[RFI] USB Speaker noise problem solved

Doug Grant dougk1dg at gmail.com
Mon Nov 17 10:26:04 EST 2008

A few weeks back, I reported to this group that I was getting massive
RFI from my wife's PC speakers. I figured it out, and maybe the cure
will help others who encounter the same problem.

First symptom was huge broadband noise when her USB-powered speakers
were on. It went way down when the speakers were powered off, but
there were still a few places in the bands where there was a loud 20-30 kHz
wide buzz. I opened up the speakers, and they were actually surprisingly
well-designed from an EMI/RFI perspective, with a big LC filter on the
USB power line and by-the-book decoupling on the audio amplifier chip.

The noise persisted if the speakers were plugged into the USB port,
even if her PC was switched off, but still plugged in to the AC line.
Unplugging the PC eliminated the noise, which told me that that
her particular system keeps the USB power output live when the PC is
off. Toroids on the USB cable and the power cord were not effective,
indicating that it was probably not a common-mode problem (yeah,
I know, Jim - alternate possibility: not enough turns on the toroid,
but I wrapped most of a 6-foot extension cord on a 2-inch core, and if
that's not enough...).

I was getting ready to open up the PC and install a line filter if there
wasn't one already, and decided to try an external filter first to see if
I was on the right track. I mounted a Corcom-type line filter that I bought
at a flea market in a small box, with a power cord to fit the PC, and the noise
went away completely. I noticed that the line cord I used was in fact
shielded, with the shield connected to the green wire - that might not be
very common, but I think it helped.

Make sure you use a filter rated for at least 6 amps...that's what
most modern PCs need, and I had a bunch of 3 amp Corcom filters
that I chose NOT to use!

73 and GL,

Doug K1DG

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