Direct Drive vs. Idler Drive vs. Belt drive

@mijostyn , comparing my big Nottingham Analogue Mentor with improved plinth, motor, and motor controller (all to Dais standard), to my DIY air bearing, is indeed no contest. The air bearing is dramatically better. Same model tonearm (TransFi Terminator), same identical cartridge (Koetsu).

But I can't hear the sleeve bearings on my little Premotec either - not until it is connected to the platter by a belt, and then only through the playback amplification. So I think that sleeve bearings are a problem anywhere in the chain, and the closer to the platter the worse it is. What do you think?

When the belt is tensioning/loading the bearings from the side (as happens with a belt), they're going to make more noise. When the bearing and the motor are synonymous, then its the thrust bearing that is likely more important.

Good point, both.

Yes, this is an advantage of a DD drive that is often overlooked. There is no horizontal force induced by a belt, thread, tape or idler wheel.

There is a toppling force if the thrust point is at the bottom of the bearing but this should be easily absorbed by a properly designed bearing. 
Invert the bearing with the rotating mass centre of gravity at the pivot and this force goes away as well. 

There is then the obsession with micro polished shafts. It might look great but technically it is sub optimal in an oil lubricated bearing. We want to maintain a continuous thin oil film. This is best achieved by a microscopically dull shaft surface finish which captures the oil film. Built properly a bearing like this has only the thrust pad as a contact point. Since this point has an extremely low radial velocity due to the small contact area, it can be made to be very quiet. Any noise in the bearing shaft/sleeve area is the shearing of the oil itself.  

cheers.