Jim,

On Oct 21 you asked about my concerns of the effect of
external DC magnetic fields on the VRDT.  This is in  the
context of my remarks about keeping the VRDT a sufficient
distance from the large feedback magnet, in this case, the
large magnet from a 10" stereo woofer.

The problem arises because of the all-ferrous construction
necessary for the VRDT to work as an inductive bridge. The 
problem arises when the moving or sensing vane gets magnetized
by induction and is attracted to the outer transformer cores.
It WILL work with a non-ferrous vane, but at about 1/100th
of the sensitivity, probably by eddy-current power absorbtion.

Having the vane magnetized introduces an astatic instability
that the feedback tries to "play" with, making noise. Since the
vane is magnetized by moving it in proximity to the large magnet,
I demagnetize it with a tape-head demagnetizer after it is installed.
along with the ferrous cores.  (a #10 wire loop in a solder gun 
will also work, with care, or course.)

There is also an induced magnetism caused simply by being within
the fringing field of the large permanent magnet: it magnetizes
the transformer cores as well as the vane. It can be minimized by
installing the VRDT so the axis of the cores is parallel and
symmetric to the fringing field. This is usually the case with
a VBB seis if both the VRDT and the magnet are in the same plane.

Magnetic shielding can also be used. I have used small fruit cans
around the magnet to contain the fringing fields, and square spice
cans around the VRDT to shunt the stray fields around it. This will
allow proximities of less than an inch to the permanent magnet; note
that the shield cans must not actually touch the magnet structure.
In the STM-8 vertical, the VRDT is about 4" from the bottom of the
magnet, and doesn't seem to have a problem. In a new instrument
with a rare-earth disk magnet, they will be 2" apart, so I am 
planning on a shield around the magnet assembly.

Regards,
Sean-Thomas

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