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Old 21st September 2005, 06:15 PM   #1
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Default Lightweight turntable plinth.

Many DIY TT seem to go the high mass route, have any of you gone the opposite way and gone for lightweight materials?
I am at the moment making a simple high(ish) mass plinth useing a LP12 bearing, sub platter and motor but will have a go at a lightweight design next. Aerolam is to expensive to have a play with so I will probably make up a constained layer plinth useing some sort of foam, (not done much research at the momment).
Any thoughts on the platter? Weight is preferable for speed stability but would a heavy platter negate any advantage of a light plinth?

Paul.
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Old 21st September 2005, 08:14 PM   #2
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Actually it isn't the weight but rather the moment of inertia which is important.

In a translational system you have Newtons famous equation F=MxA, where F is the force needed to accelerate a given mass M at a given acceleration A. Similarly, in a rotational system you have T=JxAA, where T is torque, J is the moment of inertia and AA is the angular acceleration. J depends on both the mass and the distribution of the mass.

You could have a very light platter with a large outer rim and still obtain a high moment of inertia for rotational stability.
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Ray
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Old 22nd September 2005, 07:24 PM   #3
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In my case increasing the diameter of the platter is not an option, purely for asthetic reasons. I like my turntables small and unasuming, just a personal thing.

A turned heavy ring around the circumferance of the platter would put the weight where it would do most good while still keeping the overall weight down. Perhaps I would also need to look at the motor if I were to go on a weigh saving diet. A motor with more of a flywheel effect than the generic Airpax/Premotec motor from the Linn?

Paul.
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Old 23rd September 2005, 09:17 AM   #4
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by Paul Dimaline
A turned heavy ring around the circumferance of the platter would put the weight where it would do most good while still keeping the overall weight down.
Then you will have a bell...
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Old 24th September 2005, 04:51 AM   #5
maxro is offline maxro  Canada
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Quote:
Originally posted by EC8010


Then you will have a bell...
True. My Kenwood KD 650 has just such a platter. If I pull the platter off, support it at the centre with as few fingers as will balance it, and give it a good rap, it will ring for quite some time. Place the rubber mat on top, however and it makes barely any noise with very quick decay. I'm not knocking on the mat here; the platter is 13" leaving a good half inch of bare aluminum to hit. When I put the platter back on the plinth and try a knock test, it just makes a dull thud like MDF. Can't say as it's a problem I've ever noticed when playing records on it

I guess if ringing was a concern, you could make the platter out of something that won't resonate. Wood with a lead hoop on the perimeter? Rubber platter with some sort of rebar truss inside to keeep it rigid? Asphalt?

Max
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Old 24th September 2005, 05:33 PM   #6
Tee-Rex is offline Tee-Rex  Israel
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I believe the only way to have a light platter only 12" and still maintain a high moment of inertia is having some sort of flywheel in your system - either within the motor, or external. Actually, a flywheel turning at a high speed adds to the MOI much more effectively then just making the platter heavier - as MOI is related to the square of Omega (tangential speed), but only has a linear relationship with the mass.
Ofcourse, the upside of any high MOI system is it's speed stability at steady state, where as the down side is the motor's ability to correct the platter's speed if need arises (say stylus drag, for example)

-T
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Old 25th September 2005, 02:04 AM   #7
maxro is offline maxro  Canada
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Hmm... I think a seperate, belt driven flywheel would be a bad idea. Imagine a light platter with transients in stylus drag: it will slow down easily, but the flywheel will not. This will cause the belt to bunch and the platter to pulse. It seems you'd be adding complexity with no gain in performance over a standard belt drive setup.

Max
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Old 25th September 2005, 08:56 AM   #8
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Mmmm, think your right Max. Perhaps if it were an idler drive system then you could go the lightweight platter route? When I have this one up and running I have my 401 to play with!

Paul.
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Old 25th September 2005, 10:00 AM   #9
maxro is offline maxro  Canada
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Default Hey look, it's been done!

Check out this site for a low inertia platter with a flywheel attached:
http://digilander.libero.it/bellocad...sgalleryRZ.htm
scroll down to Yamamura Churchill

Max
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Old 25th September 2005, 10:30 AM   #10
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Ahh, theres nothing new under the sun!!

Those Teragaki creations look more at home on robot wars!!


Paul.
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