It is often not acceptable or feasible to actually unplug household devices which inject a lot of AC noise onto the power line. Commercial EMI filters (normally intended to be installed in computers, etc) can be placed between the wall receptacle and AC cord of the noise-generating device and will operate just as well in reverse, i.e. to filter out much of the appliance self-generated AC noise before it reaches the house power circuits.

These commercial EMI filters typically employ symmetrical pi-type AC filter circuits, and are hermetically sealed in rectangular steel “cans” with external solder terminals. To be utilized in the suggested way, the EMI filter needs to be hard-wired to a wall plug for its “line” input and to a cord receptacle for its “load” output. Of course, the terminals must be well insulated after soldering. These units work well in this application, but are unsuitable to filter AC for most audio components due to use of bad-sounding RF chokes and capacitors.

Examples of devices which generate a lot of power line noise are high tech televisions containing switching power supplies which are constantly on even with the unit switched off, refrigerators, and surprisingly some built-in switching-type subwoofer amplifiers. The 2700-watt Sunfire subwoofer amp seems to generate an especially large amount of noise on power. We have found this to be a superb woofer even with the AC noise. Use of the suggested EMI filtering of self-generated AC power noise dramatically improves the sound of systems using this subwoofer.

There are a lot of brands of EMI filters, and they are all subtly different in their effect on a system. This is probably due to use of various inferior types of capacitors in their internal line filters. The caps in these filters are in parallel across the power line just like the suggested high quality capacitors. The following recommended type has been found to cause only a slight sonic degradation when plugged in with no load, and generally improves the sound to varying degrees when used in series with the power cords of AC noise-generating equipment.

Part Number: #562-864-03/3 (3a), #562-864-06/7 (6a), #562-864-10/11 (10a).
Source: Mouser (800) 346-6873