Cable Asylum

Can I add some explanation to what happens?

91.104.107.208

In Reply to: RE: Five questions posted by thom70 on February 04, 2008 at 16:10:52

Im not totally sure... but...

One thing that really strikes me is the earthing arrangment you have used to date will not ensure that the miniumum noise level is achived given that the source end was connected to the screen of the wire, but it was the load end that was actually connected to earth. This must have provided a return for any noise the screen of cable was picking up being returned along the cold side of the twisted pair to the real earthing point at the load end.
Eddy and other currents could be generated aswell (see below on Voltage and current interactions) this would also raise the noise floor and could capactivley couple the cold to the hot and hence the screen producing noise and low level distortion in the cable itself.

If I was you try the experiment again this time with the screen at the same end as your earthing point i.e. in this case the load end. This would ensure that any screen current and or voltage would be grounded out, instead of being transfered to the input of the load ie your amp.

Voltage and current interaction
The voltage in the wire will provide an electric field to the screen. The current flowing in the wire will provide the magnetic field to the screen. Hence my question on signal voltage and source and load impeadances as these will govern the current flow and hence the size magnetic field.

The distance the screen in from the cable pair seems reasonable given the small signal voltage and guessed at current. But you could also try increasing the distance by say 50% to 100% this would further reduce the screen's interaction with the signal wire whilst still providing a good screening effect.
Most peps here frown on screening of cables as it appears to effect the sound quality but in these days of living in an electromagnetic fugg screening is often needed.

Good luck with the experiments.
Chris.

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