Why Use Aluminum for a TT Platter?

02-09-11: Dan_ed
This is also why so many DD tables have light-weight platters. The motor on a DD is designed to directly manipulate the platter in order to correct speed. The platter is supposed to have it's acceleration constantly manipulated.

i think the reason why direct drive tables tend to have light-weight platters is for fast acceleration. club dj's are among the most enthusiastic users of direct drive turntables. when you are a club dj you want to be able to cue up the record and have it accelerate to speed as quickly as possible on the break.

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. so a higher mass platter would require a more powerful motor, which i would think, would increase the likelihood of vibration.

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.

____

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.