Are linear tracking arms better than pivoted arms?

Lewm, I built a servo for the Rabco (which uses contacts, not relays) so the contact had only to get below 1 Megohm and the motor would run. Then I put a large capacitor across the motor so it would ramp up slowly and turn off slowly. The result was that the motor was always on, and would set its speed according to the rate that the LP grooves dictated. This eliminated 95% of the hassle of that arm- it was reliable and quite precise as far as linear tracking was concerned.

Later I had an ET but after eating a few cantilevers, I ditched it. Obviously I had the wrong cartridges in it; you could sit can watch the cantilever move back and forth as the arm tried to negotiate the LP. If you have ever seen the arm 'wobble' you know what I am talking about.

There are cantilevers that are stiff enough so this effect is reduced. What is not known (IOW I have seen no measurements to this effect anywhere) is how much the cantilever actually moves to make the arm move. IOW if the cantilever flexes by only a few degrees (which will not be visible to the naked eye) than any advantage of straight tracking is lost to radial tracking where this does not occur (of course this phenomena could occur with a radial tracker too, but most cartridge designers are expecting a certain effective mass in the arm and so this should not be a problem).

Short arm tubes such as seen in the Souther have two issues. The first is that the arm bearings cannot be in the plane of the LP so tracking pressure will change as warps and bass frequencies are negotiated. The second is that warps will cause wow.

BTW to be clear about something: its impossible to have an air bearing that has no slop. If there was no slop, there would be no place for the air to be in the bearing. With precision machining and tight tolerances, the arm can be poised on its cushion of air, and not move too much- until it is disturbed by the motion of the cartridge. The fact is that the arm has to move back and forth and yet stay exactly on its locus. If it moves even slightly out of locus that will be interpreted by the cartridge as a coloration. Its a bit of a trick.

I'm pretty sure that everyone agrees that air pressure and holding tanks to promote stable pressure are important to the sound of the arm. If that is **not** the case, then I would agree that air bearings work... Right now I have a Triplanar, and I've had several pivoted arms before that, not because I think anyone of them are the state of the art, but because what I think is required in a straight tracker does not exist. I regard radial tracking arms as a temporary solution.

Atmosphere
"Short arm tubes such as seen in the Souther have two issues. The first is that the arm bearings cannot be in the plane of the LP so tracking pressure will change as warps and bass frequencies are negotiated. The second is that warps will cause wow."

My SOTA has vacuum disc clamping. Severely warped records will not suck down anyway, besides who plays warped records on a decent set up? So how much warp matters anyway? In the same vane, what sort of bass tracking are we talking about?
Wow is at a certain frequency range, or put another way, if I can't hear it, is it there?
regards,
Sam

Sometimes an LP is rare enough that you put up with what you are able to find. All LPs have some warp. When speed variations are introduced, its likely that when they are subtle you are more likely to hear them as a wavering in the soundstage.

I had a Cosmos for a long time- it is nice to have the records be really flat.

I think the difference in the stress on a cantilever caused by a straight line arm vs a pivoted arm has to do with inertia, primarily. Lets assume there is zero friction. The cantilever is therefore bearing only the inertia (in the horizontal plane) of either a pivoted arm or a straight line arm. Inertia is related to mass and is the property of a static object to stay static and of a moving object to continue to move in the same direction and at the same speed (in the absence of friction). In the case of a pivoted arm, because it is rotating with respect to the pivot, which does not move at all, the net inertial mass is lower than for an air-bearing straight line arm, where all parts of the arm from front to rear have to be moved equally by the force on the cantilever alone. Many air-bearing arms have very low mass arm wands to compensate for this issue. Then in the real world there also IS a force on the cantilever necessary to overcome friction, to add to the problem. The cantilever has a huge mechanical advantage in overcoming friction at the pivot, but no such mechanical advantage in overcoming friction at the air bearing. Obviously, all these forces are tiny, else the cantilever would not last more than a few mm of travel. I am not about to argue that these things nullify the potential goodness of linear tracking.

Lewm, Exactly!

About ten years ago I did initial sketches on a tone arm that used a mechanical track that had zero bearing slop. The are was otherwise conventional in that it used a pivot like a radial tracking arm, and a servo that was light-beam activated. Of course I never built it, instead I handed it off to a tone arm manufacturer, but it seems like its not likely to see the light of day anytime soon if it ever does. But it is a design that solves this problem.

I own an LP cutting lathe, and every time I look at the lead screws and do the setup on the cutterhead itself, I think about doing a linear tracking arm. Right now its more important to run the lathe than make a tone arm...