Why Use Aluminum for a TT Platter?

I am in the process of building a turntable made entirely of Delrin which I understand is as close to a metal as you can get in a polyvinyl product. Fabricators love it because it machines so well, and holds its shape under load. Enought that you can use it for a bearing surface without a sleeve. I hope to see what a turntable with a Delrin plinth and platter sound like compared to a similar ridgid deck.
However, a 3" thick piece of Delrin is about $250 per square foot. Dont want to make too many false cuts or holes in that.

Paperw8: "one of the drawbacks of direct drive turntables is the possibility that vibrations from the motor would couple into the turntable and get picked up at the cartridge. since the vibrations would likely be relatively low frequency, it would likely be in the frequency range that gets amplified in the phono stage."

You're talking about half Hertz at 33rpm. That frequency exists in ALL turntables. Direct-drive is as quiet as anything. Unless the bearing is bad quality, there is very little or no noise in DD, just like belt-drive. I would say it's even quieter than belt-drive. The motor turns at 33rpm, the same amount of rotations as the platter of a belt-drive or idler-drive, WITHOUT the fast spinning motor in a BD or ID that sometimes spins at 1800rpm, and that's 30Hz! Now, the DD platter is spinning at 33rpm by magnetic force and please tell me where the stereotype DD motor noise is coming from? The second people think the motor is underneath the platter, they immediately think it's noisy. Go take a look at the assembly of a DD table, it's a platter with a ring magnet underneath and wrapped around or above a series of coils centered by the bearing well. That's it, nothing is touching the platter, unlike the belt in BD or idler wheel in ID. It's "magnetic drive" - though I hate the use of that term in commercial products.

You can criticize DD having cogging issue but not noise. Noise in DD is the last thing I worry about.

Oops, this thread is about platter material. Sorry.

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When VPI's Harry was asked that question he said it was all about cost related to the platter and less torque on the motor. Remember Harry went from lead/rubber, to acrylic to aluminum. If one is not very cynical, profit would be the answer. I've heard them all and still think the original TNT platter is best.

02-09-11: Hiho
You're talking about half Hertz at 33rpm. That frequency exists in ALL turntables. Direct-drive is as quiet as anything. Unless the bearing is bad quality, there is very little or no noise in DD, just like belt-drive. I would say it's even quieter than belt-drive. The motor turns at 33rpm, the same amount of rotations as the platter of a belt-drive or idler-drive, WITHOUT the fast spinning motor in a BD or ID that sometimes spins at 1800rpm, and that's 30Hz! Now, the DD platter is spinning at 33rpm by magnetic force and please tell me where the stereotype DD motor noise is coming from?

you can't hear 2Hz so obviously i was not referring to any 2Hz rumble. in the real world, nothing is machined to perfect balance, so when it rotates there will be some vibration. if you can hear sound from a drive motor, then obviously the frequency is within the audible range. a cartridge can pick that up and if it does, it is likely going to be in the part of the frequncy range that is amplified in a phono stage.

as to the magnetic drive to which you refer, a rotating magnetic force field (which is what you need to turn the platter) doesn't just happen by magic: there has to be some motor that turns a rotor (e.g. an induction motor) that creates the time varying magnetic field that could turn a platter by lorentz force. there will be *some* vibration in that motor. if it is a reasonably well made turntable you probably won't be able to hear it, especially if you use a large signal cartridge.

"there has to be some motor that turns a rotor (e.g. an induction motor) that creates the time varying magnetic field that could turn a platter by lorentz force. there will be *some* vibration in that motor."

The rotor is PART of the motor, propelled by the stator coils. There's NO extra motor to turn the rotor. The platter and the motor shares the same bearing, hence "direct drive." NOTHING is physically touch the platter outside of the bearing and rotor, no belt, no idler wheel. You can argue the magnetic force to spin the rotor creates a reverse torque energy (vibration) on the stator. But that's still a lot less vibration from other means of spinning the platter such as belt-drive and idler-drive.

Of course there will ALWAYS be some vibration, so is planet earth. Let's not nitpick here.

All I am saying is that people seem to be brainwashed by the stereotype that just because there's a motor underneath the platter of a direct-drive turntable there will automatically more vibration than a belt-drive turntable. Simply not true. To get a better illustration, take a look at the mechanical simplicity of a, say, Brinkmann Bardo DD turntable and its coreless mtor:

http://www.onahighernote.com/images/product_categories/Bardo_Magnetic_Drive_small.jpg

[url]http://www.onahighernote.com/brinkmann/?c=2>http://www.brinkmann-audio.de/inhalt/bilder/bardo2.jpg[/ur]

[url]http://www.onahighernote.com/brinkmann/?c=2

People can argue all they want about cogging and speed hunting, etc... which are true issues. Noise, again, is the last thing I worry about in a direct-drive turntable with quality bearing, which holds true for ANY turntable. If you have noisy bearing, you have a noisy turntable, not just a DD table.

I own all genres of turntables, BD, DD, and ID. They all can sound good and I like them all. DD gets a bad rep so I am compelled to comment. That's all.

Happy listening!

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