Linear tracking vs. Pivoted tracking tone arms.

Atmasphere,
Are you familiar with the ET2 arm? Its counterweight is decoupled in the horizontal plane which lowers its horizontal effective mass. I'm not sure if any other linear tracking arms have that feature.

Ketchup, I have heard that before. And you are right, that's a helpful feature and not found in the majority of air bearing arms.

What it does is react to minor perturbations (and is thus an acknowledgment of the issue), but if you think about it, for best results it would have to be tuned to anticipate the compliance of the cartridge (I suspect it introduces LF noise too, translating the subsonic noise generated by the bending of the stylus that otherwise happens to a higher frequency). All that mass is still moving across the LP though, and its all being moved by that stylus. IOW, its a compromise but better than nothing!

There is a different issue that all air bearing arms have. Ever notice that if you have higher pressure pumps, reservoirs and regulators that the arm sounds better? There is a basic engineering problem at play. In LP playback, there can be no play between the platter surface and the mounting of the cartridge. This issue is very much akin to the steering in an automobile. If there is play between the steering wheel and the wheels of the car moving on the road, the result will be uncertain handling and could be dangerous!

In a turntable this sort of play is translated as a coloration in the sound. Anyone with an air bearing arm knows what I am talking about- the pumps and stuff I mentioned earlier. IOW, there is play that is inherent in the arm and there is nothing that can be done about it if the arm is to work.

This is why I favor the servo-controlled approach. Its a pity that there are no arms that have been developed to take advantage of recent materials advances in this area.

Atma-sphere et al, There is one type of pivoted tonearm that exhibits the property of a low vertical inertial mass with a high horizontal inertial mass, a la most SL tonearms. That is the Dynavector DV series. Rather than try to translate what Dynavector says about this feature of their tonearm, I am quoting here the DV505 owners manual:
www.dynavector.com/etechnical/505manual.html
The verbiage hints at why a high horizontal mass has some advantages as regards low frequency resonance control. As I think I mentioned once before, there are also a few vintage conventional pivoted arms that have obvious added mass on their lateral aspects, added usually at the pivot, e.g., the SAEC, to name only one brand. My point is that there are possibly some theoretical advantages to having a low vertical and high horizontal inertial mass, combined in one tonearm. Ralph, I would be interested to know what you think about this.

I just found this quote in the DV507 MkII brochure:
"The DV507 bi-axis tone arm has a large inertia for horizontal movement and a very small inertia for vertical movement. We shall now explain the reasons why this is advantageous.

It is well known that a cartridge generates an audio signal by the differential motion between the cantilever and the cartridge body. Consequently, if the supporting point of the cartridge (the tonearm) vibrates, the tonearm motion affects the audio signal.

In these conditions, the signal, which causes the tonearm to vibrate is of low frequency and large amplitude.

In the currently used 45-45 stereo record cutting procedure, low frequency signals are almost entirely recorded in a horizontal direction. This means that the low frequency signal, which can cause vibration in the tone arm, exists only as a horizontal force.

The tonearm therefore must have sufficient effective mass and rigidity in the horizontal plane in order to provide a stable platform for the cartridge.

On the other hand, for the mid to high frequencies, the effective mass of the tonearm should not be too large since the combined mass of the cartridge and the head shell need to be taken into account as well. In particular, where records have a warped surface, the vertical effective mass needs to be small enough to ensure a good tracking ability on such surfaces.

To summarise, the tone arm should have a large effective mass and enough damping in the horizontal plane and at the same time a small effective mass in the vertical plane."

I'm just sayin'...

If you look at an LP groove you see that vertical and horizontal motion are both present. Favoring one over the other sounds tricky to me at best. These days most LPs have variable groove spacing so the arm/cartridge combo has to be ready to move without inducing noise.