Serge Auckland wrote:

Maybe I have lead a sheltered life since I moved to the country.... :-) My
experience is otherwise, that decent equipment doesn't hum or pick up RF if
half-decent screened cable is used. I do accept that I've had some trouble
with SOME switched-mode powered equipment which was cured by the use of an
isolating transformer. I assumed that was an isolated example.

On the contrary, it's increasing daily and is all the more reason to go
balanced since you don't share the screen ( and any associated earth leakage
currents ) with signal return. I've even modelled typical equipment using SMPS
supplies and a very experienced colleague of mine got a shock at how much
voltage there is on the screen of much unterminated equipment these days.
Laptops PCs are a case in note. When it's earthed to its destination it
produces a ground current in the input stage, the effect of which depends on
quality of the EMC design of the receiving equipment. Namely ALL screens should
be terminated to CHASSIS. The very reverse of how it was once done.

Graham


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