Vintage DD turntables. Are we living dangerously?

Dover,
Interesting impressions of spiked mats. I'll keep that in mind, although you repeat yourself. Once should be sufficient?

" their goal is to dump excess energy from the stylus/record interaction to ground as fast as possible."

Exactly how is that accomplished, by using a mat of dissimilar mechanical impedance of a record? That doesn't take vibrations to ground, a high percentage will reflect back to haunt the stylus.

This is from a 1987 interview of Pierre Lurne who worked for Goldmund and was responsible for the design of the Studietto table:
http://www.stereophile.com/interviews/pierre_lurne_audiomecas_turntable_designer/

"From the Minimum turntable in 1979, through the second turntable which I designed for Audioanalyse in 1981 and now to the Audiomeca J1, I used the same concept of mechanics. I'll begin with the platter. I agree with other designers that methacrylate is the best material for a mat, and the shape of the platter is the same as the Minimum, which is to say that it is a little concave: it slopes from the outer rim to the center at an angle of 0.30$d. I decided on this form from a statistical survey of a large collection of records. There are actually two sheets of methacrylate, either side of a solid, 8mm-thick piece of lead, giving a total mass of 8kg.

"This construction is something very special. If you know the velocities of vibration in methacrylic and in lead, you can calculate when the vibration is reflected back to the stylus. First, the vibration induced in the record from the stylus tracking the groove goes through the record into the methacrylate, then to the lead, and so on. Each time the vibration is transmitted from one material to another, there is reflection and transmission, and the time taken for each reflection to return to the stylus can be calculated. You need not have all these delayed signals reach the cartridge at the same time. You then get the same effect as with the acoustics of a room with square dimensions—one big resonance. This is no good, and in addition, when a large reflected vibration reaches the stylus, the tracking is instantaneously different. But if you take care of the spacing in time of these delayed reflections—do you understand the concept of the 'Gold Number?'—then neither the music nor the tracking is affected, not at the beginning of the record or at the end.

"We use lead because it almost behaves as a 'magic material.' It has high mass, it has good damping with low-Q resonances, and it has a very low speed of vibration. If vibrations enter the lead center of the platter, they leave considerably later, much lower in amplitude."

Regards,

Dear Richard, For further clarification and to expound on your post above, the current Artisan Fidelity Sp10Mk2 Technics Sp10Mk2 replacement platter (~5kg.) is comprised of a magnesium alloy based lower segment and pure Copper upper section. The platter's internal cross section features concentric damping provisions to help tame stray resonances. Initially, the platters used a slightly heavier (+.4kg.) aircraft aluminum lower section but was eventually replaced by magnesium alloy possessing superior damping characteristics.

Pryso, Acute observations regarding the robustness of the factory Technics Sp10Mk2 A's DC based motor, indeed it (including supporting bearing architecture) is easily capable of accommodating a heavier platter and/or mat over stock. For reference, the factory Mk2 platter weight = 2.9kg. and a respected replacement platter mat, say for example the well known vintage Micro Seiki CU-180 weighs in around 1.4kg.

Chris74.
Thanks for the expansion of details on your SP10MK2 platter.

Further, I agree on the robust nature of the bearing assembly. Both it and the MK3 are substantial indeed.

Geoff.
I see two main structural paths in a TT... One from the record upper surface, thru the platter, bearing, plinth (or shelf for those of us who prefer au naturel ), arm, cartridge and stylus. "The loop path"
The other, from the record surface to Mother Earth "The ground path".
For now, ignoring suspended TTs

I have used lead extensively in my TT designs, but it has always been to " laminate" these two paths and has not been inserted into either of them.
I have found this methodology to work well.

Cheers.

Last month Lew brought up an aspect of performance that we read little about, speed correction timing.

"It's also the case that the L07D servo was deliberately designed to exert a much looser control on the speed than does the Technics servo, for one example. I think it only activates when there is +/-3% speed error."

Maybe it's my imagination, but I've heard this difference between my Kenny and other direct drives. Any thoughts on the timing of speed error control?
Can properly functioning non quartz servo controls outperform some quartz locked counterparts?
Regards,

Fleib.
To add a little more to your question. I took a lot of notes when upgrading a customers LO7D. One surprising observation is that the motor stator is rubber mounted via grommets. Such that it is possible for the stator to twist a little backwards in an anticlockwise direction when applying torque. This would seem to be counterintuitive but it is there in the design.

I don't know about the 3% servo thing, but it has been quoted many times in various posts. If this is how it is built, it would be a reasonable assumption that it is effectively open loop once up to speed. In this way it is relying on the synchronous motor's innate speed accuracy and the platters inertia to maintain the correct RPM.