Why Use Aluminum for a TT Platter?

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.

As a former club dj and as current employee for one of the worlds largest aluminum companies here is my two cents worth.

First regarding DJs and direct drive tables

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.

This is not true unless you are a bad dj and don't know how to mix. DJ's are attracted to the SL1200 for its ability to accurately adjust and maintain a steady speed with the pitch adjust. As a dj you want to mix the music such that the BPM's of the next track are in synch with the one playing so the change from one to the next does not disrupt the dancing of the crowd.

Second, upon purchase of a new table by a dj, the rubber mat that is provided is thrown out along with the rest of the useless packaging, then a felt mat is used to allow the platter to spin beneath the record such that the racord can be held in place or pulled back during the cueing process without disrupting the movement of the platter (it act as sort of a coupling). If you are scratch DJ you may even chose to add teflon record sleeve cut into a round shape between the felt and platter just to be sure there is no sticking.

Now that this is cleared up, high inertia could be benefitial for DJs provided the performance of the slip mat "felt coupling" can prevent the fly-wheel from slowing. So far the direct-drive quartz speed adjust is more reliable.

Aluminum or Aluminium, not much to add other then to confirm what has already been mentioned, it is relatively inexpensive, non-magnetic, easy to machine, cast, or forge and readily available in wide variety of sizes and formats. Copper has become extremely expensive. Steel is cheap but difficult to machine, magnetic and will it rust unless protected, bronze contains copper so it is also relatively expensive. Finally lead, is toxic. There also magnesium, berylium and titanium, these are expensive, difficult to machine (specialy for Mg, which the chips are flammable and potentially explosive) and toxic in the case of beryllium. So in terms of metals Al is pretty easy choice.

As for other materials such a plastic, carbon fiber or wood, I can't comment, but I would image they could be good choices for the given application.