Vintage DD turntables. Are we living dangerously?

Fleib,
I don't think the record/platter interface is understood as you claim and that's where we differ...
If it were....there would not be the plethora of platter materials, shapes and weights nor the cornucopia of platter mats (materials, thicknesses etc).
I don't believe there are even two turntable manufactures with identical platter/mat combinations...?
And if...as you declare....

The purpose of a mat is to provide a stable surface with a similar impedance of the record,

then the majority of turntable and mat manufacturers fail in this endeavour..😱

Some think the best mats are forms of acrylic, methacrylate (Delrin), carbon, or vinyl.

And some people DON'T think that....
Some people even think that NO mat is better....and on some platters I have found this to be true...
And then, as Lewm points out....some even think that the less contact the record has with a platter...the better😊

If you believe that all these opinions reflect an "understanding" of the record/platter interface....I simply beg to differ...😎

Dgarretson, although I've not seen a PD444 in pieces, probably its bearing spindle sides were cut into a shape resembling a helical gear. When the spindle turns, such a gear thread will exert a pumping action on the oil contained in the bearing housing. This pumping pressure will center the spindle (so that a hydraulic film is maintained between spindle and bearing housing), and partially unload the spindle pressure on the thrust-plate (depending on the orientation of the helical cut).

With this kind of internal structure, it would be no great surprise for the bearing to be sealed.

Other turntables incorporated a similar "Archimedes pump" philosophy. For example, Trio-Kenwood's "DL" motor (employed in the KP-880 and later models) had a herring-bone pattern cut into the spindle sides, again for the purpose of pressurizing the oil inside the bearing housing and centering the spindle.

As to why the magnetic route wasn't pursued, for sonic reasons most likely Luxman didn't want full platter levitation, which meant that simpler hydraulic partial levitation was sufficient to protect the thrust-plate from the weight of the platter that they chose to use.

kind regards, jonathan

Halcro,
If the record/platter interface isn't yet understood or a matter of opinion, why defer to original options offered for your old table? The fact that there were options implies there is/was no one right solution. Perhaps not, but I think this interface is understood better than it was 35 years ago.

Seems to be three schools of thought, damping, impedance matching - preventing reflections back, and coupling or decoupling like Resomat. Admittedly, I didn't consider that one.

I suspect platter mass and rotational imperfections might have something to do with preference. Perhaps cart/arm synergy is the biggest factor.
Regards,

Lew, The Resomat is almost weightless. It consists of a paper-thin acetate sheet that serves only to fix the locations of the small and lightweight vinyl acetate cones that couple the record through the sheet to the OEM platter. The cones pass through the sheet and their bases rest directly on the TT platter. So the idea with this mat is not to isolate the LP from the platter, but rather to couple it at two radiuses. The mat is available in several versions: short cones or long cones designed to clear the raised lip of an SP-10 platter.

With L07D I'm using a sandwich arrangement. Stillpoints LPI record weight over short-point version of Resomat over TTW Weights 4 lb./4mm solid copper platter over ERS cloth mat over Kenwood steel plate. The Kenwood's tall spindle is just long enough for all of this.

The weight of the copper mat is pretty close to the weight of an original Kenwood perimeter ring. The performance with the copper mat is improved by pressing the button on the L07D PS that was conceived for the perimeter ring.

Jcarr, very helpful & thank you for the info.

Interesting to consider torque to mass ratio with the Yamaha GT 2000. Stock platter was 5.8kg. Optional platter was 18kg and was used w/o modification. That's almost 40lb. The Goldmund Reference platter was 35lb, was said to be too heavy for a DD table.