Why Use Aluminum for a TT Platter?

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.

02-09-11: Nick_sr
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.

things have probably changed quite a bit since the days when i used to go to clubs. in the 1980's disco scratching, mixing &c were much the rage.

"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.

"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.

Y'all be cool,
Robert

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.