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Linear tracking arms and compliance...

I just finished reading the “Are linear tracking arms better than pivoted arms?” thread (http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1266367593&&&/Are-linear-tracking-arms-better-than-piv)- an epic effort which took me several hours to read and digest.

I use a Revox B795 with the Revox Linatrack servo linear tracking arm. (as well as two pivoted arm TT's)

The arm is around 2" long, has an effective mass of 4g, and is a unipivot design, with magnetic arm resonance damping.
The pivot point is what is shifted using the servo mechanism - activated by an optical sensor, when the arm pivoting exceeds a certain angle.

So in terms of force applied to the cantilever/stylus, the arm behaves as an ultra-low mass, ultra short unipivot - there is no extra horizontal mass... (this referring to a number of postings by various people stating that the weakness of LT’s is related to having massively higher mass in the horizontal plane as opposed to the vertical plane)
The entire discussion with regards to horizontal mass in that thread was focused on specific design solutions, which have become the mainstream in linear tracking designs, but which were not and are not the only solutions by any means!

Another thing I noted was there is a preponderance of focus on mid to low compliance cartridge designs. Not surprising given that this is a "high end" forum and the high end has for the most part headed down the path of mid to high mass arms with matching mid to low compliance cartridges.

However I found it interesting that in the description of the design process for the Souther arm (http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1266367593&openflup&96&4 ), all the cartridges used for testing were high compliance. - Even the DL103D - which was a high compliance version of the DL103 (all the current versions are low compliance).

It seems to me that there was far too little discussion in this thread of the issues of cartridge matching with the LT arms.

The LT arms were born in the heyday of high compliance - the Revox I use simply does not sound good with anything other than a high compliance cartridge (and preferably a very high compliance cartridge!).

It was however modified by Empire (when that company was owned by Benz) - to better suit the Empire/Benz mid-compliance MC's - to which end the Delrin tonearm was replaced with a brass tonearm, the damping magnets were beefed up, as were the supports and the drive mechanism.
Basically Benz (under the Empire banner) converted the Revox LT system from an ultra low mass arm, into a mid-mass arm to best suit their MC cartridges.
This was a non-trivial exercise, and the modified TT retailed for more than double the price of the original Revox (which was not a cheap table either.)

What I am pointing at here - is that many of these designs have their birth in relation to a particular type of cartridge (and other system parameters potentially).

Even the valiant efforts of Benz/Empire could not turn the ULM Revox into a high mass setup... but they did turn it (apparently) into a very effective mid mass arm.

If people are talking about mounting low compliance cartridges (eg: DL103, Koetsu Coral) onto arms originally designed for high compliance designs (eg: Souther) then it is not surprising to find the results less than stellar ... even if substantially modified to better suit lower compliance cartridges.

With regards to tangential error, no pivoted arm comes close to what a properly adjusted LT is capable of. (Even though it works as a pivoted arm with the pivot on a sled moving either continuously - albeit at variable speed, or periodically... the so called "crabbing")

The discussion of LT arms in this thread never really moved on to discuss the pros-cons tweaks/solutions and failures of servo armed LT's, nor the relative benefits of differing cartridge compliances with various LT designs.

So I thought I would open this thread for that discussion…

bye for now

David
dlaloum

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
12,088 posts
 
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's avatar
atmasphere
12,088 posts
 
If LPs were perfectly concentric like they are supposed to be, I can imagine that having a high lateral tracking mass would have its advantages. But in real life, concentricity is something that we hope for and often come very close to getting, but its not perfect. IOW the arm does need to negotiate such imperfections in the LP. I imagine some of the issues can be tuned out with the leaf-spring device mentioned earlier- so does that mean that you have a different setting depending on the LP?

One other issue of air bearings I forgot to mention is the coupling that needs to occur from the platter surface to the arm tube of the arm. The idea is that the arm and the platter move together as a single unit. That is to say that if there is air-borne vibration, it affects the platter in the same amplitude and phase as it does the base of the tone arm. If there are differences between the two, this will be heard as some sort of artifact, IOW it becomes something that the cartridge can react to. This is why the plinth can have such an affect on the sound of the 'table.

When you have an air bearing, this coupling is not as profound as it should be. One of the demonstrations that this is a very real phenomena is the fact that as you increase air pressure in the bearing (increasing coupling to the base) the arm sounds better.
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