A simple ferrite ring filter (OD = 1.14", ID = .748") installed on the DC power cable of a wall wart power supply can help suppress the noise interference to nearby electronic devices by suppressing the fundamental switching frequency (usually under 1 MHz) and thereby suppressing harmonics at higher frequencies.. This kit includes 5 filters useful for all types of devices powered by wall wart power supplies including laptop computer, routers, cable modems, wireless phone charges, USB chargers, etc.