[Dick Pierce[_2_]]

Scott wrote:
On Oct 6, 11:20 am, Peter Wieck wrote:
How come nobody to-date has mentioned a linear-tracking arm? I have
two in-service, a Rabco and a Revox. After the initial set-up and
tweaking, they both have performed flawlessly for many years - likely
with no more maintenance than with any given pivot-arm system - but
then I would not know that.

Sure, legacy equipment may be a bit tougher to find in excellent
condition - but not all that tough.

No!!!! Not possible!!!! And you have to deal with two resonant
frequencies not just one!!!

WHy, because it SEEMS like such arms must have
more mass in the horizontal plane than in the vertical.

With arms like the Rabco variants and the revox, those
that used a servo feedback system: guess what" they are
as much a pivoted arm in the horizontal as much as they
veryical plane, and, in fact, their measured horizontal
mass is pretty much the same as their vertical mass, and
analysis, confirmed by measurement, shows that the
resonant frequency is relatively independent of direction,
just like in a "normal" pivited arm (they ARE "normal"
pivoted arms: it's just that the pivot is moved by the
servo system, in effect).

If on the other hand, you're talking about what I refer
to as patholigical designs like the old Dennisen air-
bearing linear "tracker", well, they are just plain old
bad designs, and their operation defies any conventional
analysis. You can't simply state that they have one
resonant frequency in the horizontal plane and another
in the vertical: the behavior is much more complex than
that. It's not so much like the simple two-pendulum model
where both pendulums are coupled to the same non-rigid
pivot point, it's more like where one pendulum is hanging
off the mass of the other: the result, rather than being
a simple double-resonant system, actually acts more
chaotically. SInce it is very rarely the case that the
stimulus is strictly in one plane or another, the resonant
energy is chaotically exchanged between the two planes
in a very complex fashion. It's NOT a pretty sight, and
leads to a VERY interesting output of teh system, an output
that often has little to do with what's actually in the
groove.

Whatever inspired some of these "designers" of these air-
bearing linear arms, I really hope they got it out of
their system.

I also have a linear tracking arm. Mine may be one of, if not the most
difficult arm to set up.

WHat kind? There's a BIG difference between them. Like I said
above, the air-bearing non-servos linear trackers are nightmare
designs to begin with.

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+ Dick Pierce |
+ Professional Audio Development |
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