Direct Drive vs. Idler Drive vs. Belt drive

With any DD that uses any sort of servo feedback to maintain constant speed, the feedback circuit was designed for a particular platter mass.  If you change the platter mass dramatically, it is quite likely you will do bad things for speed constancy.  Likewise if you were to change to a much lighter platter compared to the OEM one.  Now, based on reports of others and my own experiences, there apparently IS some leeway within which some perturbation of platter mass will not do notable harm to speed constancy, but since every manufacturer had a different thought on the feedback loop and how to have it operate, there is no general rule about how far you can go.  I also don't know whether to credit some of the reports one can read on this forum, if you search the archives. Nearly everyone who replaces a 1kg platter mat on a DD with a 5kg platter mat, sometimes more than doubling the total mass of platter plus mat, says how wonderful it is. To me this says more about listener bias than it does about the physics.  And I also don't agree that there is a linear relationship or anything near to it, between platter mass and SQ. Belt drives with 200lb platters do not excite me.

lewm

Nearly everyone who replaces a 1kg platter mat on a DD with a 5kg platter mat, sometimes more than doubling the total mass of platter plus mat, says how wonderful it is. To me this says more about listener bias than it does about the physics.

Why? The physics of the situation are obviously different, so why would you discount that?

I'm really glad to see so much information and ideas being exchanged here.
The point of my post is to get a general understanding of what each drive type might sound like or what people thought it sounds like, so it's good to hear opinions from many different people. I won't be picking just one turntable as I do enjoy having many options. At some point, I will be adding a DD turntable, it should be fun to experience all three drive types in my own system.
I do agree that there are so many different variables in a vinyl setup that contributes to the final sound from the stand it sits on all the way up to the record weight, so deciphering what is contributing to what is almost impossible.

@atmasphere 

Thanks for your thoughts, Ralph. About air bearings, I think that a very heavy platter on an amorphous carbon thrust bushing is virtually immovable - at least, mine is. You need a lot of force to raise it - and more to lower it - 87N per micron, plus the inertia. Compare that to a fragile cantilever. For radial stiffness, see above. The New Way figure is 34N/u.

What you get is a noiseless bearing, and you really can hear bearing noise. I've compared a first class turntable oil bearing with New Way air in a test rig, and it really is no contest. I've demonstrated sleeve bearing noise from a Premotec 1.8W motor through a belt - subtle, but it's there.

Could you please define 'isomeric isolation/mounting'? Is this a journal bearing in an elastic mount? Thanks!

Nice Post Neil 😃
I have tried over a dozen different platter mats from copper, carbon composite, felt, suede, leather, glass, rubber etc and have found that the MICRO SEIKI Cu180 solid copper mat sounded subjectively 'best' on my VICTOR TT-81 and TT-101 turntables.
I was concerned however, at the increased weight on the relatively lightweight aluminium platters Victor employed.
More importantly....the method of Bi-Directional speed control Victor utilises might surely be compromised by tampering so drastically with the 'mass' of the platter...?
Strangely enough....of all the other mats and materials I tried, the VICTOR PIGSKIN 1mm original platter-mat sounded the closest to the Cu180 🤗
Those Victor Engineers knew their 'onions'.....😃
Jico came out with their own version a few years ago....but using deerskin.
Not as good....😕
My RAVEN AC-2 BD Turntable....because of its solid copper bonded top-plate.....sounds best without any mat.
Regards
Henry