If you were to design a tone arm, what would it look like and feature?

ct05171,

All air bearings rely on very low friction of the main bearing to allow the cartridge to easily pull the arm along.  To some extent, conventional arms also pull the arm axially to a new position, but, they employ the mechanical advantage of a pivot, something the linear tracking arms cannot do.  At least theoretically, this means more strain than a pivoting arm.  I never encountered the problem myself, nor any friends using air bearings (friend had the same ET arm as you have), so this might just be a theoretical issue.  Any sensor approach, such as that of the Reed, that then employs a motor to make an adjustment has the theoretical problem of having to catch up with a wrong angle, with some possible overshoot, mechanical noise from operation, etc.  A similar approach to that of the Reed (moving arm base) is used by one version of a Shroeder arm, but the base is moved by the arm being pulled into the right position by friction (a different compromise).  

I like the sound of the air bearing arms I've heard.  They tend to have really good bass response--full and powerful without being muddy.  I suspect that comes from the high lateral mass resisting the lateral movement of the cantilever when tracking monophonic bass signals (that means that all of the groove displacement is translated into bass signal instead of some being lost by the cartridge moving side-to-side). 

I have no idea which design or elements of an arm will result in the kind of sound I would like.  I would like to hear the Reed T-5 arm out of curiosity because it does have a lot of design elements that address theoretical issues with tonearm performance.  By using an arm base that rotates, it achieves what linear tracking arms achieve without resort to air bearing or other such mechanisms to achieve tangency across the record surface (i.e., it does this without excessive lateral mass, and without putting lateral strain on the cantilever).  By rotating the base upon which the arm is located (moving the pivot point), the headshell also doesn't need to have an offset angle, so no skating forces are created).  The motorized base's movement is controlled by a laser that measures deviation from tangency. 

But, I have no idea how all of this translates into performance and whether the arm would work well with the cartridges that I like.

The Reed arm is fundamentally, a simple, captured bearing design arm.  The only complication is the moving base that maintains cantilever/stylus tangency to the groove and eliminates skating forces.  Optical sensors have been around a long time (e.g., Beogram 6000 from the 1970's) for controlling servo motors to maintain tangency.  I don't think this makes the Reed arm unduly complicated, but, it is a matter of debate whether its advantages are worth the trouble.  The big issue, to me, is the price and possible problem with fitting such a massive arm on a particular table.

An arm from the past that is interesting to me is the Air Tangent arm.  There is one very rare model that allowed for setting VTA by remote control while the arm is playing the record.  That would be the only practical way to set VTA on a record-by-record basis.  That might be fun, although I am still to lazy to do that level of fiddling around.

I have not heard some of the ultra-exotic arms that go to extraordinary measures to damp and control vibration such as the SAT arm or the Vector Superarm 9.  These are too expensive for my consideration, but, I would like to hear them anyway.

The Thales arm is interesting in design; it is a version of the Garrard zero 100 tonearm from the 1970's.  The theoretical disadvantage is having four pivots  for movement in the horizontal plane.  I also don't know if there are advantages or disadvantages to the twin arm tubes (rigidity, inertial mass).  Also, unlike the Reed arm, the Thales does not eliminate skating force because there is an offset angle to its headshell.

I've heard some very nice setups with the Triplanar arm and I certainly think it is in play among top designs.  I also heard a nice setup with a Durand arm.  Durand makes gimbal and uni-pivot arms; their top model is a uni-pivot arm (Telos, a model I have not heard).