Linear tracking arms and compliance...

The problem you have with a really short arm is two fold. First, warps can cause audible speed variation. Second, by the nature of the device, the pivot point of the arm bearings must be placed above the plane of the LP. This results in variable tracking pressure due to warp or bass modulation. IOW, such arms will not play bass properly, as they will have less tracking pressure on bass notes.

To solve this, the LT arm must have a longer arm tube so the bearings can be placed in the plane of the LP.

A secondary issue that relates to air-bearing arms is that such arms often have much higher lateral mass than they do vertical mass. This causes the cantilever of the cartridge to move back and forth (left and right) relative to the arm. This motion can often mean that a radial arm will have lower tracking angle distortion. This issue can be reduced by decreasing the compliance of the cartridge, but doing that may not agree with the vertical tracking mass. IMO air bearings are not a good solution on account of this issue!

In case there is a concern of bias here, despite my comments above I am a fan of LT arms. I would like to see these issues solved, but I'm starting to think that I will have to do it myself since these problems have been known for over 25 years but little has happened in that time.

Atmasphere,

Hasn't Eminent Technology resolved both issues? The bearing is definitely in the plane of the LP, no question there. ET has also come up with a decoupled counterweight system which reduces the horizontal effective mass of the arm, but perhaps it's still not low enough.

Atmasphere, when I used an ET2 many years ago these issues were ameliorated by
Using as light a counterweight as possible mounted on a horizontal leaf spring. By tuning the leaf spring through loosening the clamp you could reduce the initial horizontal effective mass when eccentric records generate side force.
The other trick I used was to place a small magnet under the horizontal bearing tube ( not touching it ). This effectively provides horizontal dampening far superior to using a fluid trough. It was so effective that the volume of the preamplifier had to be reduced - very surprising.

Dover - your solution is interesting - with the magnetic damping...

The Revox setup is also primarily in the horizontal plane rather than the vertical plane (it will have some influence vertically but far less).

I am hearing a number of people posting with damping solutions for LT's focused on the horizontal plane... even with the Revox LT where horizontal mass should not be such an issue - perhaps part of the reason for the damping requirements is that due to the need to shift the pivot point (or the entire arm) - the movement itself might activate the resonance - so damping is required to keep it controlled? (just a wild guess, throwing it out there...)

Moerch's (new?) TOTL arm the DP8 has high horizontal mass, with damping, and optional damping for the vertical motion... (with effective vertical mass adjustable through armtube changes - from 4g to 8g) - again horizontal damping, without vertical damping?

JVC's servo damped arms in the 80's included the top models with both vertical and horizontal damping - but the more basic models (the bottom of the servo range) had only horizontal damping... (QL-Y3F).

Atmasphere, you are right - and the Revox can be a bitch with a warped record.... if you have a mid or mid-high compliance cartridge on it. (no point even talking about low compliance with that arm!)

But with a very high compliance low mass cartridge, eg: original early OM30 with 35cu compliance and the optional weight removed - cartridge mass 2.5g... arm mass 4g... total system mass less than 7g all told.
It rides those warps with the greatest of ease, and no audible effects.
Another option that works brilliantly on that arm is the Shure brush damped family - I'm sure the damping is helping as well - but the cartridge again has no problem negotiating warps. (In my case a V15V-SAS)

Although it is meandering off topic a little, it seems to me that high compliance cartridges handle record imperfections (warsp, eccentricity) more easily than their lower compliance brethren.

I am not sure whether this is due to the high compliance, or the combination of ultra low mass and high compliance...

Theoretically speaking - all arms must contend with the forces involved in the rise and fall of a warp (and the effects of eccentricities) - the tonearm effective mass measurement is a measure of the actual inertia at the stylus tip - ie the amount of force the stylus must cope with when warps or eccentricities are encountered.
A low compliance design, with its matching higher mass arm, must perforce cope with much higher forces applied.

With regards to VTA - it is true that a shorter arm will also have greater VTA changes when negotiating the warps - potentially with audible variations.

In the case of the Revox, the pivot point is raised when the arm us cued, and lowered for playing the record. - it obviously remains above the plane of the record (as the arm passes over the record) - but is lower than is immediately apparent when in playing position. (the entire linear gantry is lowered into playing position).

Some of the really nasty warped records are not trackable by the Revox, but still track (albeit with difficulty... and audible issues) with my 10" pivoted JVC arm. But this is not what one bases decisions on, unless one is aiming at archiving damaged records!

For the levels of warp that "normal" records (in decent condition) have, my experience has been that the variations are not noticeable/audible.

There is a Shure paper http://shure.custhelp.com/app/answers/detail/a_id/4072/session/L3RpbWUvMTI5NzAxMDc1OC9zaWQvbDl4aWhZbGs%3D

This includes a range of material presented to the AES during the 70's.

Included are discussions of "warp wow" and "stylus scrubbing" along with the pro's and con's of pivot/arm damping ... it is focused of course on their ingenious world beating solution - the damping brush... so don't forget your pinch of salt when reading(!). (mind you I really like those brushes!)

It is interesting that although they discuss both vertical and horizontal stylus scrubbing - they appear to contend that only the vertical effect is of import.... Perhaps related to the fact that eccentricity generates wow at a resonance of 1/2Hz and is therefore relatively innocuous?

There may be another advantage to ultra short arms - when it meets a warp, the cartridge rises and falls WITH the warp - actually maintaining a geometry which is much closer to ideal, than with a long arm. With a long arm, the warp changes the angle of the record surface, but the cartridge rises remaining parallel to the platter (relatively... more so the longer the arm) - but not the record...

So perhaps a case can be made that in fact a short arm maintains VTA closer to ideal than a long arm for warps?

More questions than answers?

bye for now

David

David,

I'm trying to visualize a short armed linear tracking tonearm negotiating a warp and I can't help but see the cartridge alignment doing the exact opposite of what you're saying. You are right that the long arm keeps the cartridge closer to parallel when going over a warp. You are also right in that the short arm changes the angle of the cartridge more than a long arm when going over warp, but it changes in the opposite direction that the record surface changes.

Imagine looking at your cartridge from the side with the record surface moving from right to left. A warp comes to the cartridge and lifts the tonearm. The cartridge tilts, lifting the cantilever end more than the tonearm cable pin end (although both lift, the cantilever end lifts more... and much more with a short arm). The record surface does the opposite. As the cartridge climbs the warp, the record surface is higher on the right side of the cartridge than it is on the left side of the cartridge. It's tilted in the opposite direction. For the two to remain in alignment, they must be tilted in the same direction. Please correct me if I don't have this right.