Author: Pierre Lurné - TNT France
Published: November, 2020
The first version of this article was published 12 years ago by Enjoy the Music and the Analog Audio Association of Germany. New material has been added and illustrations changed in this updated version for TNT-Audio.
Every day we use several common technical products to ease our life and for our pleasure as well. Washing machines, cars, television sets, hi-fi systems, etc. New versions of these may look different, but they are all based on the same well known technical solutions. Generally speaking, new designs work as well as the old and for the same price. Some of them could even provide better quality with improved performance. This is the case with the bearing of our turntable platter.
The platter of an analogue turntable is primarily in charge of carrying the vinyl record, to ensure it is centred about the spindle and is horizontal, to spin it at the right speed with no noise, evenly, and to deal with various external and internal detrimental vibrations, including those generated by the tracking itself. A much more difficult task than one would suspect.
A survey of the techniques in use will demonstrate their respective advantages and drawbacks. Subsequently, looking back at some Old Good Physics and philosophy will set the basics and, from these preliminaries, an answer will emerge leading to an important and surprising conclusion. This article has been written in layman's terms with the intention of being understood by anyone.
In most turntables, the spindle is mounted underneath the platter and protrudes well below onto the so called “point of rotation” which supports all the mass. The female part of the bearing is fixed on the sub-chassis. This typical bearing, commonly used by Thorens, Linn and so many other brands, provides good results, but quickly reaches obvious limitations. Such a system is highly unstable. The centre of gravity of the platter, being well above this point, has a constant tendency to topple. Imagine balancing a pencil by its tip: as soon as you remove your fingers, it falls.
The bearing sleeve holds and prevents the platter from rocking and falling, applying force on the spindle, producing friction, noise, etc. The smallest error in levelling, imbalance in the platter, the pulling of the drive belt, the tracking force of the arm and, most importantly, the mass of the platter itself, all produce unwanted pressures on the sleeve contacts. Moreover, the forces increase in spinning systems as the centrifugal force tends to eject the platter from its mounting. All this occurs in a complex spinning system with high frictions, vibrations, noise and premature wear.
Exactly what we do NOT want for our turntable.
The behaviour of the spinning system becomes chaotic. It rocks and chatters due to mechanical play and is subject to all the secondary microscopic movements of any dynamically imperfect physical body spinning on an axis. Varying pressures and a whole series of constantly changing resonances within the bearing are at work, affecting the quality of the platter rotation and producing a result that is all but smooth. Compared to the size of the engraved music modulation in the record groove, with all its delicate details, these problems are huge! The stylus cannot differentiate between music and vibrations. All that moves is inevitably tracked. The sound becomes blurred and constrained, the dynamics lose punch and low frequencies seem the same and poorly defined.
If the platter has been correctly balanced, the situation improves, but the basic instability remains. This dynamic balancing is performed in a similar way to car wheels. A very rudimentary method.
With the belt removed, spinning the platter by hand and releasing it always produces a short “run down time”. This clearly demonstrates the high level of forces and frictions within the bearing that the motor has to overcome, with more power consumption and noise produced. All things being equal, the simple comparison with the run down time of various different platter shows, without a doubt, which platter/bearing combo is better designed or manufactured (providing they have both been broken in, well-cleaned and duly oiled).
In Physics, such a platter system cannot be considered “perfect”. It is not a “PURE MASS” (direct drive turntables belong to that category).
The next article will be entitled: A Better Bearing.
Pierre Lurné - Audiomeca 2008 (revised 2020).
© 2020 Pierre Lurné - firstname.lastname@example.org - www.tnt-audio.com