Author: Pierre Lurné - TNT France
Published: November, 2020
Using a familiar design, but upside down, the less common and so called “Inverted bearing” is fixed to the chassis, pointing upward. It looks very similar, but is actually quite different. The General Centre of Gravity now moves UNDER the point of rotation (Figure 2). The system becomes STABLE. There is no tendency to topple. Friction and noise decrease. A great improvement.
Great, but limited because, unfortunately, such a situation creates a mini pendulum. Its own frequency of resonance is very easy to calculate (school basics). One could argue that it cannot oscillate for the spindle is constrained by the sleeve. This is not the case. In mechanics, “zero play” or “zero clearance” does not exist. To be effective, any spinning movement needs a “mechanical play” between spindle and sleeve or it would be at a high risk of jamming. This is even called “the working play” or “the functional play”. Problems in this tiny space begin as soon as the rotation starts and do not stop even in steady speed mode. Yet again, vibrations, chattering, friction and noise are all tracked by the stylus. Once more there is a bigger demand on the motor and more noise is produced.
Anyhow, dynamic balancing can be more accurately performed with the same “tyre shop method” or with the “tilt test” which reduces bearing friction: placed upon a thinner spindle, for example, the platter tilts in the direction of the excess of mass. Then, a few holes are drilled there and under the platter edge, removing some mass. The tilt angle decreases and after a few trials, the platter finally stays still and perfectly horizontal. This proves the Centre of Gravity is now correctly aligned on the geometric vertical axis passing through the point of rotation, and just above. Again a great improvement.
Still dynamic balancing is still not at its best because, hole after hole, the off-centred mass decreases as does the tilt angle which finally becomes too small to be located precisely enough. As a consequence, nobody is able to know exactly where to drill the next hole. The dynamic balancing remains incomplete. The run down time is only slightly better and the centrifugal force still tends, to a much lesser degree, to eject the platter away which results in the same nasty, though smaller, collection of pressures, frictions, noises, vibrations and so on, all readily tracked by the stylus.
The system is still not a “PURE MASS”.
Note that the few other manufacturers using the inverted bearing throw away its advantages by either placing the Point of Rotation too high (pendulum) or too low (instability) and/or by using too long a sleeve or a “2 sleeve bearing”. This gives the bearing several axes and more friction and noise (see figure below). In geometry, a line is defined by 2 points only.
“Hi-fi is a quest”. All audiophiles have already heard the saying. A quest for perfection, a quest for the absolute, for beauty, for the musical Truth. The Truth does not have to please or not. It simply is and that's all. Beware of the spectacular or the hype. The live musical event is the Truth and playing a recording at home is something else entirely. Any serious audiophile expects his audio equipment to sound as close to the live event as possible and he spares no pain, no time and no money to reach that point, solely for the passion of music. Step by step, as his experience and knowledge proceed, he improves his audio system, trial after trial, and goes through all hopes, satisfactions and disappointments. The game is endless.“All that is lost from the SOURCE is lost forever”.
The ideal platter should be perfectly NEUTRAL. It is not allowed to “make music” by itself as it is not a musical instrument. It must track the record without any additions or omissions and thus have no personal signature. The artistic event has been performed and recorded once for all, and so, its integrity must be respected. This is the audiophile's golden rule.
Neutrality stands on a fine line between pleasant low colorations and over-damping. Neutrality is the very first quality of a complete player, which in addition, acts as the SOURCE of the entire audio system.
Mathematics demonstrates that neutrality can be satisfied by a “PURE MASS”. Here we are! In Physics the concept of Pure Mass is defined by a particular case of the general balancing of any given body. What is it exactly?
The Centre of Gravity is a clear concept. The Centre of Inertia is a different thing because one chooses to calculate the moment of inertia of the concerned body in regards to the geometrical point in question. This leads to the “ELLIPSOID of INERTIA” which provides all possible information concerning the dynamic behaviour of the body. In short, when we wish to know everything regarding the dynamic properties of a certain body, we do not need to know what it is, we only need its Ellipsoid of Inertia.
A body is considered a “Pure Mass” or “Perfect” when its Point of Rotation is placed on its Centre of Gravity. Both points are coincident and the Ellipsoid of Inertia becomes the “Central Ellipsoid of Inertia”. This highly aesthetic and particular case gives the body a number of exceptional dynamic properties. Being in perfect dynamic balance, it has no will of its own, no personality and no signature. It is “Dead”, neutral. Such a perfect system also benefits from secondary advantages, true natural gifts: it reacts perfectly to any input of energy; any parasitic force originating from outside (vibrations, belt drag, arm tracking force, resonances, etc.) is reduced to a minimum and all microscopic movements are cancelled. It is obvious just how significant a role this notion of Physics will play in arm design.
A turntable platter designed in this way benefits from all theoretical properties of a Pure Mass. More than stable, it becomes “INDIFFERENT”. This means that if one removes the little low sleeve, the platter is free to fall, but it will stay still. Point of Rotation and Centre of Gravity remain coincident in all cases. If one tilts it a bit, it will neither fall over nor will it oscillate like a pendulum, rather it will simply stay in that new position. Dynamic balancing can be achieved to near perfection for a very small mass, and let's say, a butterfly (see figure below), would be enough to topple it. So, the possible accuracy is at a maximum and experience shows that it can reach up to only half a gram on a 10 kg platter. An incredibly low value and a far cry from typical standards. The sleeve therefore has a smaller load to handle and it is there purely as a simple “keeper” and can be reduced to a little ring. As a result, rotation is exceptionally free and the platter will spin several minutes with just a push. There could be no better demonstration of very low bearing friction.
Such a smooth rotation obviously means that the motor has little work to do and noise decreases. It merely provides the slightest top-up to keep the platter spinning. An old tip of engraving engineers was precisely to reduce the drive motor torque close to the breaking point in order to improve the signal noise ratio and the rotation quality of the lathe.
To summarize, the correct application of the Pure Mass concept brings a number of qualities: neutrality, quality of the rotation, less motor noise, less bearing noise, lower wear rates, reduced significance of critical levelling, no complex resonances and so on and on. All this thanks to the so simple Inverted Bearing Principle, with coincident Centre of Gravity and Point of Rotation of the platter, and a single low-down ring as sleeve.
Elegant and Simple!
In practice, it is almost impossible to get the Centre of Gravity and the Point of Rotation coincident because of the “mechanical tolerances or margins”. Inevitably one will always be a tiny bit above or below the other, leading to instability or making a mini pendulum. For best results, the solution is to compromise with a tiny difference, as tiny as possible, but kept under control, between the Centre of Gravity below and the Point of Rotation above. So doing, the system is safe and controlled by both the sleeve and the platter inertia.
Now the question is: would you prefer a well-designed platter or a common one affected with all the defects and weaknesses described above? From now on, the virus is growing in you and every time you will see and listen to a turntable, even a great and expensive one, you will hear a little voice murmuring: “How much better it would be if Physics would have been respected”.
Too many products in the market ignore fundamentals such as Mass Distribution and Acoustic Impedance, to name just two. Obviously the laws of Physics are demanding, but they are the laws of nature and one single weak parameter can set the overall limit of the results. Invested time, experience, love and even luck add the final touch in producing an outstanding analogue source.
Many other parameters are involved in platter design, but whether it is light or heavy, made of aluminium or composite, built as one solid piece or as sandwiched components, there is, basically, ONLY ONE WAY to design a platter bearing.
Pierre Lurné - Audiomeca 2008 (revised 2020).
© 2020 Pierre Lurné - email@example.com - www.tnt-audio.com