Why Use Aluminum for a TT Platter?

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!

____

No Viridian it was not missed at all. As usual your need to be better and smarter than anyone else came radiating above all others. My apologies for insulting your imperical command of both language and knowledge.
Cheers

Aluminum (or aluminum alloy) were chosen many decades ago for a number of reasons depending on the company and its intentions. Aluminum is non-magnetic and easy to machine, shape, and balance. Copper or bronze is heavier, but is also more expensive, would take longer to spin up, and is more likely to bend out of shape.

And get this straight: The direct drive turntables were not designed for the DJ market; they were designed for the high end home audio market. Adjusted for inflation, the SL1200 was $300, equivalent to $1200 today. Most "nice" turntables at the time were $200-300. The SL1200 was designed around the SME 309 tonearm, and the SL120 version came with an SME armboard instead of a tonearm. DJs adopted SL1200s gradually because they're rugged and spin up to speed quickly. In the '70s, a 4-to-5 lb. platter was considered a heavy platter.

There are many things to commend an aluminum alloy platter, some from a design standpoint and several from a manufacturing standpoint. To control ringing Technics dampened the underside with a thick rubbery coating, and furnished a 17-oz 1/4" thick turntable mat to dampen the topside. This was later dropped to half the weight and thickness because the DJ market favored it. I have a heavy Oracle Groove Isolator sorbothane mat and it quieted my SL1210 M5G nicely.

Before belt drive really took off, aluminum was the platter of choice, inherited from the rim drive 'tables of Garrard and Dual. As mentioned before, it was easy to mill, balance and shape. It also has plenty of tensile strength for creating a thin rim to accommodate the pressure of the motor's idler wheel. Acrylic, MDF, and glass would not make a good platter material for an inner rim drive as implemented by Dual and Garrard.

So most of the reasons for aluminum are historical. If Technics had continued making turntables for the high end home audio market, they might have come up with an acrylic platter and better damping and isolation, but they didn't, so economy of scale dictated that they stay with the original design and materials and shift their market to the dance clubs.