Why Use Aluminum for a TT Platter?

02-09-11: Dan_ed
This is also why so many DD tables have light-weight platters. The motor on a DD is designed to directly manipulate the platter in order to correct speed. The platter is supposed to have it's acceleration constantly manipulated.

i think the reason why direct drive tables tend to have light-weight platters is for fast acceleration. club dj's are among the most enthusiastic users of direct drive turntables. 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.

one of the drawbacks of direct drive turntables is the possibility that vibrations from the motor would couple into the turntable and get picked up at the cartridge. since the vibrations would likely be relatively low frequency, it would likely be in the frequency range that gets amplified in the phono stage. so a higher mass platter would require a more powerful motor, which i would think, would increase the likelihood of vibration.

02-09-11: Hiho
You're talking about half Hertz at 33rpm. That frequency exists in ALL turntables. Direct-drive is as quiet as anything. Unless the bearing is bad quality, there is very little or no noise in DD, just like belt-drive. I would say it's even quieter than belt-drive. The motor turns at 33rpm, the same amount of rotations as the platter of a belt-drive or idler-drive, WITHOUT the fast spinning motor in a BD or ID that sometimes spins at 1800rpm, and that's 30Hz! Now, the DD platter is spinning at 33rpm by magnetic force and please tell me where the stereotype DD motor noise is coming from?

you can't hear 2Hz so obviously i was not referring to any 2Hz rumble. in the real world, nothing is machined to perfect balance, so when it rotates there will be some vibration. if you can hear sound from a drive motor, then obviously the frequency is within the audible range. a cartridge can pick that up and if it does, it is likely going to be in the part of the frequncy range that is amplified in a phono stage.

as to the magnetic drive to which you refer, a rotating magnetic force field (which is what you need to turn the platter) doesn't just happen by magic: there has to be some motor that turns a rotor (e.g. an induction motor) that creates the time varying magnetic field that could turn a platter by lorentz force. there will be *some* vibration in that motor. if it is a reasonably well made turntable you probably won't be able to hear it, especially if you use a large signal cartridge.

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.

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.