Tables That Feature Bearing Friction

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.

Dertonarm, the heavy platter, slip coupling approach focuses on only the issue of motor cogging. This approach does not deal with the issue of stylus drag, or any other variability in drag. Contrary to popular beliefs platter mass changes how stylus drag affects speed but does not correct it. A massive platter will reduce the magnitude of the variation but extends it over a longer period of time. A light platter will conversely allow a larger speed variation but it enables more rapid recovery. Heavy vs. light platters exhibit different sounding degradations but they are still degradations.

Intimate coupling of the motor to the platter is the only way to effectively deal with stylus drag. But intimate coupling also makes problems from cogging worse. So in the end a compromise between the two is needed. A DC motor needs less isolation than an AC motor so the compromises will and should be different. Personal preferences also will dictate the ideal compromise. For example idlers with AC motors have poor isolation from cogging, but more intimate coupling. The result is excellent rhythm and timing but finesse and low level detail are sacrificed. Some like the idler compromise and others don't.

If you start with a very low cogging motor then a better compromise can be achieved.

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.

Dear Teres,
stylus drag is only an issue if the record is not firmly clamped down to the platter.
If the record is not firmly clamped down, we do not need to talk about correct application or technical issues anyway. This is basic parameter. If securely clamped down it becomes part of the moving system and its mass - hence: heavy platter with high inertia.
As I said before - this is only one approach and certainly not the only one in igh-end audio, but it is the approach of physic and technical engineering.

A heavy platter will have no variation once it is on speed.
Any possible loss in speed is avoided before it occurs - by correct allpied coupling with string (= very low grip but enough to avoid loss of constant speed). Thus the error does not occur but the only task for motor and string is to hold the speed - nothinh else.
Stylus drag do only have an effort when the record itself "slips" on the platter surface (and believe me - I do use a cartridge which really can "drag". But of course it is only going on a record which is firmly - really firmly - pressed down on the platter).
Sometimes it really helps illustrating forces in motion with vector diagrams on a sheet of papaer. Visulising what really is going on does set some points clear really fast.
This is physics - thus it can fairly easy be determined when you allow the facts to spread.
Trying to correct any variation in speed as fast as possible ............
The result is constant back and forth in speed.
In other words - you implement unstability by doing so.
Every technical engineer into dynamics or constant torque will tell you that this is futile.
Turntable is pure physics - not taste, not opinion.
Too often in High-end audio people get the impression that physics laws have been invented during the development of audio components.
Not so.
Extremely few audio components - mechanical ones like tonearms, cartridges and turntables - do really take correct applied physics into account.
Otherwise we would have much more better components around.