I believe the switch mode power supplies in our DVD players are just as nasty as this fluoro and looking up filters on the net I found these things called shunt filters. They plug in to a socket adjacent to your HiFi (or your noisy equipment) and 'absorb' the rubbish that is fed back into the mains by the errant device. I was attracted to this shunt filter concept because you are not putting the thing in the circuit that feeds your system.

I found that the parts were readily available for a few cents at my local Jaycar/Dick Smith toy-shop. Why not build one and see what happens? This is the circuit I went with:

The 'absorbing' is done by capacitors connected between the live and neutral and the neutral and earth. The 100nF capacitors are X2 style metallised polypropylene 250VAC (Jaycar part no RG-5236) $0.95. The MOV is a gizmo that absorbs voltage spikes (to protect your equipment). I figured it was worth spending $1.50 on putting one of these in (Jaycar part no RN-3404). The big 8uF capacitor turned out to be a 'motor start' unit and broke the bank at $12.95 (Jaycar part no RU-6602) and the resistors were easy at $0.32 for a pair (Jaycar part no RR-2840). The job of the resistors is to discharge the capacitors when you unplug the unit (so you don't get a jolt when your put your fingers across the plugs) Finally I bought a 3 pin plug case to put them all in for $7.30 (Jaycar part no HB-5900). Total cost under $25.

I few minutes of soldering and I was ready to go. First I plugged it into a socket adjacent to my DVD player in my home cinema. This is a mid-fi system consisting of Rotel pre-processor and a Rotel 5 channel power amp fed by a top line Pioneer DVD player (with a fancy clock, special capacitors and other special bits) going out to DIY speakers containing VIFA drivers. The effect was immediate - stereo image became focussed like that of a proper system, and high frequencies lost their harshness, midtones (voices) became more life like and bass seems to go deeper. I quick unplug confirmed these changes. This is a great party trick, even non-hifi friends hear the immediate difference the magic box makes. Not bad for $25!

Next I took the magic box to my music room and plugged it in next to my real Hifi system (Quad electrostatics, Quad valve power amps, Audio Research pre-amp and modified Shanling CD player). The difference was there, not as marked as in the cheaper system, but certainly worthwhile.

Needless to say I have built another couple of these wonder boxes, the guys on the net seem to say that more boxes give more improvement and they plug them in next to all their noisy equipment (fridge, cordless phone, clock radio, computer, etc)

If you are not up to soldering a few companies make these shunt filters and you can buy a completed unit...

The Quietline™ Noise Reduction System http://www.audioprism.com/quietline.html you can read a review at http://www.soundstage.com/revequip/db04.htm, this review was done in Sept 1998 and the price of the filters was $200 USD per set of eight.

Enacom plug-in power line filter. Review at http://www.positive-feedback.com/ambackissues/enacom.htm and these were $84 each in 2002.

Here are some photos of my home brew unit:

Sadly, like all things I HiFi, this was just the beginning of the journey. I went on to build an inline power filter (which gave even more spectacular results for even fewer $$).

Tom Burke
(tomburke@netybay.com.au)