Tables That Feature Bearing Friction

Dear Dgarretson, before I get off for holiday, - first of all: very nice set-up you have done with your modificated TNT. There is much more insight and mechanic knowledge displayed in your set-up, than in many comments posted on that topic here in the last days.
As for the denier of the aramide thread.
It depends on your intention regarding the amount of grip you wish to have on your platter. With your set-up the thread does entangle the platter for 5/6 of its circumfence. I would start with a thread about the "thickness" of a sewing-linen. This will run smoothly through the pulley groove of the VPI motor and will give "fairly" good grip without becoming too massive. Aramide thread have become so inexpensive and widely available ( it was a very different situation 18 years back.....) - just get a selection of 3-4 different strenghts. For best comparism - why don't start with a aramide thread excatly the strength/thickness of the string you are using right now. That way you get a direct "result" as for any sonic benefit/or possible drawback of the aramide in your given set-up.
I really am interested to learn about your findings.
Please let me know.

Dear T bone,

>Would not the drive (pull) side have a different tension than the 'lag' side - thereby making same resonance on 4 strings impossible?<

You are correct if the thread has stretch and is under tension. Dertonarm suggests the use of a thread with no stretch and advocates low enough a tension to allow for slippage(if I misunderstood, I apologize). Both longitudinal and transversal waves contribute to the vibration of a connecting medium(be it a rubber belt or a thread), modulating the load "seen" by the motor. The ratio between the two differs though. Since Dertonarm was approaching the discussion from a theoretical point of view and has emphazised individual aspects, I thought it would be fair to point to conceptual flaws as he has asked everyone to do.
But he has built a turntable following his dogma, appearently without running into a problem in this area as he said that achieving and maintaining correct(or what he considers correct) thread tension was not difficult. Complextity and resulting chaotic behaviour of elements working together can come to rescue here, rendering a theoretical problem nonexistant in praxis. Dertonarm was asking for input on how to improve upon existing solutions and my suggestion was merely aimed at that.
As a designer my aim has always been to avoid a source for problems rather than quantitatively minimizing the problem. I've built turntables with thread drive and counterpulley in the early eighties and later commercial implementations of the mirror image positioning of the counterpulley all fixed some major issues - side thrust on the platter bearing, uneven operation of the subchassis(Audiomeca Roma) - and introduced others - noise, belt "flutter", slippage(where none was intended to occur).

BTW, a platter driven by a constant stream of air will be harder to build and not necessarily better than one with a pulsed supply. If the pulsed supply works against, lets call it "teeth" or "wings" of appropriate shape, a very even drive force in agreement with Dertonarm's dogma can be achieved.
If the system losses are small enough and the inertia very high, such a pulse can occur only once per revolution or even less frequently.
One could see an analogy between tts and high precision timekeeping devices. The astronomical regulator with a "free" escapement and heavy(high inertia pedulum) driven with minimal force(keeping the drive system's influence small) stands for Dertonarm's thinking, the low mass high frequency oscillation of a quarz or even Cäsium atomic clock for the quarz-"locked"(it's not locked) instantaneous(it's not instantaneous)correction direct drive with light platter.

The atomic clock produces ultimately less deviation from perfect accuracy. Now why then do the best DD-PLL turntables with light platters(i.e. EMT 948) sound inferior to the best DDs with heavy platter or the best belt, tape, thread or rim driven turntables? Lack of proper execution or fundamentally flawed? This seems like a rethorical question, but it isn't.

If your motor has infinite inertia(god brought it up to speed initially...) and your drive is lossless, then your platter can be infinitely light, will be infinitely inert and therefore rigid as well. No more energy storage or mechanical impedance matching problems(topics for another thread), Yeah! :-)
I'm gonna run, create an avatar to build this theoretically perfect turntable in cyberspace. Not shure I'll be digging the music over there though...

Cheerio,

Frank

Frank, thanks. My understanding, from Mark Kelly's interesting writings on the subject, is that no matter the tension, some kind of slippage is, by definition, necessary; and that slippage induces differing tensions on either side of the platter, by definition - or perhaps I misunderstood his various writings - I would not put it past me :^)

I am intrigued by your pulsed air and finned/winged/etc underside of platter concept. Personally, I would expect substantial cogging potential from the airpump power supply, and other sources, so would (not being burdened by any practical experience in the area) avoid intentional pulsing. I would expect that running the pulsed supply through a series of buffer tanks would get you to your constant air stream with more accuracy, and if you ran that through the same 'toothed' underside which had a very tight tolerance for the space between the downward-facing 'top' of the tooth and the upward-facing air inlet area, it would serve to pulse the air as well, but as long as the air-pressure post buffer tanks were constant, and applied at several (or more than several) places under the platter in regular syncopation, it would go a long way towards lessening the impact of variations in the air pressure coming out of the tank, and if it was a high-inertia floating platter, the 'teeth' could be oriented to receive air pressure which would self-center the platter. The amount of air pressure necessary for the drive system would actually be quite low. One could set it up so that the platter got to speed through some other system, which was then clutched 'off' when the air pump took over. I guess the question is how one would 'brake' the platter using that system, other than using its own inertia, if one needed to...

As to your non-rhetorical question, my gut is that the qualitative differences between DD-PLLs with light platters and the best of the other types with heavy platters may have to do with the audibility of the speed of speed correction, which is where the inertia comes into play...

In any case, have fun in virtual reality

I would think that an "air drive" platter would not opnly be difficult to get started, but if it got spinning too fast would also be hard to slow down to the proper speed. The platter would have to have a wee bit of friction I would think.

A push with the hand is really not an option to start off unless there was some minor friction. You would want to have the speed build up and stabilize, so low to proper speed is the goal rather than high to proper + friction.

It is a rather intruiging proposition though, although the "friction" of the air on some form of wing under the platter could cause a rumble of sorts and a cogging effect I would assume. Not without its flaws, but probably the next $100,000 turntable idea. Anyone want to come up with a business plan?

Bob

Berlinta, T_bone, Raul, Dave, et al: Do you all except Dertonarm's contention that belt drive is the inherently superior way to motivate a platter, compared to all forms of idler- or direct-drive and notwithstanding theoretical air propulsion? It seems as though you all do. I don't, yet. T_bone, I think it's very significant that Chris Brady reports that his belt drive tt, and his rim drive motor when applied to his heretofore belt-drive tts, outshine any of his belt-drive creations, which are themselves held in very high esteem among end-users. I don't think his real-world experience should be dismissed BECAUSE it is anecdotal, if I understood you correctly. Rather, these are good data that have to be explained. The escape clause for anyone who wants to hang onto his or her own bias, is that perhaps none of us has heard the "ultimate" belt-drive tt.