Alternative platters ?

[Oldpinkman]

Think about a seesaw.
Put a fat kid and a skinny kid on it. The fat kid near the middle and the skinny one near the other end. This gives the skinny kid greater downward inertia.
Now move him in to the same relative distance as the fat kid. The fat kid now has greater downward inertia.

Now move him back a bit and they balance, so they have the same downward inertia (in this case, zero)

Hey that's my variable effective arm mass analogy! You mean I forgot to copywright it? Bugger!

[Oldpinkman]

Agreed, I did mention momentum further up the thread. Using mass simplistically in place of momentum, you'd have to have a very unusual design of platter to be high mass but low momentum.

How about a platter like the LP12 platter with effectively a hollow middle and a big rim round the edge where the mass is concentrated? Compared with a flat even platter with a big record weight in the middle? I'm not the engineer here doing the sums, but I think the point is that the design constraints for the motor and its servos are based on inertia not mass, and there are upper and lower boundaries for that which you cannot measure with mass alone. The boundaries are a range not a number (hence the Funk acrylic is still in range at a lighter 1.2Kg). But the range is inertia not mass. And inertia is greater where mass is concentrated at the edge of the platter rather than the centre. He is saying (I don't do sums) that the inertia of the Funk 2 is inside the inertia range (just) for the original Technics motor specs. Another platter with the same mass, but that mass concentrated at the edge would not be. A heavier platter with the mass concentrated at the centre, might be.

I think that's the idea

[YNWaN]

What effect would plonking a heavy record weight of high mass have?

Very little in real terms as the mass would be at the rotating centre and so add little to the overall inertia (though record weight manufacturers do sometimes make this spurious claim). Concentrating mass at the periphery optimises the inertia for a relative mass. For a given speed of rotation, mass (or more specifically weight) and inertia are inherently linked - raising the weight of the platter will increase the inertia unless the distribution of that weight is moved away from the periphery.

Putting aside inertia for a moment, one other aspect that strikes me as pertinent is the radiated magnetic field generated by the motor, both the ferrite magnet attached to the underside of the platter and the motor field as a whole. A metal platter will help reduce the area of radiation, but a polymer platter will have little effect (though making the platter thicker will help). I haven't studied the specifics of the Technics that closely but I have done quite a lot with magnetic bearings and certainly with these, both shielding and structure are relevant.

[Oldpinkman]

Very little in real terms as the mass would be at the rotating centre and so add little to the overall inertia

Yup - the point I was making. The opposite is also true - if your mass is concentrated at the rim, the inertia is consequentially higher

[YNWaN]

How about a platter like the LP12 platter with effectively a hollow middle and a big rim round the edge where the mass is concentrated? Compared with a flat even platter with a big record weight in the middle? I'm not the engineer here doing the sums, but I think the point is that the design constraints for the motor and its servos are based on inertia not mass, and there are upper and lower boundaries for that which you cannot measure with mass alone. The boundaries are a range not a number (hence the Funk acrylic is still in range at a lighter 1.2Kg). But the range is inertia not mass. And inertia is greater where mass is concentrated at the edge of the platter rather than the centre. He is saying (I don't do sums) that the inertia of the Funk 2 is inside the inertia range (just) for the original Technics motor specs. Another platter with the same mass, but that mass concentrated at the edge would not be. A heavier platter with the mass concentrated at the centre, might be.

I think that's the idea

The Linn platter, and others like it, are specifically intended to maximise inertia, but their drive systems are inherently unlike that used by the Technics.

How do you know what the 'inertia range' of the technics is – with respect, that is a meaningless term. Presumably you mean the maximum torque of the motor but that itself means less than it may at first appear as the torque is continuously being modulated by the control circuit.

Ideally, you don't want to generate as much inertia as you can get away with, you want to provide the best possible environment for the control circuit to operate under and that assumes that the platter is able to react instantly to the inputs it is given.

[loo]

How about a platter like the LP12 platter with effectively a hollow middle and a big rim round the edge where the mass is concentrated? Compared with a flat even platter with a big record weight in the middle? I'm not the engineer here doing the sums, but I think the point is that the design constraints for the motor and its servos are based on inertia not mass, and there are upper and lower boundaries for that which you cannot measure with mass alone. The boundaries are a range not a number (hence the Funk acrylic is still in range at a lighter 1.2Kg). But the range is inertia not mass. And inertia is greater where mass is concentrated at the edge of the platter rather than the centre. He is saying (I don't do sums) that the inertia of the Funk 2 is inside the inertia range (just) for the original Technics motor specs. Another platter with the same mass, but that mass concentrated at the edge would not be. A heavier platter with the mass concentrated at the centre, might be.

I think that's the idea

I think you would make a good ley preacher Richard as that's a great explanation , I think we should trust Arthur on this he has previous , imo one reason the original PT acrylic platter was so revolutionary and good apart from the use of acrylic was its shape, the man knows his physics
Paul

[Oldpinkman]

The Linn platter, and others like it, are specifically intended to maximise inertia, but their drive systems are inherently unlike that used by the Technics.

How do you know what the 'inertia range' of the technics is – with respect, that is a meaningless term. Presumably you mean the maximum torque of the motor but that itself means less than it may at first appear as the torque is continuously being modulated by the control circuit

I don't know the range. I pointed out I wasn't doing the sums. I was referring to what Arthur had put in his post, and some comments he made to me, and what I percieved as a failure to understand them in the manner I had. The principle, as you pointed out, holds good. If you accept that inertia not mass is the issue in determining the response of the servos, then the distribution of that mass will be relevant to the systems ability to operate in range. I think Arthurs reference to "out of range" when developing products for the professional market, suggest he has measured or experienced this.

As you also note, and Arthur did - you don't get these problems with a high speed DC motor driving the platter via a belt

[YNWaN]

Yup - the point I was making. The opposite is also true - if your mass is concentrated at the rim, the inertia is consequentially higher

Yup, I also made that point - glad you agree though...

[MartinT]

What effect would plonking a heavy record weight of high mass have?

Good question and very little effect - I've tried it. I added my heavy weight to the centre hub of Arthur's two-part platter to see if there was enough momentum there to make it spin properly. It doesn't.

So a heavy record weight will stress the bearing, as Arthur pointed out, but will not add appreciably to the momentum of the platter.

[Oldpinkman]

I think you would make a good ley preacher Richard as that's a great explanation , I think we should trust Arthur on this he has previous , imo one reason the original PT acrylic platter was so revolutionary and good apart from the use of acrylic was its shape, the man knows his physics
Paul

Thanks Paul. Apart from being friends with the guys at PT, and other coincidences along the way, I think one of my contributions was that I trained as an auditor. No technical, physics, or engineering skills. An auditor needs to be able to understand "just enough" about a clients systems or a transaction to satisfy himself, and assure others, that they give a "true and fair view". It's a mind-set. I remember a final exam question about corruption and influence on audtior independence, and the quote "Independence is an attitude of mind"

In developing methodologies and tests to prove that a statement (financial statements) give a "true and fair view", you develop a nose for bullshit!