in them that is read by a tape head-like device to keep the speed correct. If you were to change platters, then the control would be gone.
Max
Basic physics:
The motor is only 'working against' the rotaional inertia (which depends on the mass and its location) of the platter when it is changing speed. At a constant speed the motor works against friction of the system, in the bearing, between the stylus and record, maybe eddy currents from the motors magnets and something, all depends on the design. The inertia will effect how the motor responds to changes, more inertia gives a slower responce, and therefore is very important to the overall design of the drive system and therefore the speed control.
There were DD turntables that would run without the platter installed that would 'violently cog' (probably one reason for the cogging ideas) the motor was responding faster than the feedback system and thus 'hunting' around the correct speed. This does not mean the design was bad or that the system would have any behaviour like this with the platter installed, it just means that the system was designed with the platter inertia as part of the system. If you are going to make a system with feedback it is a lot easier if you know and control all the parameters. So changing the platter inertia is likely to make the performance worse (assuming the designer got his stuff right and somebody else didn't spoil the design with random parts replacement).
Boy that was a little long winded, sorry.
Bill
The motor is only 'working against' the rotaional inertia (which depends on the mass and its location) of the platter when it is changing speed. At a constant speed the motor works against friction of the system, in the bearing, between the stylus and record, maybe eddy currents from the motors magnets and something, all depends on the design. The inertia will effect how the motor responds to changes, more inertia gives a slower responce, and therefore is very important to the overall design of the drive system and therefore the speed control.
There were DD turntables that would run without the platter installed that would 'violently cog' (probably one reason for the cogging ideas) the motor was responding faster than the feedback system and thus 'hunting' around the correct speed. This does not mean the design was bad or that the system would have any behaviour like this with the platter installed, it just means that the system was designed with the platter inertia as part of the system. If you are going to make a system with feedback it is a lot easier if you know and control all the parameters. So changing the platter inertia is likely to make the performance worse (assuming the designer got his stuff right and somebody else didn't spoil the design with random parts replacement).
Boy that was a little long winded, sorry.
Bill
Bill,
I hear what you are saying, when you add mass to a platter, you change the speed and magnitude of changes. I can see where lightening things could be a problem, but I don't see that making things heavier would hurt. With a heavier platter changes would happen slower and with a smaller magnitude. Would this not allow a servo feedback system more oppertunity to compensate for things like stylus drag on heavy grooves? In effect you would be able to reduce the wow and flutter of a table by increasing the platter mass.
The only thing I can think of is if the motor responce to input is calculated to avoid overcorrection, the motor may not respond properly. Hmmmm.
DaveM
I hear what you are saying, when you add mass to a platter, you change the speed and magnitude of changes. I can see where lightening things could be a problem, but I don't see that making things heavier would hurt. With a heavier platter changes would happen slower and with a smaller magnitude. Would this not allow a servo feedback system more oppertunity to compensate for things like stylus drag on heavy grooves? In effect you would be able to reduce the wow and flutter of a table by increasing the platter mass.
The only thing I can think of is if the motor responce to input is calculated to avoid overcorrection, the motor may not respond properly. Hmmmm.
DaveM
Surely if one assumes constant motor speed then compliant belts and massive turntables do all this feedback loop thing mechanically no need for transistors or magnets any were near wires carrying micro-volt signals!
I.e. TT slows down slightly ,belt stretches slightly increases torque on TT , TT speeds up slightly etc,etc, If the TT mass to belt
mod of elasticity is chosen correctly. If this works to the point that
the deviations are such that the unmeasurables like attack etc are
satisfied why all the electronics?
I.e. TT slows down slightly ,belt stretches slightly increases torque on TT , TT speeds up slightly etc,etc, If the TT mass to belt
mod of elasticity is chosen correctly. If this works to the point that
the deviations are such that the unmeasurables like attack etc are
satisfied why all the electronics?
But why would you assume that the motor (relatively small moment of inertia) has speed any more constant than the platter (relatively high moment of inertia) ?
Well because the motor is in the simplest form (synchronous ) tied to the mains frequency or in its more advanced dc controlled guise
controlled by the laws that govern its speed of turning for a given voltage / current the electronics of the supply to the motor can be as fiendish as you wish! But again the high compliant belt cuts both ways and will iron out minor deviation in motor speed also ,working against the high inertia of the turntable .
controlled by the laws that govern its speed of turning for a given voltage / current the electronics of the supply to the motor can be as fiendish as you wish! But again the high compliant belt cuts both ways and will iron out minor deviation in motor speed also ,working against the high inertia of the turntable .
But don't these factors that tend to make the motor speed constant apply equally to a DD system, where the motor shaft _is_ the platter spindle?
Yes, I did an experiment with my DD motor, by placing a large mass on the platter. The only effect I could discern was that it took longer to spin up. But once it was at speed, it didn't seem to have any trouble maintaining constant speed. The amount of current it draws with a large mass is the same as with the normal mass, so it's still battling the same friction.
However, removing the platter induced severe speed fluctuations. It's exactly like changing the amount of filter capacitance in a power supply. The more you add, the longer it takes to get up to voltage. If you have too little, you will see ripple.
My motor controller runs open loop, so does not use any feedback from the platter. It's up to the human to close the loop by adjusting the little knob until the strobe bars stop moving.
No because the piece of elastic between the driven and the driver
is a short (stiff) flux gap not a long supple belt.
is a short (stiff) flux gap not a long supple belt.
You seem to be saying that if one increases the compliance of the belt, then the platter speed will become more constant.
No Im saying that if the modulus of elasticity of the belt and the rotational mass of the TT are optomised most of the problems :-
cogging ,minor speed fluctuations etc are no longer a problem so why
throw loads of electonics and complication at them.The bonus being no fields near your sensitive cartridge.
cogging ,minor speed fluctuations etc are no longer a problem so why
throw loads of electonics and complication at them.The bonus being no fields near your sensitive cartridge.
pixpop said:
I once saw some old school footage of a DJ doing a handstand on a spinning turntable. How's that for added mass? I think he was going slower than 33.3 RPM though.
Max
Platter mass
I have tried different platter-weights (drilled & mass added) to Technics & Kenwood DD, without any practical problems...
BTW; I always imagined the drive on these motors as a linear motors (in a circle).
Arne K
I have tried different platter-weights (drilled & mass added) to Technics & Kenwood DD, without any practical problems...
BTW; I always imagined the drive on these motors as a linear motors (in a circle).
Arne K
That's funny. If you look up practically any description of a linear motor, it will say it's just like a conventional motor, but unwrapped.
😀
Just did another experiment. I applied power to my DD motor, without platter attached. It began it's herky jerky oscillations, with the large current spikes. I then put the motor right next to the cartridge on my belt drive TT. I turned the volume up to max, but couldn't hear anything.
I repeated the experiment while playing music on the belt drive TT, but couldn't hear any difference when the motor was very close or far away.
If there is some radiated magnetic field, I think it's either too weak, or too low in frequency for the cartridge to pick it up.
I repeated the experiment while playing music on the belt drive TT, but couldn't hear any difference when the motor was very close or far away.
If there is some radiated magnetic field, I think it's either too weak, or too low in frequency for the cartridge to pick it up.
Here's a picture of my Sansui DD motor.
The metal plate is 3.5" x 4" so the whole thing is pretty compact.
If you apply 18V DC to it, it spins at 33 or 45, depending on a front panel switch.
There's a trimpot for each speed, and another pot for the front panel speed control, that connects through the white connector.
The metal plate is 3.5" x 4" so the whole thing is pretty compact.
If you apply 18V DC to it, it spins at 33 or 45, depending on a front panel switch.
There's a trimpot for each speed, and another pot for the front panel speed control, that connects through the white connector.
An externally hosted image should be here but it was not working when we last tested it.
I think my Technics SP-25 got speed control feed back because with the original platter, there was no speed fluctuation. When I add a second platter on top then the speed started to fluctuated just a little. The extra mass required more torque apply to the platter which caused the speed to overshoot then the torque removed and the platter slow down. This on and off actions from the feedback control caused speed fluctuation. Of course, the VPI TNT platter adds about 20 lbs. to the original platter. If I use the Technics SP-25 as a motor to pull my VPI TNT platter in a belt drive setup, then no speed fluctuation. The massive momentum of the VPI platter stabilized any jerky motion from the Technics.
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