Pierre Lurne: Audiomeca's Turntable Designer Page 2
I obviously looked puzzled.
"To determine the ellipsoid of inertia, you put an axis anywhere through the center of gravity of the system, and calculate the moment of inertia around this axis. You find the two points on your chosen axis, either side of the center of gravity, where the moments are equal. You then choose another axis through the center of gravity and find two more points. At the end of the process, these points trace out an ellipsoid in space around the center of gravity. This ellipsoid gives you an image of the effort in every direction applied to the system, so in the ideal case, if it takes the same effort to move a tonearm, a cartridge, a needle, in any direction, that's better than the contrary, where maybe it is easier to go up and down than to go right or left."
Pierre showed me some of his findings from that original L'Audiophile article, where, with a tonearm with a J-shaped arm-tube, an eccentrically mounted counterweight (most of the mass under the arm-tube axis), and a unipivot bearing point too high above the plane of the arm, both the ellipsoid of inertia around the bearing and that around the stylus/groove contact point are very asymmetrical, implying a very irregular distribution of the forces around these two points. I asked if that is why he had always gone for a straight tonearm tube in all his designs.
"These days, everybody knows that a straight tube is better than an S-shaped one for reasons of rigidity. But, by chance, the straight line also agrees with this calculation, giving a symmetrical ellipsoid of inertia. Maybe Nature and God are in accord. This seems very elegant to me, even beautiful."
We moved on to the subject of turntable design.
"From the Minimum turntable in 1979, through the second turntable which I designed for Audioanalyse in 1981 and now to the Audiomeca J1, I used the same concept of mechanics. I'll begin with the platter. I agree with other designers that methacrylate is the best material for a mat, and the shape of the platter is the same as the Minimum, which is to say that it is a little concave: it slopes from the outer rim to the center at an angle of 0.30$d. I decided on this form from a statistical survey of a large collection of records. There are actually two sheets of methacrylate, either side of a solid, 8mm-thick piece of lead, giving a total mass of 8kg.
"This construction is something very special. If you know the velocities of vibration in methacrylic and in lead, you can calculate when the vibration is reflected back to the stylus. First, the vibration induced in the record from the stylus tracking the groove goes through the record into the methacrylate, then to the lead, and so on. Each time the vibration is transmitted from one material to another, there is reflection and transmission, and the time taken for each reflection to return to the stylus can be calculated. You need not have all these delayed signals reach the cartridge at the same time. You then get the same effect as with the acoustics of a room with square dimensionsone big resonance. This is no good, and in addition, when a large reflected vibration reaches the stylus, the tracking is instantaneously different. But if you take care of the spacing in time of these delayed reflectionsdo you understand the concept of the 'Gold Number?'then neither the music nor the tracking is affected, not at the beginning of the record or at the end.
"We use lead because it almost behaves as a 'magic material.' It has high mass, it has good damping with low-Q resonances, and it has a very low speed of vibration. If vibrations enter the lead center of the platter, they leave considerably later, much lower in amplitude."
I asked if a clamp was necessary; if the record is not in intimate contact with the platter surface, you won't get the optimum pattern of reflections.
"Yes, I try to get the best contact . . . The bearing is an inverted bearing, with the contact point between the tungsten carbide ball and the bearing surface on the center of gravity of the platter/bearing system. Actually, the center of gravity is just under the ball because it is dangerous to make something only according to the theory. Mathematically, with only paper and pencil, you can make the bearing point and the center of gravity identical. In reality, however, your center of gravity will certainly be in a different place, maybe above, maybe under the bearing because of the tolerances involved.
"If the center of gravity is above the ball, you have no stability, but if you arrange things so that it will always be under the bearing point, you create a very small pendulum which will restore the situation, acting at frequencies which you don't care about, frequencies out of the audio band. In fact, the J1 bearing also has a ring, a sleeve, under the platter so that this small pendulum cannot be excited. But if you take such a precaution, you can sleep better at nights. This ring is adjusted by three screws, in order to adjust the friction and keep it from having any play.