Hi guys. My wife bought a new HP laptop. We've hooked up Harmon Kardon's HK695 speaker system (two desktop speakers and a small sub), the same type I have on my PC. When it's running the speakers emit a low-frequency rumble, kinda sounds like audio on a video that's streaming. I tested these speakers on my system and have confirmed they're 100% fine.
Is there anything I'm doing wrong by having them on the laptop, or is there perhaps a step I'm missing? Thanks.

Home computers and HiFi systems are often supplied with unscreened speaker systems which are proving very prone to radio station interference pickup (breakthrough). RFI is often picked up on a combination of the amplifier signal cables, speaker wires and the 120V / 240V power lead. Unless the interference is severe, this type of problem is relatively easy to cure 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 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.

RFI Solution

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). 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 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.