Why Use Aluminum for a TT Platter?

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.

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.