This is no doubt the easiest method of making a good quality mains filter suitable for any audio or visual use be it a HiFi, home studio or TV system. These also make great mains filters for computer based audio systems. I have made and used these on all my computer and audio based setups since they are better than the basic ones you buy for a computer and compare quite well to the cheaper dedicated HiFi ones.
Parts Needed :-
- Full height surface mount plastic mains pattress box (very common in any DIY store).
- Double mains socket (switched or un-switched).
- 1 or 2 varistors (250V to 275V AC operating voltage) (eBay).
- Small mains chassis filter (3A or 5A is usually enough) (eBay, RS, CPC).
- Either an inline IEC socket (eBay) or a length of mains flex with a mains plug fitted. (i.e use a lead with a molded mains plug and cut off the iec plug end).
- Rubber mains cable grommet.
- 4 x rubber feet or foam / felt pads (foam pads pound shop).
- Some mains cable for L-N-E (i.e. strip some old mains flex for the 3 wires needed).
- Solder and a few tools (optional basic mains polarity tester from any DIY store. These are fairly cheap).
Step 1 : Drill a hole in the side of the deep pattress box big enough to fit the mains cable grommet. I start with a 4mm hole and then use a step drill cutter to widen to 10mm or 12mm depending on the grommet size. (Picture 4)
Step 2 : If you are using an inline iec socket; solder 3 lengths of wire to the L-N-E terminals of the iec socket pins. You can use the screw terminals if you wish but solder is more secure so I remove the screws and solder directly. You may need to sand or file the zinc plating off the terminals. (Picture 2). If using a ready made mains cable with a mains plug already fiited then assuming you have already cut off the iec plug end, strip back the outer sleeving at the cut end to reveal the 3 L-N-E wires and skip to Step 5.
Step 3 : Twist the L and N cables tightly to make a twisted pair. Wind the E around the L-N pair in a longer twist. (Picture 3)
Step 4 : Pass the 3 twisted L-N-E cables through the grommet in the pattress box. The inlet grommet on the inline iec socket should fit over the grommet used on the pattress box. (Picture 3 and 4)
Step 5 : Solder the L-N and E of the mains wires to the terminals of the small chassis filter input side. You may need to sand or file the zinc plating off the terminals. You can use push on spades if you wish instead and solder the wires to the spade terminals. The earth terminal on the chassis filter is a common terminal for the input and output wires. (Picture 5)
Step 6 : Solder 1 varistor to the L-N input terminals of the chassis filter. (Picture 5) THIS IS THE RED VARISTOR IN MY BUILD.
Step 7 : (Optional) Solder a second varistor to the L-N output terminals of the chassis filter. (Picture 6) THIS IS THE YELLOW VARISTOR IN MY BUILD.
Step 8 : Twist a length of L and N wires tightly to make a twisted pair long enough to reach from the output terminals of the chassis filter to the double mains socket. (Picture 6)
Step 9 : Solder one end of the L-N pair to the output terminals of the chassis filter and screw the other ends into the terminals L-N of the double mains socket. (Picture 6)
Step 10 : Solder one end of a length of earth wire to the common earth terminal on the filter and screw the other end into the E terminal of the double mains socket. (Picture 6)
Step 11 : Screw the mains socket to the box. Stick some rubber, foam or felt feet to the base of the box. Add a yellow warning sticker if you wish. (eBay, RS, CPC, Maplin) (Picture 8) Optional : Add a serial number or date sticker to the base of the box or use a permanent marker pen to date it if you wish.
Step 12 : Test with a basic mains tester for missing or wrong wiring.