For my birthday last month, Dave gave me a nice pair of Bose powered speakers to replace the crappy broken set I'd been using. Your basic computer speakers, except I actually use them primarily for a little portable radio I listen to while hacking.
Only one problem: they had a major hum as soon as I turned them on. Even when I turned on the radio, I could hear the hum in the background. It got better if I turned the speakers way down and the radio up -- it wasn't coming from the radio.
After about a month it was starting to irritate me. I mentioned it on #linuxchix to see if anyone had any insights.
Maria and Wolf did, and narrowed it down pretty quickly to some sort of ground problem. The speakers need to get a real ground from somewhere. They don't get it through their AC power plug (a two-prong wall wart). They also don't get it from the radio, which is plugged in to AC via its own 2-prong wall wart, so it doesn't have a ground either.
How could I test this? Wolf suggested an alligator clip going from one of the RCA plugs on the back of the speaker to my computer's case. But it turned out there was an easier way. These speakers have dual inputs: a second set of RCA plugs so I can have another cable going to an MP3 player, radio or whatever, without needing to unplug from the radio first. I ran a spare cable from these second RCA plugs to the sound card output of my spare computer -- bingo! The hum entirely went away.
I suppose most people buy this type of speaker for use with computers, so it isn't a problem. But I was surprised that they'd adapt so poorly to a portable device like a radio or MP3 player. Is that so uncommon?
[ 21:49 Oct 11, 2009 More tech | permalink to this entry | comments ]