My wife recently bought a new PC. The (conventional analog) speakers
that came with it were defective, and when we returned them, the shop
was out of stock, but offered to give us a pair of HP-branded
USB-powered speakers at no charge.
They work fine.
However, they also appear to be the source of some really ugly RFI
that I hear on 160 and 80M. Even with the PC powered off, it appears
that the USB port still delivers power. With the PC off, and the
switch on the speakers turned OFF, there's no noise. If I turn the
power switch on the speakers ON, a broad 20-kHz-wide (+/-) signal
appears, spaced every 150 kHz or so, and drifiting around slowly. A
quick test of a clamp-on ferrite on the USB cable, then wrapping 10 or
so turns of the cable around a bigger toroid did not make a dent in
it.
Not sure if it is noise on the USB power or if the speakers have
poorly-decoupled Class-D amplifiers in them causing the noise.
I am sharing this so that anyone with a similar noise problem might
benefit, and also to see if anyone has encountered this problem, and
has a better solution than mine (which is to ask my wife nicely to
turn off the PC speakers when I am on the low bands).
Does anyone make a USB filter that cleans up the power line without
compromising the data lines?
73,
Doug K1DG
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