Why Use Aluminum for a TT Platter?


Mass I am told is a good thing when it comes to TT platters. Lloyd Walker for one extolls it's virtues and as a rule some of the better turntables like to brag about their big ..Platters. Why then would aluminum, known for it's light weight (low density) turn up as frequently as it does as a platter material. I know it is easily machined but isn't there anything better and much denser.
mechans

Showing 3 responses by atmasphere

Aluminum is used for several reasons. The first is that it is non-magnetic. This keeps it from messing with the cartridge. Aluminum is also relatively cheap, as it is one of the most abundant metals. Its also easy to machine.

An aluminum platter should be damped in some fashion for best results. If this is done it will keep up with any other material with no worries.
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.
Copper is the best for platter top!

There are problems with any metal for being the top of a platter. The top of the platter should be available as a means of damping the vinyl as it is tracked by the tone arm. Metal is ill-suited to this task.

Copper is a special condition that likely should be avoided. In the presence of a magnetic field (perhaps the one in the cartridge itself), the spinning copper will form a sort of primitive generator, and could easily be responsible for extra noise in the playback.