Tonearm mount to the plinth vs arm board vs rotating arm board vs isolated tower

The OP in this thread, is asking about an idler.

@kanchi647
The idlers are one of the crudest, oldest form of turntable, and with them comes.....(based on my personal direct experiences)

The most vibrations, least isolation, the most noise.

That's what you are dealing with.

For these reasons I would recommend you try a design that works to eliminate those three (at least) design symptoms.

When done well they (idlers) can be quite good. I own a Jean Nantais Lenco that contains reference level (Parts) see my virtual system. His design was called the best turntable by blogger A. Salvatore at one time.

His armboard design specific to your thread question, has gone to great extent to reduce the symptoms mentioned above.

To give you and others reading an idea of how far he goes; here is a picture of mine showing the extent he goes to....to reduce vibrations, noise and provide for isolation. 

https://photos.app.goo.gl/MdYZd7sBGJ1cCHyC8

Similar cavities exist under the motor. Even with these huge cavities, it still weighs 100 lbs.

Those who intend to build an new plinth for their TT's are in different
position from those who want to keep their TT as is. My Kuzm Stabi
Reference accept only 9'' tonearms. Triplanar can be mounted
because the arm(wand) is on the side of the VTA tower allowing
for longer lengt. I wanted an second arm and was forced to use
an armpod + 12'' arm. I ordered both by Reed. My Kuzma and
armpod rest on aan sand filled base resting on 3 spikes 
in an professional rack ( German ''Copulare'') . I have no idea
what ''optimal solution'' means but am satisfy with the situatiom
''as is''. 

Halcro, if your arguments are good, it’s really not necessary to be insulting.

The OP in this thread, is asking about an idler.

@kanchi647
The idlers are one of the crudest, oldest form of turntable, and with them comes.....(based on my personal direct experiences)

The most vibrations, least isolation, the most noise.

That's what you are dealing with.

For these reasons I would recommend you try a design that works to eliminate those three (at least) design symptoms.

If you look above, I said pretty much the same thing. 
But "crude" does not equal "inferior". 
Take the first ten vehicles featured here  https://www.townandcountrymag.com/leisure/sporting/news/g2165/best-vintage-cars/
Each and every one is "crude" by today's standards and yet they are-imho-more desirable and more "worthy" than anything available today at any price. You can argue all you want that a top Tesla is far more "precise" (think "accurate") but where will your Tesla be in ten years. Likely in a scrap heap. 
As crude as they may be, idlers are built to last and capable of giving incredible amounts of joy and pleasurable reproduction of music in the home. They possess a color, more-so than top direct drives, but I would rather have the slight color they provide. 

I agree with @halcro , it is essential to isolate the tonearm from the platter bearing and the motor.

I also agree with @atmasphere , it is essential to have the tonearm rigidly connected to the platter bearing.

I also agree with @lewm , bolt the tonearm to the plinth if possible, but compromise as necessary.

There is a material which is highly rigid, on the same order as aluminum plate, which is also very highly damped. That material is panzerholz. I built my platter board out of panzerholz and glued a layer of carbon composite to the top for further rigidity and constrained layer damping. Then I bolted my tonearm to the platter board.

Bearing is all air - high pressure amorphous carbon bearings in three dimensions. Bearing is grounded by offset steel columns connected through panzerholz blocks from (slate) tabletop to platter board. Platter bearing noise is nil.

Outboard belt drive with a flywheel allows excellent isolation of the motor. I say, "Let's have it all !"