Abstract: A device and a method for its use to attenuate electromagnetic noise, such as that caused in power supply line current by back-emf from devices being powered, in order to provide cleaner, or more regular alternating current and voltage waveforms. A permanently magnetized magnet is held close to a conductor cable for an alternating current, in a case, which may also hold a permeable material extending from the magnet and at least partly surrounding the cable. Multiple magnets may be included, arranged parallel with each other and with like poles adjacent or facing each other, forming a tubular arrangement about the cable.