Why Use Aluminum for a TT Platter?

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.

T_bone, very good post. This DD sub-topic came about after some general comments about dd/bd and mass, and thus my own generalized comments. It is most definitely a much more complicated discussion. I think if Chris starts talking about eddy current braking and such we'd soon leave the dj talk behind. ;-)

Livemusic, Yes Cocobolo is still the best platter material I have heard, but Garolite sounds nearly identical and is far less expensive.

Tbone, Coreless, slotless motors have very low cogging, but it is not zero. A top quality DD table is remarkably sensitive to even extraordinarily small amounts of cogging. I have no first hand experience with the fine motors you cite, but suspect that the very low cogging they exhibit is still audible.

Higher platter mass generally translates into better sound. But as is often the case with audio there is no consensus. Heavy platters sound different than light platters. Most but not all prefer the sound of heavier platters. I have experimented with platters up to 75 lbs. For my tastes I found that heavier was always better and never found a point of diminishing returns. But anything beyond 75 lbs starts getting very impractical.

An interesting discovery was that both belt drive and DD implementations seemed to benefit from heavier platters. But with DD the benefit from a heavier platter was significantly less than with belt drive.

I had a Technics SL-1100. I think it had the distinction of being the first DD machine made. It also had the heaviest platter of any DD machine, including the SP-10 and certainly heavier than any Dual, Garrard, Thorens, or the like.

The 'cogging' thing is not an issue on the better DD 'tables. However I should point out that this is a problem with any servo-controlled motor (often found on belt-drive machines) if the servo is not designed properly. The result can be that if you graph the overall speed of the table over time, the result will be something that looks like a sine wave, centered around 33.33 rpm. The slower the sine wave and the less amplitude it has the better.

Subtle speed variations like this are heard as a loss of focus in the sound stage due to variable skating forces on the arm. This is why a belt drive, using an AC synchronous motor, might be preferred over some of the cheaper DD machines. But to assume that all DD machines have this problem would be a mistake.

"But compared to other 200-300 turntables of 1975, the platter of the SL1200 was pretty heavy."

That may be but I remember thinking something like a 12 pound platter was heavy.

"Sorry. I meant the SME 3009"

I should have thought of that. Response from SME:

Regarding the SME 300 Series we can confirm production began in 1988 as an addition to the SV ‘range’ which of course includes the SME SIV. The 309 arm remains much the same apart the introduction of magnesium arm tube approximately 11/12 years ago. This change was to bring the arm tube into standard production alongside the SV and IV but clearly is also an upgrade to the 309. The 309 today is one of our most popular arms and provides many of the sonic benefits so often referred to in HiFi press of the SV tone arm.