I just finished reading the thread - 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 a ultra low mass, ultra short unipivot - there is no extra horizontal mass...
The entire discussion with regards to horizontal mass in this 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, 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.
bye for now
David
