A Copernican View of the Turntable System

Dear In_shore, What you say is also well and good. I further still maintain that the tt and armpod ought ideally to be in unison, so use similar strategy to isolate both tt and tonearm, a la the Da Vinci. I would not favor using a Da Vinci or similar armpod with a tt that was sitting on AT616 feet, even though I know that others here are doing something akin to that and getting what they interpret to be great results. I think that might mean that the level of our concern regarding structure borne and internally generated forces amounts to massive overkill.

I dont agree with pods but I have thought about mounting a cantilivered armboard off the plinth using 3 mini spikes machined from grub screws so they are adjustable ( for levelling the armboard relative to the turntable bearing ) and using a nylon nut and bolt through the centre of the triangle formed by the grub screws to anchor the board. This arrangement would give you true 3 point mounting, energy dissipation and levelling capability.

Dover: For whatever reason, cantilevered armboards appear to increase the bass volume and power, compared to non-cantilevered armboards. This is from my own experience, as well as from discussing these issues with other turntable designers.

Still, if I were to design an ultimate turntable, it would have a low-noise platter bearing, an ultra-rigid connection between platter bearing and tonearm pivot (ideally machined as one piece), a helluva lot of moment-inertia in the platter, and even greater moment-inertia in the plinth (which would be designed for low structural and cavity resonances).

The complete turntable would be integrated with a self-levelling air isolation platform, which would rest on a stone column that goes directly into the ground, without any contact with the floor of the listening room. Surrounding the turntable (but not contacting it) would be a double-wall acoustic shield, vacuum-evacuated between the walls to minimize energy transmission.

cheers, jonathan carr (using 60kg solid zinc plinth to connect platter to tonearm)

Hmmm Jonathan,
Sounds like a typical listening room to me? :^)
Lewm,
The shrinkage of concrete is greatly determined by the water content.....the greater the water content, the more the shrinkage. (George knows what I'm talking about).
I doubt that concrete designed to set under water has much shrinkage.
My experience has been that concrete, as it cures, tends to stick to its edge formwork tenaciously, with most shrinkage occurring within its main body via shrinkage cracks.
That's why great care is usually taken to coat the formwork with 'releasing' agents so that it can be stripped away fairly easily.
If one were worried, or had experiences of the concrete shrinking from it's mould.......one could ensure the internal face of the cylindrical mould was coated with a rough epoxy film to which the concrete would cling.

You actually pre-empted another design I had which was for exposed concrete armpods.
In this case, the metal cylinders would not be permanent formwork, but would be coated with form-release agents so that after initial cure took place, the concrete could be released from its mould by a hammer tap to its top or bottom. This would allow many pods to be cast from the same mould and even pods of different heights to be cast.
The bare exposed surface of the concrete pod cylinder could then be left 'as-is'......or painted as one wished.

Jonathan - nice to hear from a fellow 60kg SPZ plinth owner - only point I would debate would be the air suspension, still think the tt has to be grounded no matter how much mass you have in the plinth. Rf shielding in the wall & perhaps a sealed room with an assistant inside to turn the record over.. One point I note is the number of people who locate their turntables in resonant room corners or between the speakers where sound travels free and fast.