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

For practical reason this type of Steve Dobbins plinth for Garrard 301 is universal for different tonearms if you're going to use many.
Same construction in Stereophile article

Aesthetically Artisan Fidelity 301 is the best (imo), but you need many armboard to swap tonearms.

Well, I know Steve Dobbins. I have met him face to face and have had extended conversations with him about optimization of the venerable Garrard 301. Steve will tell you that the swiveling arm board is a compromise and is not optimum. It is a useful convenience feature. A very useful and very convenient feature but still a compromise. For optimum performance, he advocates mounting the tonearm directly to the plinth. No replaceable cut-outs, no extensions, fixed or rotating. Why should this be surprising when extensions resemble diving boards?
Everything needs to be kept in perspective. The Garrard 301 is not the ultimate in terms of being quiet or in terms of speed accuracy. Instead, it has it's own sound, a very good one. In light of that, one should not sweat bullets about having or eschewing a rotating arm board. They are clever and indispensable for those that want to use lots of different arms. The compromise in overall SQ is likely negligible. I would argue that it is only when using the very finest MC cartridges with the 301/401 that one should avoid any otherwise avoidable compromises. 

We were discussing the Garrard 301 at the Axpona 2019 room featuring his turntable, a 301 on a Dobbins plinth with a Reed arm and a top of the line VdH cartridge and Magico speakers. I don't recall the electronics in that room. The room sponsor was Van den Hul's former distributor who has since then been replaced with VPI. 
My opinion though, fwiw, would not change with any type of drive system. 

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. 

Outboard belt drive with a flywheel allows excellent isolation of the motor. I say, "Let’s have it all !"
Actually, a huge "THANK YOU" to all three of the above, who guided me to the above design. Also to the late Tom Fletcher. I certainly could not have done it without you

I don’t mean to pick a fight, but your post is self-congratulatory, don’t you think?
There are those who would argue-correctly imho-that there is no such thing as perfect vinyl playback and not one option provides "it all", particularly any design that is belt drive.
As between idler, dd, and belt, belt is the worst in terms of speed stability and musical propulsiveness/dynamics. This explains why VPI plays around with using not just one, but two, and even three belts. This explains why some belt drive designers incorporate dental floss or non-stretch thread to drive the platter rather than a rubber belt. The very same material that decouples the motor through elasticity and vibration absorption-rubber-introduces other sins that are arguably worse.

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

Among TD124 cognoscenti, this is a subject of debate. According to Greg Metz of STS who studied the TD124 in Switzerland under an original engineer involved in the design of that iconic table, coupling to and not isolating the tonearm from the bearing/motor is essential to the design. This is why the TD124 chassis encircles the armboard mount and the armboard is to be tightly screwed down to the cast iron chassis-the same chassis upon which the platter bearing and motor are mounted. The idea if I understood Greg correctly is that everything vibrating is concert is better than vibrating out of synch. The same school of TD124 experts state that a minimal mass plinth-really nothing more than a frame-similar to the stock base is best for the very same reason-a heavy solid plinth decouples the motor from the tonearm and would be the reverse what was contemplated by the engineers for best possible sound. So I am only saying that there are no absolutes. It all depends upon the design.
And yes, many people rave about the sound of their TD124’s mounted in slate and other high mass plinths. But how do we know that they have done valid comparisons? How do we know they are not judging based upon their eyeballs rather than being unbiased? The stock hollow framed base for the TD124 sure does not appear nearly as impressive as a huge hunk of slate, granite, or solid birch ply. IMHO, even the sorely missed Art Dudley got things wrong on his TD124 in this regard. He made an assumption and not an empirical decision as to what is best for the TD124. I say all of this because I used to have a heavy birch ply plinth for my hot-rodded restored TD124 and now have a hollow framed base not too dissimilar from what came stock back in 1959 (see pic in my profile of my present TD124 set-up). 

@terry9; "offend" is not the appropriate word choice here. . 
I simply felt your post was a bit strong and worthy of comment. 
I will never challenge someone's love of their own table no matter how humble it might be. 
But when someone claims that their deck is perfect, my ears perk up. 
You took pains to claim that with the help of three people you had achieved perfection. 
Halcro by the way obviously has far more experience than I do and I have a feeling you quoted him out of context. 
I don't claim to have superior knowledge to anyone here. 
In fact, I know that I have less than many. 
But I do have some opinions that are only worth your 2 sense if you happen to agree with me and I don't mind those that disagree as long as the basis for the disagreement is rational. 
This is a topic that particularly interests me and I have some first-hand experience with it. 

The arguments just go round and round like a record.
Most of us manage to get good sound and most of us have no idea why.
Most of us think we have things figured out as to why our systems sound good to the point we will pick up our figurative sword and vehemently argue our point on boards like this and yet what we won’t admit is that we are always looking for the change that will make everything better.
I don’t claim to be an exception. I fit all of the above.
That said, I think that a cast iron or similar inert plinth weighing 200 lbs or more will NOT make a motor unit-bearing-platter-tonearm interface sound better. If someone gets good sound with such an inert massive plinth it is by fluke, not design. I am thinking of Oswald Mills cast iron plinth for the SP10 as I write this. It might very well sound great with that particular direct drive deck but I doubt the same would hold true for most other designs, particularly idlers.
I like to give this example-John Atkinson can measure loudspeaker enclosure resonance and predict the impact of same on the speaker’s sound quality til the cows come home and Mike Fremer can tap on the plinth while a record is playing and tell us what he thinks this means until the swallows return to San Capistrano but neither test means squat. And further, a totally inert loudspeaker enclosure built like a sarcophagus of poured concrete will likely sound.....dead. Aluminum rings and yet people listen to Magicos and swear to themselves that with all that expensive space age technology and expense and machining, well they gotta sound good. I don’t think they do but if you do, I would suggest it is despite the space age aluminum enclosure, not because of it.
Vibrations have to be tuned and managed, not eliminated. Just as a microphone transducer vibrates, just as a loudspeaker driver vibrates, just as soundwaves emanate from vibrations, so too does a stylus/cantilever. No one argues that those vital vibrations need to be deadened, but then many try to deaden everything else.
Fremer gave Rega’s latest top turntable a rave review. He quoted them as stating their design principle was to keep in mind that a turntable, at the end of the day, is a vibration measuring device. And then out of habit he rapped his hairy knuckles against the plinth and noted it was ’lively" Arghhh. When will we ever learn? Harbeth, Volti, DeVore, Audio Note, and many others have this figured out when it comes to loudspeakers. I am not a Rega guy but Rega has if figured out. But on and on it will go, round and round.