I have some cheap computer speakers with a headphone jack. The speakers themselves sound fine, but I always hear an annoying background static when using my headphones. Strangely, when I turn the volume on the speaker itself all the way down, I can actually hear a very faint radio station on my headphones! The speakers came with my Sony Vaio computer, and plug directly into the AC (there is no power converter like most other speakers I’ve seen).
My question: why am I picking up radio stations when the volume is turned down completely? and can I get rid of the static, which I hear at all volume levels?
I’m assuming your computer speakers have an amplifier inside them, otherwise the noise is coming from your sound card. Amplified speakers will take small signals and amplify them (that’s why they call the amplifier the amplifier… duh) which means that it only takes a small signal to get into the sound path and it will be amplified into a big signal.
It’s very easy to “accidentally” make an AM radio receiver. All you really need is something that conducts better in one direction than the other, like a diode, and something to filter it, like a capacitor. Coincidentally, all you need to make an AC to DC power supply (like the wall wart that powers your speakers) is a transformer, a diode, and a capacitor. So, either the wall wart or part of the amplifier circuitry itself is acting as an AM radio receiver and coupling a small amount of noise into your audio path, and from there the amplifier is making it loud enough for you to hear it.
You can try moving around the speakers and cables, but there’s not much else you can do without adding some filtering circuits to the amplifier in your speakers, which is not something that a typical home computer user can do. If you are handy with a soldering iron, try soldering a couple of small disk capacitors between the + and - where the power supply goes into the speakers.