"Andrew Rossmann" wrote in message
.net...
I have a weird question. I recently bought a second Linksys BEFSX41
VPN router to test out some new firmware before deploying it elsewhere.
I noticed that when it was on, there was a very audible buzzy/hissing-
like noise coming from it. I ended up opening it up and tracing it to
two chokes on the 12VDC input line. They look like Fig D in the
following link:
http://www.jwmiller.com/pdf2/FB2000.pdf

By moving them around, I could sort-of make the noise worse or better,
but couldn't eliminate it. As a temporary workaround, I simply shorted
them out with a solder bridge on the solder-side of the board.

On the other router, you can barely hear the hissing only when your
ear is pretty much presed up against it.

My question is: What is causing the hissing, and why is it doing it on
DC? Any fixes to stop the hissing without losing the EMI suppression?

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My guess would be that these chokes filter ripple current from a high
frequency switch mode PSU, the best way to stop them ringing is with fast
setting araldite epoxy - allow it to run into the holes through the ferrite
where the wires run through.