J=F3n Fr=EDmann wrote:
(snip)
>>This difficulty in doing this may be that it is already grounded at=20
>>some other point (say, through the computer power plug), so that=20
>>adding this second ground contact will produce what is called a
>>ground=20
>>loop.
>=20
> I hope that doesn't happen. The shield is not connected to the amp boar=
d
> it self or to the geophone.

But that means that the cable shield (that is intended to keep any=20
external electric fields from driving capacitive current signals into=20
any of the shielded conductors) is not necessarily grounded to a=20
potential that the amplifiers consider to be zero volts.  The=20
instantaneous potential to one point on the Earth can be quite=20
different than the potential at some other point, since atmospheric,=20
subterranean and the electrical power system dump current into various=20
points on the Earth at any particular moment, and those currents drop=20
voltage as they spread out in the Earth.

The only way a cable shield keeps external signals out of the shielded=20
conductors is if the shield is held at whatever potential the=20
receiving electronics considers to be zero volts.  You need to find=20
the signal common (zero volts as far as the electronics sees the=20
universe) and tie the shield to that.  If the geophone has a shield=20
connection, it also should be tied to that potential through the cable=20
shield, or a spare conductor running through that shield.

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