My German friend was keen about building an inline filter for his audio system back in Germany, He had searched the net and found this page http://www.lcaudio.com/index.php?page=12. The site sells the kit for 64.48 Euros, about $105. The unit looked very cheap in a plastic housing (its one of those plastic things you buy a Bunnings for $6 to waterproof the connection between two mains plugs).
I thought I'd house my filter in a nice metal box (more RF shielding, and a better look). I bought Jaycar part no HB-5603 (111 x 60 x 54 mm) for $10.45. With a metal box it is important to earth the case for safety reasons - just screw an earth wire onto the case.
This was the circuit for the unit
X-Class capacitors are specially designed to be connected across the mains supply and Y-Class are built for connection between the supply lines and earth. The components MUST be rated as X and Y-Class for safety. The resistor at the end in the circuit discharges the X-Class capacitors when the unit is switched off. The VDR protects the equipment from voltage spikes. The exact values of the X-Class capacitors in this circuit are not critical but care must be used as not to use too high a values. There are safety limits on what can be connected across the mains and even more so between the lines to earth.
As you can see the value of the capacitors are far smaller than those in the shunt filter. Between line and neutral there is 0.1uF here and 10uF in the shunt filter. Between earth and active there is 0.001uF here and 0.1 in the shunt filter. Cleary the shunt filter has a role to plain in cleaning your mains.
The small components were easy to source
100nF X2 type capacitors were Dick Smith part no R 2628 @ $0.95 each
The coils were another story. These were 1.8 mH, 10 amp, 14mW, 250 volt suppression chokes made by EPCOS. (EPCOS part no B82725A2103-N1) link: http://www.epcos.com/. We could not find them anywhere! In Germany they could be had for aprox $6 - $7 each, but by the time you added postage to Australia they became very overpriced. In the end I rang EPCOS. (9560 5544) They were happy to do business with me if I was interested in buying lots of 100's or 1000's of units. Hmm! Somehow I managed to convince the nice man to send me some 'samples' to try out. He sent these at no cost - don't ask me what I said to him! So total cost for the in line filter came out to $15.81.
How did it sound? It cleaned up the grunge in the treble and created blacker spaces between the notes like the shunt filter, but its effect extended down through the midrange and into the bass. Deeper (and tighter) bass extension was the big surprise with this unit.
This is what the finished product looks like
Notice that the Shunt filter is there to absorb the crap that the DVD player is pumping back into the mains. Also you'll notice that one component is not going through the filter. That is my power amp. Why you might ask. The filter description on the LC audio page says this:
"A mains filter will always mean a little bit higher mains impedance. A common Class A/B amplifier with limited power supply capacity may get problems reproducing a firm stable bass, if a mains filter is added to the system. Therefore we don't recommend a mains filter in systems where a common Class A/B amplifier is used."
I also wonder if my 5 x 100 Watt Rotel power amp can get enough juice through those two 10 amp filter coils. There is some talk on the net of filters causing a loss in the dynamic character of the music. I have concerns about running all my components through the one little filter.
"Some filters designs do not use chokes claiming that they increase the mains impedance. Well that is correct; a choke does that, which is exactly what they are designed to do, but only at very high frequencies. At mains frequencies a choke will appear as a piece of copper wire with no attenuation at all. The choke will only come into effect at high frequencies, i.e. where the unwanted noise is. The higher the frequency then the greater the attenuation level. The idea is that the choke blocks all RFI noise but does not alter the AC sine wave at normal (low 50/60 Hz) frequencies and presents no load."
"However the choke size can be critical depending on what the filter will be feeding. If you are supplying a CD Player or DAC then a large value choke or multiple stages with chokes seems to work very well, where-as using this setup in a transistor power amplifier can 'flatten' the sound slightly. In the case of a power amplifier a much smaller value choke and series inductance with a larger current capability is a better choice."
This is interesting reading, maybe I have to build a custom mains filter into the power cord of every one of my audio components. Looks like that'll have to be my next project...
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