These days we are using dozens of electronic devices with switching power supplies working at different frequencies and producing large spectrum of disturbing signals. Sometimes their internal filters are not effective enough to block this electrical noise, and therefore, notable EMI and RFI via power cords and other connections will occur — with consequences for  sensitive audio and measurement equipment. Because it is not so easy to “upgrade” such devices with better filters, an alternative is an external filter, which is described in this article.