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Design Philosophy For Turntables:
A Paper From Pierre Lurne Of Audiomeca

Forward by Scott Faller

Goldmund T3 Tonearm  For those who are unfamiliar with his name, Pierre Lurne has been an integral part of cutting edge turntable design since the late 1960's. Pierre designed his first arm design in 1968. He later was commissioned to do design work for a then major player in the audiophile turntable market.  Pierre is responsible for the now legendary T3 arm which sat atop a well known reference table. The T3 design dates all the way back to 1978. Pierre later went on to design and produce the T3's little brother the T5. The T5 is also know as the SL5 under the Audiomeca name.

In the years since, Pierre has designed several turntables and arms marketed under the Audiomeca name. Starting with the Audiomeca J1 turntable in 1979 and his (now) familiar SL5 linear tracking tone arm, then proceeding to his J4 turntable, Pierre has created a legacy. In 1989 and after many years of research, Pierre debuted the Audiomeca Romance turntable and his latest arm design, the Romeo.

After nearly twenty years of production, Pierre has refined his design once again. His latest creation is the Belladonna turntable and the Septum tone arm. In the forty year process of designing tables and arms, Pierre has formed some very strong opinions regarding turntable and arm designs. His design thoughts are more than simple opinions. He bases all of his designs on his knowledge of the elemental, physics. In Pierre's own words, "I have worked a lot on these points (basic physics, vibration control and channeling) which seem so forgotten in our analogue field. After all this is Physics again. I base my work on strong physics foundations but Physics is not enough, experience, time, love, and even luck all play their role.".  For those of us who are truly into high end turntables, we know how damaging vibrations can be to the fragile signal that our cartridges pick up.

As you will read, vibrations go far deeper than isolation or absorption.  Vibration control at its root should be the focus of proper turntable design. The flow and proper draining of any noise within a turntable is absolutely critical to produce an uncolored signal from a piece of vinyl.

Though Pierre's position paper submitted within is primarily about turntable and platter design, his philosophy extends to tone arm design also. Later this year I will have the pleasure of spending some time with the Septum tone arm. This should prove to be a very interesting experience. I look forward to mounting the Septum to my Opera LP5 and comparing it to my Dynavector 507 MkII. With some luck, I'll be able to have Pierre write a subsequent paper on tone arm design.

So without further adu, Here are some thoughts on turntable design from the master designer, Pierre Lurne.

 

Platter Design
There are many parameters involved in platter design but this article will focus primarily on the dynamic behavior, namely, mass distribution, dynamic balancing and bearing systems on the mechanical side. A complete study would have required an entire book and anyway, this is just a kind of excuse. In truth, this area of platter design proves to be the perfect example to highlight much more important questions which is the very intention of this article. A survey of the techniques in use will point out their respective advantages and drawbacks. Subsequently, a short reminder of some good old physics and philosophy will set the basics and, from these preliminaries, an answer will emerge and lead to an important and surprising conclusion. This article has been written in layman's terms with the intention of being easily understood by anyone.

The platter of an analogue turntable is primarily in charge of handling the vinyl record, to ensure its centering and horizontality, to spin it at the right speed with a high degree of accuracy and silence, and to deal with various external vibrations as well as those generated by the tracking and bearing. A much more difficult task than one would suspect. The platter acts as the heart of the machine. All of the other components ultimately play a supporting role and are comparatively of a lesser importance.

 

The Usual Bearing
In most turntables, the platter is supported by a bearing point at the end of a long spindle that is mounted on the platter and sticking well below it. The female bearing remains on the chassis. This usual and conventional bearing, intensively used by Thorens, Linn and so many other brands, can provide positive results but quickly reaches obvious limitations. Such a system is highly instable. The General Center of Gravity being above the supporting point, also called the Point of Rotation or the Pivot, has a constant tendency to topple. Without the female bearing sleeve it will inevitably topple. The sleeve holds and prevents it from rocking and falling. The smallest error in leveling, imbalance in the platter, the pulling to one side by a drive belt, the tracking force of the arm and, most importantly, the mass of the platter all produce high pressure on the bearing contacts. Moreover, the pressure increases in spinning mode as the centrifugal force tends to eject the platter away. This translates in a complex spinning system with high friction, noise and premature wear. Exactly what we do not want for our turntable.

In fact, the behavior of the spindle, hence of the platter, becomes quite chaotic. It rocks and chatters inside its mechanical play and is subject to all secondary microscopic movements of any dynamically imperfect physical body spinning on an axis. As a final result, varying pressures and a whole series of constantly changing resonances within the bearing are at work, affecting the platter rotation and producing a result that is all but smooth.

If the platter is well balanced, the situation improves but the basic instability remains. This dynamic balancing can only be performed in a way that can be witnessed at your local tire shop - a very rudimentary method.

With so many drawbacks, it is no surprise to get a rotation of poor quality. Spinning the platter by hand and releasing it always produces a very short "run down time". This clearly demonstrates the high level of effort and friction within the bearing that the motor has to overcome, not too mention the increased power consumption and noise produced. All things being equal, the simple comparison with the run down time of another 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).

The stylus inevitably tracks all noises, friction, efforts, micro-movements and resonances. The music becomes blurred and constrained, the dynamics lose punch and low frequencies sound the same, becoming badly defined

In Physics, such a platter cannot be considered "Perfect". It is not a "Pure Mass". Note that Direct Drive turntables belong to that category.

 

A Better Bearing
Designers have another choice, the less common "Inverted Bearing". In this case, the female part, or sleeve, is built as one solid piece into the platter and the spindle is fixed to the chassis pointing upward. The General Center of Gravity Sonderzeichen now passes under the point of rotation and the system becomes stable indeed. There is no more will to topple. Both friction and noise decrease - a great improvement.

Unfortunately a pendulum is so created which limits the improvement. Its own resonance frequency is very easy to calculate (school basics). One could argue that it cannot oscillate for the spindle maintains the sleeve. This is not the case. In mechanics, the "zero play" 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 to jam. This is even called "the working play" or "the functional play". Parasites randomly take place in this tiny interval as soon as the rotation begins and do not stop in steady speed mode. Yet again, vibrations, chattering, friction and noise are all tracked by the stylus and once more, there is a bigger demand on the motor and more noise is produced as a result.

The dynamic balancing can be more accurately performed with the same "tire shop method" or with the "tilt test" which gets rid of bearing friction. Placed upon a thin 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 adequately horizontal with the center of gravity aligned on the geometric vertical axis passing through the point of rotation, the pivot, just above.

But again, the accuracy is still insufficient because, hole after hole, the off-centered mass decreases as does the tilt angle which finally becomes too small to be located precisely enough and consequently, nobody is able to know exactly where to drill the holes. Although the dynamic balancing is slightly better, it is far from being complete. The run-down time is improved as well but it remains rather short. On a lesser degree the centrifugal force still tends to eject the platter away which results in the same nasty collection of pressures, friction, noises, vibrations and so on, all readily tracked by the stylus.

Once again, the system cannot be described as "Perfect".

It is not a "Pure Mass".

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 resulting in friction and noise. We recall that a single line is defined in geometry by 2 points only.

 

A Question Of Philosophy
All audiophiles have already heard the saying "HiFi is a quest". It is a quest for perfection, a quest for the absolute, for beauty, for the musical truth. The truth doesn't have to please or not to please. It simply is and that's all. Beware of the spectacular, the hype. The living 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 living event as possible and he spares no pains, no time and no money to reach that point solely for the passion of music. Who is there for other reasons is not an audiophile.

The audiophile always looks for the best. Step by step, as his experience and knowledge proceeds, he improves his audio system, trial after trial, and goes through all hopes, satisfactions and disappointments. The game is endless.

Logically, the audiophile favors the source because the perfect electronics and speakers are not able to recover something already lost. "All that is lost from the source is lost forever". The turntable acts as the source of the complete audio system and the platter is the heart of the turntable. On the other hand, arm and cartridge also make an all–important combination, a whole. They never solely track a record, they always track another whole, that is, the record and platter combo and how a poorly designed platter damages the musical quality has just been discussed above. To achieve real dynamic range, transient response, deep bass, image, clarity, credibility, emotion or any other musical characteristic requires an absolutely neutral reference.

 

A Question Of Physics
In theoretical terms the ideal platter should be perfectly neutral. In contrast to a musical instrument, it is not allowed to "make music" by itself, but rather it must track the record without any additions or omissions and thus no personal signature. When the artistic event has been performed and recorded, 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 for the entire audio system.

Mathematics demonstrates that neutrality can be satisfied by Pure Mass. Here we are. In Physics, the concept of Pure Mass is defined by a particular case of the general balance of any given body. What is that exactly?

Center of Gravity is a clear notion. It is almost intuitive. Everything has a Center of Gravity (or Barycenter) that we can roughly locate and therefore claim, "It is there".

 

Sonderzeichen
The Center of Inertia (always a big Sonderzeichen) is different because one chooses to calculate the Moment of Inertia of the body in regards to the geometrical point in question. It is then that the mathematician is able to calculate the so-called "Ellipsoid of Inertia" which provides all possible information concerning the dynamic behavior of the body. In fact, 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 it is perfectly balanced on one point, the Point of Rotation, which is also its Center of Gravity. Point of Rotation and Center of Gravity are coincident. 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. In perfect dynamic balancing, the body has no will of its own, no signature and no personality. It is "dead", neutral. It is obvious just how significant a role this notion of physics will play in arm design. 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.

 

Application
A turntable platter designed in this way profits from all theoretical properties of a Pure Mass. More than stable, it becomes "indifferent". If one ignores the bearing sleeve and tilts the platter, it will neither fall over nor will it oscillate like a pendulum, rather it will simply stay in that position. Point of rotation and Center of Gravity remain coincident in all cases. Dynamic balancing can be achieved to near perfection for a simple butterfly landing on the edge is enough to topple it. To reiterate, the platter reacts perfectly to any input of energy and experience shows that the balancing accuracy reaches up to only a half of a gram. This is an incredibly low value in comparison to the platter mass and a far cry from the standard. The sleeve therefore has no load to handle and as it is there purely as a "keeper", it can be reduced to a simple 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, resulting in decreased noise. It merely provides the slightest top-up to keep the platter spinning. An old tip of the 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 lath.

So we are able to see here that the correct application of the Pure Mass concept brings various benefits with it including neutrality, quality of the rotation, less motor noise, less bearing noise, lower wear rates, reduced significance of critical leveling, no complex resonance and so on.

To summarize, the "Perfect Platter" would require an inverted bearing principle with only one low-down, short ring as sleeve and the General Center of Gravity coincident with the Center of Rotation - elegant and simple.

 

A Little Secret
In the real world, it is simply not possible to exactly coincide the Center of Gravity and the bearing point and attempting this can have its disadvantages. This is simply due to the limits of mechanical tolerance and any attempts at exact coincidence will either leave the pivot too high or too low. If the pivot is too high, the platter will be a mini pendulum with a high (audible) resonance and if it is too low, the platter will be unstable. The solution is to compromise with a difference of 1 mm, which in production results in approximately 1.0mm +/- 0.1 mm. In this case, the mini pendulum theoretically oscillates within a safe area and is easily controlled by both the sleeve and the platter inertia.

 

Conclusions
There are far too many products on the market that are faulty and designed with the "Objet d'Art" in mind rather than musical reproduction. To ignore fundamentals such as mass distribution and acoustic impedance, to name just two, can only end in a compromised product.

Obviously the laws of physics are demanding but they are the laws of God and remain un-changing. One could nevertheless maintain that good physics are merely the foundation of good design and that time and experience, love and even luck are what add the final touch in producing an outstanding analogue source.

Now the question is: Would you prefer a well-designed platter or a common one affected with all of the defects 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, "Remember just how much better it would be if physics would be respected".

There are hundreds of other parameters involved in platter design but even so, this cannot be used as an excuse for defects in design. Even with the utmost care to sound propagation, correct grounding and to the acoustic impedance of the materials etc., all will be lost or at least corrupted if a weak point sets the overall limit of the results.

Whether it is light or heavy, made of aluminum or composite, designed as one solid piece or as sandwiched components, there is basically only one way to design a platter.

Pierre Lurné
Audiomeca

 

Audiomeca
Route de Tessy
74370 Pringy, France

Voice: +33 450 27 20 64
E-mail: audio@audiomeca-hifi.com
Website: www.audiomeca-hifi.com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     
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