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Audio & Design M9BA tonearm

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Back in the days of pre-stereo high fidelity, when a 6-gram phono pickup was considered to be "featherweight," the best universal-type tonearm we knew of was a bulky, very professional-looking device made by Gray Labs and designated the Model 108. One unusual thing about it was that, instead of using sleeve or cone-face bearings, it had a single up-ended needle—a so-called unipivot—for both the vertical and lateral modes of motion. The other unusual thing about it was that the pivot system was viscous-damped, and it was this, we suspect, that was largely responsible for the arm's ability to make any pickup sound somehow sweeter and cleaner than it did in any other arm.

With the advent of lightweight pickups, though, viscous damping fell into ill-repute, allegedly because it caused too much pivot friction. In truth, the criticism was valid only in that the existing viscous-damped arms simply had too much pivot damping for the new pickups.


The main purpose of tonearm damping is to control the low-frequency peak that normally develops through interaction of the pickup's compliance and the combined mass of the arm and pickup. Depending on its frequency (and this in turn will vary from one arm-and-pickup combination to another), this bass resonance can cause acoustic feedback, exaggerated turntable rumble, heavy, muddy bass, and extreme sensitivity to floorborne vibrations. Unfortunately, there is usually no way of predicting, from published specifications, the frequency at which an arm and pickup will resonate (footnote 1), so the safest thing to do is suppress the resonance itself, and this is what tonearm damping does. One way of going about this is by applying a viscous fluid to the tonearm pivots.


The fluid has virtually no effect on slow movements of the arm, as when it is tracking a warp or an eccentric groove. But at the bass resonance, where the arm would normally vibrate rapidly from side to side, the damping fluid applies a certain amount of braking power, so the movements, and the resulting resonant peak, are much reduced in severity.


A second way of damping the tonearm resonance is by using a slightly flexible coupling between the arm and its counterweight. When the arm vibrates very slowly, it and its counterweight will move in unison. But when the movements become more rapid, as at the bass resonance, the inertia of the counterweight tends to make it stand still, and the flexible coupling allows it to do so while the rest of the arm vibrates, so that the mass of the counterweight is effectively isolated from that of the rest of the arm. Thus, the more the pickup's compliance tries to resonate against the arm's total mass, the more this value of mass will be diminished by the isolation of the counterweight. Instead of a single, well-defined resonance, the resonance becomes smoothed out and spread over a wide range of frequencies.

Both damping systems are reasonably effective, but each one has its shortcomings. Pivot damping tends to increase tonearm friction if enough damping is used to completely suppress the resonance. And while the decoupled counterweight adds no friction to the pivots at all, it so happens that a certain amount of pivot damping is useful for controlling torsional resonance.


If the stylus in a pickup system were right at the end of the arm, and in line with it, we could probably forget about torsional resonance. In fact, though, the stylus is always a little bit below the axis of the arm (it has to be, to contact the record), so whenever it traverses a modulated groove, there is a tendency for it to tilt the arm back and forth. The arm pivots prevent any actual tilting from taking place, but there is always a certain amount of flexibility in the arm, so its pickup end will tend to tilt while the pivot end will stay put. And here we have the old familiar mass/compliance situation, between the mass at the pickup end of the arm and the flexibility of the arm.


The result is a twisting or "torsional" resonance. This is by no means as intense as the main bass resonance, but it can have a subtle effect on the cleanness of the sound, particularly if the pivots have enough play in them to allow them to chatter when the arm is twisting back and forth. This is the second useful purpose served by pivot damping; it cannot eliminate the torsional resonance itself, but it does prevent the pivots from rattling. All of which brings us to the Audio and Design tonearm, which is the first design ever to incorporate both forms of damping in a single arm.


The A&D toneamrm
Superficially, the A&D arm looks rather like the SME 3009, minus the plug-in shell. The head is a lightweight "skeleton" mount, the arm itself is a light alloy tube with an offset counterweight at the rear end, and the pivot section of the arm is a small inverted cup with a tiny ball-bearing race at the center and four metal pins protruding from beneath.


The base of the arm has a single vertical needle-shaped pivot, which fits into the ballbearing race, and a short tube inside the pivot cup dips into a small well full of viscous damping fluid surrounding the pivot. Around this well are four other small wells, each with a contact at the bottom, and each filled with mercury which surrounds the arm's contact pins when the arm is assembled. This neatly eliminates problems of pivot drag due to the stiffness of the usual wire-type signal leads.

 

The arm base itself fits into a channeled slide, for tangency adjustment, and this plus the base height are both adjustable via small lever screws, thus obviating the need for adjusting tools. A lift lever is fitted to the arm for gentle handling, and this too is easily adjustable for precisely the same amount of lift all the way across the disc surface. The arm is not too difficult to mount—it requires a small rectangular cutout and four screw holes, and a precut board is available for use with Thorens turntables. Templates are supplied for getting the channeled base located in the right place, and for adjusting tangency.


Set-Up
The height and arm lift adjustments had us chewing our fingernails, though. They were spelled out clearly enough in the instructions, but since the lift lever only lifts the pickup about 3/8", the lift position was adjustable, and the arm base would fall down the instant we loosened its height adjustment, it took us a little while to arrive at the proper setting.


The necessary mercury and viscous damping fluid are supplied in small plastic vials, along with a tiny medicine-type dropper for handling the mercury. Once the mercury is installed the phono unit should not be moved, or the mercury may roll out of its wells. If you wish to move the unit, it's safer to take the few seconds necessary to suck the mercury into the dropper and replace it in its vial.


The viscous-damping fluid won't exactly spill, but it will tend to crawl out of its well if the arm base is left lying on its side for more than a few minutes. So if you have to ship the arm somewhere, seal off the bottom well with a wad of used chewing gum. The top sleeve won't retain enough fluid to run all over the place.


Two adjustments are provided by the counterweight. Besides the usual force adjustments, the offset counterweight can be rotated around the arm to counteract the effect of the offset head, which would normally cause the arm to tilt to one side on its unipivot when playing a record. With careful adjustment in the initial setup, the arm will hold the pickup perfectly vertical to the record surface.


Stylus force is adjustable by a vernier thumb screw at the rear of the counterweight, and no force gauge is needed. You set the arm for perfect balance, locate a small red dot on the adjusting screw, and use the dot to count turns. A full turn gives ? of a gram change, and the adjustment proved, not surprisingly, to be more accurate than any of the scales we have on hand. One suggestion, though: set the stylus force before adding the viscous damping material. That way, the position of perfect balance is easier to see. It can be done with the fluid installed, but it takes a bit longer.


Mounting the pickup was easy, and we noted that even the Ortofon pickups can be accommodated by bending the tabs at the rear of the pickup and installing the extra, heavy counterweight that is supplied with the arm.


Connecting the arm was a bit less simple, because its output leads are only 2 feet long. Just on a hunch, we checked the lead capacitance (by resonating it against an inductance of known value), and found it to be in the vicinity of 180pF. This explained the short leads, as 180pF capacity is already high enough to resonate within the audible range with a few pickups. Making the leads longer would cause serious resonance problems with many currently available pickups. There is really no excuse for using such high-capacity leads, though; shielded cables with less than 20pF capacity per foot are readily available, and we see no reason why they weren't used, to allow at least four feet of cable length.


We found one other mild problem, too. When the lift lever is lifted, the arm swings to one side by a good half an inch, making it impossible to lower it into the same groove it was lifted from. It would also be a great help if the lift lever had some damping in it, to prevent the pickup from being lowered too rapidly onto the disc, for the main purpose of a lift lever is to minimize the chances of pickup and record damage due to accidental dropping.


Performance
Aside from the lift lever and cable problems, both of which should be easily remediable by the manufacturer, the A&D arm performed better than any tonearm we have ever tested. With a pickup that resonated with the cables at beyond audibility, the A&D arm made it sound sweeter and smoother than any other stereo tonearm we've tried to date, and it did a remarkable job of tightening up the low end, while reducing tendencies toward rumble and acoustic feedback.


Our Thorens/SME combination, for instance, has always been somewhat prone to feedback, not because of any intrinsic flaws but because the side-mounted tonearm panel (or plinth, as the British call it) acts as a sounding board. We were able to hold both problems to a minimum with the SME arm, but the A&D arm virtually eliminated them. The A&D arm is also unusually resistant to physical shocks that knock every undamped pickup right out of the groove. It doesn't make shock isolation unnecessary, but it does make a given amount of it much more effective.


The carrying arm is easily lifted off its base to facilitate cartridge replacement, but we doubt that many people will have the spare cash on hand to follow the manufacturer's recommendation and use a separate "pre-calibrated carrying arm" (at $45 each) for every pickup cartridge. We'd like to have seen the arm equipped with plug-in shells, even if the fittings did add a gram or so to the total arm mass. Of course, if you can afford one arm per cartridge, it's an ideal way to achieve quick changes, but when removing the arm, hold its pivot above the damping fluid well for a few seconds after you lift it off, to give the pulled-off strand of fluid time to drip back into its well.


The A&D arm uses a unique system of magnetic bias compensation which is built directly into the base of the arm. This is not adjustable, and A&D's data sheet states that it is optimized for elliptical pickups tracking at between ? and 1 gram. Apart from the fact that we have yet to find a pickup that would perform at its best at a gram or under, the bias compensation did appear to be right for ellipticals tracking at between 1 and 2 grams.


Summing Up
The basic design of this arm, then, appears to be markedly superior to anything else on the market, in that it can make most pickups perform better than in any other arm. But the very fact that it is so good in most respects makes us less tolerant of its minor shortcomings than we would otherwise be. There's been some highly successful pioneering in the pivot and counterweight design of this arm, but other manufacturers have solved the cable and lift lever problems long ago, so there does not really seem to be any reason why A&D couldn't have paid closer attention to these details. We don't expect perfection in anything, but for $150 we should at least be able to expect freedom from problems that have been solved in $70 arms.


As it is now, we will probably adopt the A&D arm as our primary test standard (with suitable pickups), but not as gleefully as if it were debugged.


2019-09-17