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An Interview with Pierre Lurné on Analogue Thoughts

[Italian version]

Interviewer: Geoff Husband
August, 2002

One of the joys of doing my series of turntable tests has been discovering the enthusiasm that seems universal with anyone connected with vinyl. One person stands out and that is Pierre Lurné of Audiomeca.

This man has been deeply involved in vinyl reproduction for well over 30 years now (the first arm was in 1968...), but for the last ten years his company has concentrated on cutting-edge digital products, and there haven't been any developments in their Analogue range for that time. Analogue sales now account for a tiny part of Audiomeca's output and yet on contacting Pierre for the first time I found a man who's heart and soul remain steeped in Analogue. As well as sending me a very fine turntable/arm, the Romance/Romeo, he bombarded me with emails and reams of hand written A4 sheets setting out his thoughts on getting the best for LP records. His roots are firmly in an engineering background, and unlike many 'snake oil' practitioners he can back all his ideas up with the Math...

I'm no editor and so I've collected together these writings and put them together as a series of thoughts and ideas in an interview form. As much as possible I've stuck to his excellent English which if you read it with a French accent is even better :-)

Oh and before I start Pierre added the following to one of his early emails, which tells you all you need to know... "Except between ourselves I really don't want to criticise any other manufacturers by name as I consider all of them as Analog friends."

TNT-AUDIO >
So, first question - What started you in hi-fi and particularly arm building?

PIERRE LURNÉ >
When I was a student my pocket sounded very empty... I saved coin by coin to buy records, vinyl records of course. This story starts in "old Analogue good times". At home I had a very cheap turntable only, you know the sort that looks like a suitcase, all in one, the cover being the speaker - a 'Teppaz Oscar'!

Every time I listened to one of my darling records I was able to hear the quality degradation. It was clear that this was due to the cartridge/arm combination. Also I realised that a good turntable was the key point as it works as the very source, nothing can be restored afterwards, as it respects or destroys the record integrity. Finally I decided to buy a good cartridge and build an arm and a turntable (impossible to make a cartridge for a beginner). I made a lot of mistakes but the records of that period are still with me in a rather good shape.

Then I progressed step by step, making other models and becoming little by little more familiar with the hi-fi world, shows and so on.

I have always been a "do-it-yourself man" and at that time I was a student in Physics. Many designs on the market seemed to me incorrect in terms of physical laws. Besides I always felt very confused after visiting a hi-fi show - you surely know the following situation: you meet a guy who says "the best sound that year was in room ...X". Ten minutes later you meet another person who says "I'm just out from room Y and it sounds fantastic" etc etc. So confusing! As a young man it was difficult to admit that the 'truth' does not exist, and cannot exist - all is subjective! BUT this fact cannot be an excuse to design things upside-down... I though "OK, all is subjective and all can be discussed. That's alright to me but I shall design things right, correctly, with respect to Physics and, at least, one will not be able to criticise my designs"... Is there any better way?

Intuitively I guessed that there are fair advantages in taking that hard way. Who can seriously suggest that 'crazy' designs are superior? Apart from a few rare exceptions "the exception that proves the rule", there is no doubt to me that you will think like me after reading all these boring pages :-)

Later we shall go through simple examples that will explain a lot and will appear beautiful to your mind.

So little by little I developed my theory? Wrong! It is not my theory in any case - I simply learnt about it in school :-) The funny thing is that it took me five years to go from theory to correct application - you know the famous difference between theory and practice - this was it!

Obviously the laws of Physics don't change everyday, they are laws of God, they are still all right today and I'm still using them. But on that point I'd say that good Physics are not enough, they are merely the foundation of good design. We see that if there is only one way to design correctly, the applications can still be numerous. Sometimes you have simply the choice, you will go one way and someone else will go the other. You can choose that material for a given purpose someone else will choose another. One day at the Frankfurt show an old professor of physics came and asked to meet me. Then he shook my hand and with a smile and enthusiasm because he was a fan of hi-fi and had given the problem of the turntable platter to one of his students who has ended up with the same results as me. I will always remember that day as mountain-fresh air...

TNT-AUDIO >
On the proportions of a product (you never though of that eh?)

PIERRE LURNÉ >
When you make furniture like a bookcase, you must take care of dimensions. Not only regarding the ceiling but also the doors and windows (it might have to go through). When you design an arm it's the same story, all dimensions must fit together. This might seem obvious but I know of one Japanese manufacturer who made a turntable with the platter flush to the top-plate. No arm would fit! So they had to lower the arm mounting plate to fix the problem - really strange looking and really stupid... Turntable arm and platter exist in 3 dimensions - no one can reduce them to 2...
And before you wonder at the example, consider CD mechanisms (CDM) which suffer from the same engineering mistake. That design has surely been done by an electronic engineer, because no mechanical engineer on earth would have designed a CDM with only 2mm clearance between disc surface and lens - many potential improvements cannot be done because of that basic fault. And whilst on the subject of CD some remark on speed of rotation: A CD disc varies from 500 rpm to 200 rpm. The purpose of this is to get a constant tangential speed in order to end up with a regular (more or less) datastream That is really crazy when you consider the problems of regulating even one speed (like 33 1/3)! With such variations you are bound to sweep all over a wide range of resonances - an open invitation to problems, especially jitter. There are much better ways of doing this!

TNT-AUDIO >
On hi-fi, neutrality and the 'pure mass'

PIERRE LURNÉ >
If the least you can do is look after the overall dimensions, it is also very important to define the target, the aim in terms of philosophy and physics.

If the word 'Hi-Fidelity' means something for you, you must respect the music as best you can. An audio designer (despite what some think) is not an artist - that is the musician him/herself. The designer is not allowed to change the music, to alter it, to add something to it, this way of thinking is very important. I think this is the true basis of our job as designers.

So a good turntable and arm should be neutral, have no personal character, no overt behaviour, to be 'dead'. The concept is defined in Physics as the 'Pure Mass' concept. A pendulum for example, is not a pure mass by definition as it has its own resonance frequency - so many arms and turntables work as pendulums...

To know if a given body is a pure mass, and by the way, in order to know anything about its dynamic behaviour, one must calculate the 'Ellipsoid of Inertia'. Everything has it's ellipsoid of Inertia, a chain, a car, a pencil. My dear Geoff, I am pleased to inform you that even you have an ellipsoid of Inertia... What is it? To simplify; suppose you want to know the ellipsoid of a book. OK - start by taking a line or axis through the book's centre of gravity - any line, any direction but through the centre of gravity. Then you calculate the moment of Inertia of the book as regard to this axis. This will give you two values + and - either side of the axis - thus you have two points plotted. Now draw another axis through the centre of gravity and plot two new points. Keep doing this over and over again, or speed up thanks to integral calculus, and you will end up with a mass of points forming an ellipsoid, sort of egg shaped - this will always be the result! More or less long, more or less flat but always that egg shape - an ellipsoid... That's physics, that's nature... And that egg is the mathematical representation of everything regarding the dynamic component of the given body. You will see straight away that with a very regular body, the egg will have a special form - a sphere called "the Central Ellipsoid of Inertia"...

OK so if we apply this test to a tonearm we'll get an ellipsoid around the arm's centre of gravity - approximately around the pivot of the arm. Now project the ellipsoid forward to the stylus so an ellipse (or circle) surrounds the stylus and you'll have a representation of all the forces acting on the stylus, and by the way, to the arm cartridge combination. See Fig.1 [Lurné ellipsoid]

The perfect arm would have the ellipse as a circle with the stylus tip in its centre. That means that the stylus will move equally easily up, down and left/right. Neutrality is complete.

If the ellipse is flat then the system is not neutral, it is easier for the stylus to go up and down than left to right. In this case the combination of the arm's effective mass and cartridge compliance will give various frequencies of resonance rather than one which is easier to control. This is the design weakness of all air-bearing parallel-tracking arms, the resistance to up and down movement will be much like a bearing tonearm, but side to side will be much more difficult as the totality of the arm must move - around 20 times more difficult than vertical movement. . Depending on the eccentricity of the record (no record is perfectly centred) the arm movement can take any direction and speed each time adding a different resonance. At best such arms can be symmetrical, at worse just crazy - by that I mean that to have an ellipse so distorted with the stylus way off centre it means that all movements are different, adding different resonances, different behaviours etc.

The question is, would you prefer a well designed arm with a round ellipsoid which moves equally easily in each direction, or one that prefers to go right than left and up than down with a crazy ellipsoid?

Just looking at an arm will tell you whether an arm is correct or not: If the arm is straight with the stylus well along the main central axis and a simple counterweight on the same axis this looks about right. If it has a curved armtube, a high centre of gravity, a low counterweight or a mass of parts then it is not correctly designed. A word of caution - the respect of these rules doesn't guarantee a good arm, it's still possible to make mistakes elsewhere (flexible armtube, poor joins etc) but it is certain that a good arm could be even better if it respected the basic laws of physics.

There are also secondary advantages to such a perfect system - if a parasitic force is applied to it (anti-skate design, lead out wire drag, any kind of vibrations coming from outside) then you cannot ignore it or get rid of it, but you can minimise the effect because it is applied to the system on its own, rather than working through another force (couple) caused by the poor ellipsoid - so you get a minimum value.

TNT-AUDIO >
And on turntable platters.

PIERRE LURNÉ >
Normally the spindle is mounted on the platter and the female bearing stays still. Like the wheel of a car the platter is dynamically balanced. But what about the accuracy of this balancing? With a conventional bearing the point of rotation is well below the platter's centre of gravity - such a system is not stable at all, it will be trying to 'fall over' all the time. Remove the female bearing sleeve and it'll topple. If the platter is well balanced the situation is improved, but the basic instability remains and how do you know if the platter is dynamically balanced???

Now make an inverted bearing where the female part spins with the platter and the solid shaft of the bearing is fixed to the chassis. Place the point of rotation at the centre of gravity. You now only need a short sleeve below the bearing to control the much lower 'falling' effect. It's important that the support sleeve is as short as practical, as 1 point and one small sleeve define one point of axis. Put two sleeves in, or a long one and you define two geometrical axes, one of them is doing nothing and can only add problems of adjustment, friction etc.

Now to dynamically balance the platter all you need do is temporarily remove the support sleeve and the platter will tilt gently to show any imbalance. If the bearing is 10mm above the centre of gravity the tilt will be of a certain magnitude. Move the bearing to 1mm above and the tilt becomes much greater. If the bearing is at the centre of gravity a single butterfly landing on its edge will tip it. Therefore the bearing should be as close as possible to the centre of gravity in order to ensure the most accurate balance. Using this method the J1 platter, which weighs 8 kgs can be balanced to an accuracy of less than 0.5 grams. Once made like this the platter will spin extremely smoothly, with very little friction (as the sleeve is doing very little), little noise, no reaction on subchassis etc.

In the real world, trying to make the centre of gravity and the bearing point exactly co-incident is not possible and trying can have disadvantages. This is simply due to the limit of mechanical tolerance, which if you aim for exact co-incidence will either leave the pivot too high or too low. If the former the platter will be mini pendulum with a high (audible) resonance, if the latter the platter will be unstable. The solution is to compromise with a difference of 1mm, which in production results in figure of 1.0mm +/- 0.1mm. In this case the 'mini-pendulum' oscillates around 5 Hz, a safe area and easily dealt with by the supporting sleeve.

TNT-AUDIO >
Lurné on tonearms, first Unipivots and centre of gravity

PIERRE LURNÉ >
If centre of gravity is low as regard to bearing point of rotation, this creates a nice pendulum. Its own resonance frequency is very easy to demonstrate or to calculate (School basics). With such a character the arm cannot be neutral, it has its own behaviour and adds its own personality to the music. This seems very clear but... why designers continue to make this mistake is out of my understanding. Their usual answer is that they tried and listen and choice is clear. Far from serious.

Centre of gravity is a clear notion. Almost intuitive. Everything has a centre of gravity (point of or barycenter) and we can say it is 'there'. Centre of Inertia is different because you choose to calculate the moment of Inertia (always a big I) of a body as regard to the geometrical point which matters for you. This point of reference can be inside or outside the body, it can also be the centre of gravity and that peculiar case provides a lot of information on the dynamic behaviour of the body. (See above).

On a correct design, stability simply comes from Inertia ( plus from a few advantages given free of charge by God for correct design). We made thousands arms and nobody never mentions bad stability. Well designed, well balanced, well adjusted, a right arm has no tendancy for anything anymore, it is dead !

TNT-AUDIO >
And pivot height?

PIERRE LURNÉ >
If bearing point of rotation is too high as regard to the horizontal line going through the centre of gravity, on warped records the stylus moves up and down of course, but front and back as well, adding a low wow, wow to the music, exactly similar to speed variations.

The 2 (pivot and centre of gravity) "slug" the sound as you say with a poor image. If you look at the stylus during tracking, you will see on warped records ( all records are warped and off-centred, the perfectly flat and well centred record does not exist ) that it is alternately smashed down and relaxed. This shows that the tracking force and the VTA varies all the time( anti-skating force too). I do not think that one single person on earth would claim this is good.

TNT-AUDIO >
On arms and cartridge compliance.

PIERRE LURNÉ >
An arm has no compliance. A cartridge has one. And who has ever stated that horizontal compliance and vertical compliance have to be different? To be neutral a cartridge/arm combo must be considered as a whole and must have the same behaviour/comportment in all planes, in all directions. If not, you make a "living" system which, for instance, prefers to go down rather than up and so on. The neutral system must be dead ! No confusion with stability and neutrality - see notes on Ellipso´d of Inertia.

TNT-AUDIO >
Question: Unipivots, the Romeo for example, can be influenced by the tonearm cable, it's a fiddle to get the arm to sweep across the record and keep azimuth correct - ideas?

PIERRE LURNÉ >
Naturally any kind of arm must deal with its wires stiffness. A good reason to chose good but also thin and flexible wire, but you know that the perfect wire does not exist, as the best would be no wire at all. I use this Litz wire for many years. Another long story. The manufacturer has hundreds different Litz wires on his catalogue... You guess - listen! A true unipivot appears to be influenced a bit more than a ball bearings one, but this is not sure at all. I explain: on a unipivot you see what's going on and you can manage to get the better. You place the wires like you did yourself and so on. In all cases you get an improvement easily. On 'usual' arms you see nothing so you can do nothing. Some unipivots do not allow that by construction. There are 2 effects : on azimuth and on "Moment of torsion" (do you understand?) which works as a tracking force and as an anti skate or the reverse in the other direction. Anyway I have to admit with what you say. I am afraid it is a slight drawback of the Analogue system in general. Wires are there and the best we all can do is to limit their effects. I use to build much more expensive arms with the wires right under the pivot, along the vertical axis.

TNT-AUDIO >
On turntable motors...

PIERRE LURNÉ >
In my opinion, the motor is a limitation factor of performances, for any designer. I used to spend hours and hours looking for the perfect motor with no success. It simply does not exist. The only solution left is to make it ! You imagine this is quite complex. Motor design is another speciality for a full time job! Besides, Physics does not bring us what we need re motor design. A pity ! I'd thought I'd caught a pretty and elegant solution years ago but this would need a too long development and would cost a lot. Pity again. The idea also allows us to get rid of some other limitations. Maybe one day we shall talk together about it. Today it is a little out of the subject.

Let's go back to the Romance motor and power supply: the motor is a synchronous type. It has 2 stages of coils. That means a simple condenser can delay one coil and impose the direction of rotation. No mechanical "clicks" like some others. Absolutely no noise from that. Another advantage is that double coils gives double pole quantity obviously. Consequently movement is smoother. The more poles you have, the better. To absorb vibrations (all that moves, makes noise) the motor is mounted on a piece of lead and suspended in a very simple but effective manner. I often say that lead is for us a "magical" material. Power supply is very simple. Only a few passive components. I use a well known old connection to get 120/60Hz or 230/50Hz thanks to only one other condenser to be added or not. Just needs a special motor voltage.

TNT-AUDIO >
And finally is it all art or science?

PIERRE LURNÉ >
Many years ago I had rather disappointing article published - The reviewer wrote that a good turntable is not only a question of good mathematics. I felt a bit hurt (although it is rather difficult to get on my nerves) as this is exactly what I've been saying since the early days. I even described in the old l'Audiophile, a French magazine, what I called "the minimum intention" which is a personal feeling or state of mind necessary for a good job. Yes it is important to design in a way that respects physics, but getting the best from such a design is an art in itself...

TNT-AUDIO >
I'd just like to thanks Pierre for such an interesting overview, I'm painfully aware of how space and time limitations have only allowed us to scratch the very surface. The good news is that at some time in the future Pierre has hinted that he might contribute more - here's hoping :-)

Copyright © 2002 Geoff Husband - http://www.tnt-audio.com

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