It's not a new idea. The Garrard 301 used an "eddy current brake" in much the same way. The degree of braking was used to fine adjust speed, but it also stabilized the speed. Don't know of any other "modern" turntables that do this, though. |
I don't know of any direct-drive tables that deliberately use this strategy, except possibly in their electronics. Can anyone think of any? |
Come to think of it, Win, I see your point. I observed the herky jerky movement of my SP10 motor without the platter. It almost threw itself off the benchtop where it was sitting. So clearly, the mass of the platter smoothes out the operation. Further, the mass of the platter is constrained to within certain limits, if a servo dd table is to operate well. Of course, that is quite different from an eddy current brake, but the principle is perhaps similar. I just had not thought of it that way. |
Dertonarm, Are you saying that the vertical magnetic suspension of the La Platine per se simultaneously offered a form of eddy current braking of the rotation of the platter in the horizontal plane? If so, why would that not also be a feature of the current version of the La Platine? (I am going to check my physics books to verify that such a phenomenon would occur, but for now I accept the principle, if that's what you meant.) |
Thanks. Very interesting, indeed. I do recall hearing rumors about La Platine platters "falling down", due to loss of magnetic field strength, but none of my (two) friends who own the table have had that problem. There are a few other brands that now sport magnetic suspensions, but as far as can tell from photos, none of those has magnets of nearly the same size as found even in the current La Platine. Nor are the platters as massive. |
Dertonearm, I don't know whether you have already done so, but you might like to go over to Vinyl Asylum and search on the musings of Mark Kelly, a very smart fellow, on the various platter drive mechanisms and their pros and cons. Of additional interest is his work to develop drive systems for AC motors that reduce motor noise and cogging. |
Dertonearm, You wrote, "Stylus drag is a very small sliding force in constant motion and is - coupled with any serious platter (of course not if the LP lays just on the platter and is not firmly clamped down) - really neglectable. Its a force smaller by several magnitudes compared to the energy the stylus puts into the platter while modulating the groove information. A force smaller by several magnitudes compared to any motor generated vibration."
In writing thus, you are implying that you know the approximate magnitude of the force of stylus drag. I would be interested to know what that is, so I can compare it to the magnitude of the other forces you mention. I tend to agree with you, that the force of stylus drag cannot be THAT great, since, if it were, cantilevers would be ripped from their insertions into the cartridge.
I don't think anyone would take issue with your definition of the function of a turntable. The problem is that many of us have heard or think we hear deficiencies in LP reproduction that are attributable to "something" that is at least partly overcome by the use of direct- or idler-drive, as opposed to belt drive. In this tiny little world of vinyl audiophilia, that "something" has been identified as being the result of stylus drag, because there are no data to indicate what else it might be. By the way, I don't think you meant to infer that stylus drag is a constant. Would it not be expected to vary according to groove tortuosity and distance of the stylus from the spindle? |
What I wrote was that any effect of stylus drag on speed will in part be a function of the distance of the stylus from the spindle (not "distance from the stylus"). I was referring to the mechanical advantage gained by applying a force some distance from the center of rotation vs near to the center of rotation. Kirkus got it right. |
The string "belt" on the mammoth turntable in the photo is WAY too long for optimum control of platter rotation, even if the string has no stretch at all, but I guess the inertia of the humongous platter mass compensates for this problem to a degree. I am also guessing that the very long distance between motor and platter is an exteme attempt to isolate the platter from motor vibration, a la the Verdier on a grand scale. Nevertheless, it is a "compromise". IMO, it is impossible to avoid all compromises in any design and in any other human endeavor. Wherever there is a choice between two options that each has its own justification, there will have to be a "compromise" with respect to some feature of the desired outcome of the project. God help the perfectionist. |
Syntax, In that third photo, is the platter on the right driving the platter on the left, as it appears? This is in keeping with some of Mark Kelly's teachings on "belt creep" and how to prevent it. Anyone interested in this topic should search on the Vinyl Asylum archives for Mark's ideas. He is a brilliant guy who applied math and physics to belt drive and who compared the different qualities of idler-, direct-, and belt-drive to each other, also on a math and physics level. Far be it from me to speak for him, but based on my understanding of his findings, a very long thread between a very narrow diameter pulley and a distant platter is not a good idea, likely to result in significant belt creep compared to other arrangements, such as using one platter to drive another, using capstan type devices to force the driving string to conform to as much of the circumference of the driven platter as possible, etc. |
Dertonarm, You have written that there should be no force on the bearing in the horizontal plane, and elsewhere you have written that use of more than a single motor is a no-no. (I agree in both cases.) But how would you achieve the former goal in view of the latter principle? |
Dertonarm, For your consideration, an idler-drive in which the motor force is applied to the underside of the platter, in the vertical plane. Thus no horizontal force needs to be cancelled. No string needs to be chosen or "adjusted". In short, I offer you "Super-Lenco". Take a look at the Saskia turntable. (I know you will dislike the possibility for idler wheel "noise" to be transmitted into the platter, but this is the real world where choices have to be made. Believe me, that turntable is silent.) Put a Saskia on a Minus-K or an industrial isolator for an electron microscope, and you might be in vinyl heaven. |
Dertonarm, You are entitled to your opinion, but please acknowledge that you DO have an opinion - that the ultimate turntable must have a humongous platter driven by a string or thread. That is an agenda in itself. You have closed your mind to other ideas, no matter how well executed. We are just beginning to see how good idler-and direct-drive tables can become, in my opinion and assuming the vinyl renaissance continues for a whle. After all, belt-driven tts have been researched almost exclusively since at least the early 80s, whereas these other technologies were all but abandoned at that time. Anyway, thanks for all your insights; I do not mean to be argumentative. By the way, I would guess that a "cheap" idler- or especially a direct-drive tt costs more to make than a "cheap" belt-drive tt, which is a part of the reason those drives were abandoned. |
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. |
