PIERRE LURNE J1 TURNTABLE with SL5 arm
sl5 just supplied by Pierre Lurne
full working order, the only blemish a crack at the back of the lid
Audiomeca - Pierre Lurne J1
Belt driven sprung chassis turntable featuring electonic speed control, metacrylate and lead platter, inverted main bearing, 3kg lead block for housing the motor, and double compliance suspension.
The Audiomeca J1 platter is a masterpiece of physics and precision machining which has no competitor in the high end field.
Type: belt-drive with counterpulley
Motor: integrated and independent motor enclosed in a 3kg lead block
Control: electronic speed controls, integrated panel controls
Platter: 8kg machined from metacrylate and lead
Power supply: separate, connected with neutrik connectors
Finish: black metacrylate
Dimensions: 500 x 450 x 200mm
record clamp and cartridge not included
"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 dimensions—one 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 reflections—do 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.
"In terms of mathematics, to have the center of gravity very near to the point of rotation gives you a lot of advantages. In one sentence, forces acting on the system are not applied through a couple, but only as pure forces. There are no distances involved. That is to say, the system is as simple as possible."
The Audiomeca J1 is one of the more elegant turntable designs I have seen, being constructed from a black methacrylic sheet. In common with the Oracle, it dispenses with the traditional box construction for the plinth.
"Most turntable bases are like the sounding box of a guitar: we use two U-shaped forms that fit together without resulting in any box resonance. Inside, there is a subchassis, again fabricated from a three-layer sandwich of methacrylate and lead to optimally disperse vibrations from the bearing, sitting on a three-point suspension. You can adjust the springs from the top."
Had Pierre taken the same care with the subchassis that its center of motion is the same as the center of mass?
"Yes, the center of gravity is always in the same place. Let me explain the problems of spring suspension. Some designers hang the subchassis from the springs, and say that this is better: if the subchassis is hanging, then the restoring force will be back toward its natural position of rest. They are right if the springs are long and very thin, but if you use large springs, this is not so good. The Audiomeca J1 is the proof. The mass of the subchassis and platter is centralized between the springs, and when everything is set up correctly and balanced correctly, and the tonearm is balanced with the appropriate counterweight, you can push the subchassis, and the response up and down is quite correct. It just bounces vertically."
The drive system of the J1 appears to be unique, for the belt doesn't just rotate around the motor pulley and the subplatter; there is also an idler pulley on the opposite side of the platter. The only thing like it that I have seen is the two-motor arrangement featured by the Alphason Sonata.
"If you have no counter-pulley, the belt will pull the platter toward the motor, resulting in wear. If you use a counter-pulley on the other side of the platter from the motor, the platter is in balance. This has been a feature of my turntables through the years since the Minimum."
Pierre showed how the motor itself is quite independent of the rest of the turntable. It is locked for transport, but when the J1 is set up, the motor, coupled to a 3kg block of lead to absorb vibration, sits on three Delrin—a high-density grade of nylon—feet that protrude through the U-section base to couple directly to the support; it has no contact with the base at all.
"The motor and its big lead block are twice decoupled because we try to create an artificial ground. Vibrations should be removed from the turntable system so that the stylus/groove relationship is as undisturbed as possible. I take the motor vibrations to the lead and try to earth them essentially out of the system. The counter-pulley offers a similar kind of advantage because it is also decoupled from the turntable base. In the accessory pack supplied with the turntable, there is a threaded spike which screws in to the base of the turntable underneath the counter-pulley to touch the surface that the turntable is sitting on. This takes any vibration from the belt, vibration from the bearing, and so forth, and in effect takes it to ground.
"The motor itself is a dual AC type, but rather than relying on the stability and purity of the mains frequency, what's presented to the motor is a synthesized sinewave based on a clock pulse generator, similar to what Linn is doing with their Valhalla board. To specify direction, this is then flipped by a capacitor right at the end to give two out-of-phase signals to the two different sections of the motor. You also get the smoothing effect of multiple poles, and can change the speed by varying the frequency to drive the motor. There is both a coarse and a fine speed adjustment. I chose this solution because I have used it for many years. But, of course, this is only one point among many that contribute to a good turntable design. You can actually make a very good turntable with any system of motor, even with the old idler-wheel drive, but it's more difficult then, of course.
"I also machine the Delrin motor-pulley after it's been mounted on the motor. If you machine the motor pulley separately from the motor, you have a problem with eccentric rotation. We put the pulley on the motor, and we finish the machining of the pulley with the motor turning so that the pulley becomes absolutely round."
The J1 can be fitted with any tonearm, not just the parallel-tracking Audiomeca SL5 (a derivative of the Goldmund T5). As well as supplying dedicated armboards, Audiomeca supplies counterweights to adjust the mass of the subchassis to correspond with that of the arm so that the three springs can be adjusted to give a good balance with any arm. Stereophile will report on the success of the various design philosophies outlined by Pierre when we receive a J1 for review. Before returning to my hotel room in New Haven, I concluded our conversation by asking Pierre what he felt to be the future of the analog turntable.