So recently a friend at work (the esteemed DJ Hedflux) had a problem with his speakers – he has a pair of powered speakers, and he outputs sound down a normal stereo cable into the speakers. Being a professional DJ, nothing in his system is particually cheap (he’s not spending £100 on cables, but he’s not buying £1.50 speaker modules from the market) so we can quickly rule out shoddy work and bad connections inside of the devices.
Anyways, he worked out that it was a combination of his computer and his touch lamp causing it – doing things on his computer that required lots of processing, and having the dimmer lamp “dimmed” caused the noise to appear and dissapear.
This is very common, and also, very easy to fix.
How to fix:
The solution is very simple, and very cheap. You’ll need a ferrite ring, which you wind the audio cable in just a couple of times, like in the above picture.
Maplins sell these for a couple of pounds, usually find them in the Radio section:
Image stolen from http://www.gbrcaa.org/ntoa/Filters,%20Chokes%20and%20OIs.htm on which site you can also find more information about other kinds of chokes.
In Hedflux’s case, its possible that his computer internals arn’t all grounded to the case correctly, or the case to the power supply, so its generating more electrical noise then ideal. The dimmer lamp however is a common source of Electromagnetic Interferance (EMI).
Because he’s using a set of powered speakers, the loudspeaker cable is acting as an antenna, and the amplification circuit is having a side effect of acting as a little radio, which is picking up the EMI from the computer and the lamp, and turning it into the annoying hissing noise.
The ferrite bands that he added to the audio cable basically change the frequencies of that cable, effectivly filtering the annoying frequencies out.
Why don’t these cables come with the bands installed? Well, the point is that it dosn’t STOP the frequencies, it shifts the resonance frequency of the cable. The cable and speakers will still produce noise if the EMI comes in on a different frequency, so it was just bad luck that the EMI in his room was the same frequency as the cable / speaker setup.