"Andrew Rossmann" wrote in message
.net...
In article ,
says...

"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?

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.

The power supply is your basic plug-in brick. I tried both the
original and a RadioShack that is heavy enough to NOT be any form of
switching supply. Both are standard 120V in, 12VDC/1A out.

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I have several old modems laying about that use 230V to 12V wall-wart
adaptors and SMPSU circuits on the modem board to provide the variety of
voltage rails required by the modem - if an old modem has this level of
technology, don't you think it could be found on a modern router?