"gndloops" and Maxwell

[Martin Czech]

Ok, how important are magnetic fields, concerning loops?

What are the figures we encounter in studios?

Example: Normal power chord (distance of conductors 10mm) @ 1.0 A peak
current, 50 Hz.  maximum field at a distance of 1m is: 2nT. (2e-9
Tesla, a single wire of 1A makes a B of 200 nT at the same distance,
the formula is B=u0*I/2/PI/r=0.2e-6*I/r (I in Ampere, r distance in meters)

In my example the ground loop had an area of ~ 1m^2, PHI=B*A worst case
(orthogonal, B-Field constant 2nT): PHI(t)=2e-9*sin(w*t) Am

E=-d/dt PHI : |E(t)|=2*pi*f*2e-9 V~ 0.63 uV

The loop in the example had an resistance of 0.1 Ohm : Rloop 6.3uA

This is not much, compared with the assumed current dump of 10mA.  This
example shows, that the internal error currents are much more important
than external induced currents via E=-d/dt PHI. (Of course only in
normal environment, on stage with light show, or with computer crt or
other sources of ac B-fields, or near power transformers this may be
different, but in studio we can mostly avoid these fields. Switch the tv of,
and keep the computer away from single coil pickups, or try to find
orthogonal directions of B-Vector and A-Vector).

By the way: almost the same loop is build up by the signal path and
ground, so disconnecting the shield may help a bit, but it will not even
eliminate all loops.

m.c.

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