Why Use Aluminum for a TT Platter?

In my experience aluminum is not the best platter material. But there are good reasons that it is common and popular. It is easy to machine, reasonably inexpensive and when layered with another material or at least a good mat it performs quite well. Brass is a far better sounding platter material than aluminum, but costs 4 to 6x as much as aluminum. So as good as brass is, it's not a great value. I find that paper based phenolic, like Garolite XX, makes an exceptionally good sounding platter.

The discussion about vibration from motors is missing the most important point. It is not vibration that degrades sound but cogging. Motors with a lot of cogging usually, but not always have detectable vibration. Because of the isolation afforded by the belt, most belt drive motors have a lot of cogging. Because DD motors have no isolation they must be designed to have dramatically less cogging or they would sound terrible. Cheap DD implementations often have mediocre motors that have too much cogging (still far less than belt drive motors) and hence sound ragged. It's not that DD is bad but it's that a poor implementation of any sort will sound bad.

I find that DD has the potential for goodness that cannot be matched by even the best belt drive implementations, but bring your wallet because it is both expensive and difficult to get right.

02-09-11: Robob
"In the '70s, a 4-to-5 lb. platter was considered a heavy platter. "

Considered heavy by who? Certainly not any of the dealers or audiophiles I hung out with.

i used to have a dual turntable, which was a fairly popular, and reasonably well-regarded, brand at that time (at the time it was better regarded than panasonic, but that might be a matter of who your circle was). i believe that the platter was in that range.

02-09-11: Robob
"In the '70s, a 4-to-5 lb. platter was considered a heavy platter. "

Considered heavy by who? Certainly not any of the dealers or audiophiles I hung out with.

I was a dealer at an audiophile store in 1975 when the SL1200 and relatives came out. The SL1200 platter was heavier than the platters on Garrards and Duals of the time, and heavier than platters on the Rabco, Philips, and B&O turntables, and at least as heavy as a Thorens platter, all upscale from the Dual. The really heavy platters started coming along with the Micro-Seiki and similar massive belt-drive turntables and showed up soon but were not as widely known. But compared to other 200-300 turntables of 1975, the platter of the SL1200 was pretty heavy. I oughtta know; I held most of these platters in my hands as we assembled them for demo models.

"The SL1200 was designed around the SME 309 tonearm"

I'm thinkin' the SL1200 was designed and brought to market before the 309 existed. Anybody know? I have sent an inquiry to SME.

Sorry. I meant the SME 3009, which existed before the Technics DD 'tables appeared. It has a J-shaped tonearm with standard detachable headshell. The SL120 was an armless DD turntable with a calibrated SME 3009 cutout. In fact, the Technics tonearm on the SL1200 variants has an effective mass of 12.5g, the same as a SME 3009 Series II.

"I find that paper based phenolic, like Garolite XX, makes an exceptionally good sounding platter."
Hi Cris, do you still think cocobolo wood is the best platter material available? I'm personally a big fan of your wooden platter; happy with my DIY wood/bronze platter inspired by your design.

If you use enough aluminum, it can get pretty heavy. It is a lot easier to work,
and a lot cheaper than using stainless steel, bronze, brass, copper, feric
alloys, etc. When you are trying to mass produce (and keep in mind, in the
70s, companies made turntables like today they make ipods), ease/speed of
creating the shiny metal piece was paramount for many.

Moving off-track...
There are DD tables with heavy platters. I have a couple with platters which
weigh 20+lbs.

Not all DD motors are built to instantaneously bring the platter to speed if
there is the slightest speed variation. Some are built to bring back to speed
more slowly.

Cogging is an artifact of certain motor types (generally, those with iron cores).
Not all motors cog. The top brushless, slotless, coreless motors by Pioneer,
Yamaha, Denon, and others are (generally) extremely smooth (zero cogging).
They'd be a bear to make again in some cases. In general, these motors will
have less torque than slotted motors like Technics.

From what I can tell, the first really heavy platter came on a belt drive Melco.
Micro copied soon afterwards. That said, DD platters gradually got heavier on
the high-torque motors. The Sony PS-X9 of 1976 has a decently heavy platter
which is very wide (more inertia). The Technics SP-10Mk2 platter is not light.
The Yamaha PX-1 of 1978 is also a decently heavy platter, and the higher-
end tables from Exclusive, Onkyo, Lo-D, and others from the early 80s mostly
had high-mass platters even though they were DD tables (they all had
coreless linear motors too). The lighter platters on high-end tables came on
Denon, JVC, and most other Sony tables, which operated with substantially
lower torque than other high-end tables.