Tables That Feature Bearing Friction

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

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.

Hi again,
Mark Kelly is correct about the tension differential, provided the thread/belt does stretch and that there are frictional losses in the platter/bearing system. I don't think he said that slippage was neccessary, but rather that it was unavoidable in real life(maybe I got that wrong...)
Slippage can be avoided, idler, belt or tape creep can't.
My comment on the counterpulley symmetry relates to Dertonarm asking for a debate in the theoretical domain. I made it the sake of debating idealized concepts rather than compromised implementations(called reality), please take that into account.

A single constant airstream working against "wings" will cause cogging. Two(or more), 180°(x°) perfectly out of phase air vents with proper orientation of the wings will cancel the cogging(don't want to be responsible for the maschining of that one...). But this isn't all that different from a conventional multipole motor.
A single leaf and a one impulse(or 0,5, ...) per revolution is different. One could achieve the same without the complexity of an air supply, the turbulence issues, etc. by using a very heavy platter, direct driven with the motor being switched on only infrequently , be it for a very short duration or ramped up, then down again, but only when the speed drops below a set deviation threshold. Oops, I see the word feedback on the horizon, mmh...
The inertia of the platter will be an intergrator again, the constant motor noise/mechanical jitter will be exchanged for an occasional pulse and the bearing losses ought to be zero ideally.
Low frequency pitch stability will once again be less than perfect :-(
There again, it contains compromises... I'm still tempted to build such a device(and sell it for --- ONE GAZILLION DOLLARS, buahahaha...!).

A pulsed air supply with transfer function matched "wings" won't cause cogging IF you solve the problem of syncronisation(and if there was no such thing as turbulence, a.k.a. CHAOS).

Now back to "real" work :-)

Frank

P.S.: Related to the original post: There are at least 6 different currently manufactered turntables that partially rely on creating additional drag on the platter for maximum speed stability.