Home computer RFI problems
RFI Ferrite Rings
This article focuses on how to cure radio interference (RFI breakthrough) on computer speakers as well as radio station pickup on HiFi equipment and telephone handsets, with practical tips and advice on the correct use of ferrite rings.
Home computers and HiFi systems are often supplied with unscreened speaker systems which can be prone to radio station interference pickup (breakthrough) where high RF field strengths are present.
RFI is often picked up on a combination of the amplifier signal cables, speaker wires and the 120V/240V power lead. Unless the radio interference is severe, this type of problem is relatively easy to fix using ferrite rings as a simple RFI filter (see tips below).
Many computer speakers are very susceptible to interference from mobile phones and from nearby radio transmitters, including amateur radio stations using SSB transmission on the HF bands and AM broadcast stations (broadcasting on the short wave or medium wave bands). Interference from AM radio stations may be intelligible, whereas SSB radio breakthrough will not be (it may sound like "Donald Duck".
In severe cases it is also possible that a local AM radio station can breakthrough onto computer speakers, with the speaker cables acting as an aerial. This RFI interference may be heard continuously whilst the computer speakers are turned on. Moving the speaker cables may change the level of the interference.
Bunching the cables with cable ties to reduce their length will reduce the interference, as will winding each speaker cable around ferrite rings (winding at least 10 to 15 turns around the ferrite ring for shortwave radio station or HF amateur radio interference).
The ferrite rings act as RFI filters, making the cables inefficient aerials. This reduces the level of the radio signal on the amplifier wiring. Ferrite rings can be purchased from Maplin.co.uk under stock code QT26D.
It is important to place the ferrite rings close to the amplifier unit itself for best interference suppression (see the picture left). Don't forget to feed the mains power cable to the computer speaker's amplifier around a ferrite ring too, as it is very common for most of the interference in these cases to emanate from the 120V or 240V mains power supply.