This is a copy/paste of a question I have asked on What HiFi, in case someone here has any ideas. It's a problem I have no experience of dealing with because I've not had it before. My wife has an Asus Xonar Essence STX feeding a Sennheiser BT100 via a headphone extension cable, this sends to a pair of Sony WH1000-XM3s. I believe the cable is picking up a mains hum. We have tried to route it away from power cables but, with only limited success. The cable is the one supplied with the headphones, would a better cable, or a clip-on ferrite suppressor be likely to help?