Tables That Feature Bearing Friction

Lewm, two magnets rotating in a horizontal = planar sphere do indeed produce an eddy current field.
The Platine Verdier folks and dealers will hate me for this .......anyway:
The current La Platine does feature somewhat lower quality magnets (compared to the old Focal magnets used till 1990/91 - that particular magnet was no longer available when Focal changed to the "6-tablet-magnet" - design invented by J. Mahul for the 15" woofers in early 1991) - thats why they promoted the ball to be inserted in the top bearing shaft hollow. To stabilize the vertical movement of the platter (in mid-90ies production was a tendency to instable magnetic field and often in loss of magnetic force causing many Platines in europe (and I suppose elsewehere too) to "oscillate" (= being unstable in height of platter)). To solve this problem the "top ball bearing shaft" was introduced (well, the hollow was there before, so they just put in the ball - smart move). That particular problem never occured with pre-1991/92 Platines. However the eddy current brake effect is no longer as dominat as it was in the original version with much better and more homogenous magnets.

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.

As to my knowledge all La Platine Verdier sold after 1995 do already feature the ball supported bearing. So those can't actually "fall down", as the vertical position is already determined by the ball bearing and no longer by magnetic force. The Platine verdier was imported and introduced to the USA fairly late (it was originally a DIY-project presented in french L'Audiophile magazine (with detailed schematics and description how to built) in the late 1970ies and the first offical built retail version was tested in summer 1980 in a german magazine). I do not know, whether there were any Platines delivered to the USA via the offical importer before 1995.

Thanks for qualifying your position, Dertonarm. I understand now that you are referring to a specific approach taken by Verdier. That approach as you have outlined it does seem to be unique, but costly and hard to control from a manufacturer's point of view. As you documented there were other issues with this approach, so perhaps it wasn't all that to begin with. Seemed to be a good idea.

The eddy current breaking is a solid, proven approach. The problem is that it still seems to be beyond the financial means of most of us, so we are left with belts or idlers. To Chris's point, the non-compliant mylar belts and lower torque motors do sound very good. This type of belt along with a decent control mechanism can provide speed stability that is better than most belt drives and I believe it is very close to your average idler. I suspect this is because we are starting with a no-cog DC motor and the mylar provides a much tighter coupling to the platter than any stretchy belt. But that is going off on a different topic.

If nothing else this discussion should show that the designer must take the entire drive chain into consideration, regardless of whether the bearing or motor is chosen first. They still have to work together to produce a speed stable platform.

I agree with you Dan_ed. However - the eddy current brake approach as shown by J.C.Verdier isn't all that costly. All you need are 2 old 15" woofer dirivers magnets which never suffered from shock or extreme low temperature. Thats about all you need to get those 3 features mentioned above all in one.

BTW - idler drive was introduced by and for the broadcast service turntables first. Kind of heir from the days of the grammophone. For good reason. It was of paramount importance that the platter had full 33 1/3 rpm after less than 1/2 turn. You need it for broadcast. For timing the tune played next. When direct drive had grown to full mature (by mid 1970ies the latest) the idler drive TT all vanished from broadcast stations.

Idler drive needs extreme care in execution to supply good results for turntable application. It however always gives some problems as the idler wheel itself is a source of direct noise transmission to the platter.

The idler drive is the direct counter-approach to the belt drive - one favours direct coupling between motor and platter to have very direct and immediate control over speed. The other favours as little influence and as little coupling between motor and platter as possible to minimize any possible vibration and speed shift in the motor being transmitted to the platter.

If the time frame till stable speed is actually reach is of little to no importance, - a very heavy platter coupled by string (= little grip) and driven by a very good motor will give the most stable speed for a turntable. Huge inertia combined with "slip coupling" or a kind of "cumulative coumpond motor drive" (read: very good motor coupled via string to a platter with little grip). If correctly done, it will take fairly long to get to stable speed, but once there, the speed will be extremely constant and little changes in the motor have no effect on the speed of the platter due to the - wanted" slip/low grip of the string.

Of course, this is one of many approaches in today and yesterdays turntable design. It is however the technical engineers approach if absolute stable speed to the prime goal. And if the time taken to reach this stable speed is neglectable.
The whole scenery is worth musing about. However this approach does ask for fairly expensive components (= high quality = expensive motor and very large mass in platter and extremely precise manufacturing and tooling) and huge weight in platter. Nothing that can come cheap.