Mains Hum

Mains electricity cycles at 50Hz across the UK and Europe and 60Hz across North America, and any of it finding your signal path arrives as a constant hum with harmonics stacked above. Ground loops are the usual culprit: two pieces of gear earthed by different routes, so a small current runs down the audio cable between them. Dimmer switches, chargers and fluorescent tubes all contribute. Fix it at the source before reaching for a plug-in. Run the laptop on battery, move audio cables away from power leads, and swap the cable. A notch filter takes out the hum and a piece of the voice with it.