Skeletal vs Plinth style turntables

Ct0517 and RichardKrebs, have you considered filling the webbed aluminum casting of an SP-10 with a dense hard mixture of industrial epoxy and crushed granite(similar to Verdier) or brass powder? This might give the casting the properties of a massive plinth, while performing in conjunction with a rigid skeletal "plinthless" sub-frame. I've been think of this for my SP-10 MkII, together with a turnbuckle between a threaded bearing well and the sub-frame similar but different to the Porter concept.

Dgarrestson.
I have removed the motor housing on my MK3 from the original square chassis
See the triangular shaped TT on my krebsupgrade.com web site.

The rather flimsy naked motor housing has been laminated ( epoxy glued and bolted) into a machined duralumin housing of 15mm wall thickness. This in turn is adhered to a 15mm thick lead disc. ( the piece that was cut out of the plinth to accommodate the motor)

In my original post on this thread, I talked about minimizing material changes and joins in the platter arm loop along with consideration of propagation speeds, absolute dimensional stability and stillness.
This leads to the conclusion that the chassis has to go.

I also replaced the platter with an acrylic duralumin lead composite.

Regards

Hi Dgarreston,

Your idea for damping the SP-10 frame is similar to what a friend (physicist) suggested to me some time ago to damp unwanted vibrations. He suggested using Devcon Flexane 60 to the underside of a surface to be damped. The listed properties include "absorb noise and machine vibrations". http://www.itw-devcon.co.uk/index.php?/devcon_mro/flexible_urethane_maintenance_and_repair_systems/devcon_flexane_60_80_94/

It would seem a simple matter to apply. It would not have the mass of your ingredients but if I understand the product, as an elastic material (does not fully harden) it would server more as an absorber for vibrational energy.

Comments?

This is/was essentially my longstanding argument with Halcro vis a vis outboard arm pods.

Lewm seems a nice sort of guy but I can't bring myself to continually explain why his analogies are often theoretically inappropriate.
In fact Lewm, over the years has demonstrated a clear preference for 'theoretical' analyses over actual 'listening' experiments.
Virtually all the turntables I have listened to over the last 40 years have had a plinth and firmly connected armboard...
In fact I have just such a table mounted on exactly the same wall-mounted shelf as my skeletal table so I can actually LISTEN to both versions sitting on exactly the same "row boat"....

Here is the issue in mechanical engineering terms. It is not a lot different from the steering and suspension of a car.

In a nut shell:

the plane of the platter must be consistent with the plane of the cartridge. As the arm moves the cartridge must remain in this plane. Since the platter must revolve, there can be no slop in the bearing such that the platter can deviate from said plane.

As the arm must be set at a fixed point, it can then be seen that if there is any difference that occurs between the point of the arm and the surface of the platter that is will manifest as a coloration of some sort.

For this reason, the coupling between the surface of the platter and the locus of the cartridge cannot have any slop of any sort. To this end, the coupling between the bearing and the base of the arm must be as precise and tight as possible; IOW of a singe piece which will not respond to vibration, as if any differences can occur they will be interpreted by the pickup as coloration.

What this means is the more dead and the more rigid the plinth is, which also holds both the arm and the platter, the better the 'table will sound.

Now there is essentially nothing in Atmasphere's Post that I disagree with..
In my case, the rigid plinth is the wall-mounted shelf which is disconnected from any wall and floor Structure-Borne feedback...and that is really the most common problem with most turntables' support systems.
They are usually connected to racks sitting on suspended wood or concrete floors which are structurally deflecting under their own weight as well as live loads. These structural deflections induce low-frequency soundwaves into the suspended floor systems which are often below 5Hz and this low-frequency 'movement' cannot be stopped by racks or plinths.
That's why specialised stands designed to cancel out 2-5Hz Structure-Borne movement are used to support electron microscopes.
Air-Borne feedback is virtually a non-issue in all but a few extreme examples.
Once your rack and plinth are experiencing 2-5Hz structure-borne movement....you are on the Titanic and thinking about the safety of individual row boats..for one thing is certain....
The ship is going down...👀

Hi Henry, I was hoping you would come out of your recent shell. Once again, you've written nothing that comes remotely close to changing my mind about outboard arm pods. However, I do agree with you on some of the other issues... Where was it exactly that I was talking about "airborne feedback". That's your pet bugaboo; you raise the issue in order to beat it to death. I've never used the term.

The way I read Atma-sphere's post, I should think you would have to disagree with some of it. He is saying what I have always said. To wit, "To this end, the coupling between the bearing and the base of the arm must be as precise and tight as possible; IOW of a singe piece which will not respond to vibration, as if any differences can occur they will be interpreted by the pickup as coloration."

Take a look at an L07D some time. There is the epitome of what I and Atma are talking about in terms of "coupling".

All of this folderol aside, I have come to think of you as a friend and I would love to argue these silly issues with you over a Foster's, if and when I ever make it back to Oz.