[Phil Allison[_4_]]

Bob Simon wrote:
-------------------

When I connect audio out from my cable box to my pre-amp, I get hum.
I was able to eliminate it for a couple of years by inserting an isolator
(ISO-MAX VRD-1FF) just ahead of the Contour cable box.

Two days ago, the cable company disconnected my cable TV service because
they determined interference was coming from my house. Yesterday, a
Cox technician looked at my connections and stated that the source of
the interference was my isolator.



** Very dubious....



He directed me to remove it, performed a measurement, and said that
everything was ok now, so he reconnected my cable TV service.
No surprise - the hum is back.



** The isolator is probably just a simple 1:1 ratio ferrite balun that passes radio frequencies only. The hum you hear is 60Hz plus various harmonics.

The radiated RF interference is due to having no low impedance path to ground at the cable outlet - allowing items that follow to radiate easily.

The fix is ridiculously simple - re-fit the isolation balun and ADD a small capacitor across the input and output grounds. I may be possible to fit it internally but if not, is does not matter much.

Try a ceramic cap of about 100nF, voltage rating is not important either.



..... Phil