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FOR HI FI SYSTEMS Are you irritated by clicks, pops, spruigles and sundry other noises in your hifi system? Chances are they are getting in via the mains wiring. Here’s an easy-to-build mains filter which should put an end to those annoyances. By Ross Tester W hat we have done here is taken a commerciallydesigned mains filter kit and modified it ever-soslightly to make it more acceptable – not only for Australian wiring rules and practices but also more acceptable as far as the end user is concerned. As well as making a few mechanical changes to the circuit (which we will cover shortly), we also added a four-way powerboard so that the filter goes between the mains plug and the four outlets. That way, all (or at least four!) devices could be plugged into clean mains. Incidentally, there is nothing to stop you using a six-way or even eight-way power board if you wish. But four-ways are now really cheap – often less than four bucks each at Woolies (and no doubt other stores!). 36  Silicon Chip The documentation and PC board both state a maximum loading of 2300W – this would be more likely 2400W in Australia with our 240V system. But if your hifi system draws anything like 2kW, please let me know before you turn it on so I can leave town! Where’s it from? The kit came to us from Leon Gross of the Soundlabs Group, a purveyor of high-end sound equipment. The actual design (and kit) is marketed by LC Audio Technology, of Denmark, as the “NETfilter”. On first inspection, we were quite impressed with the layout and quality of the kit. As our circuit (Fig.1) shows, it is an LC design with the chokes formed on toroids and www.siliconchip.com.au Fig.1: the circuit consists of two LC filters and a voltage-dependent resistor. All sections of the circuit are at 240V so take extreme care. pre-potted. We imagine that these chokes might contribute a significant portion of the overall kit cost. Similarly, X2-rated capacitors are used, giving a high margin of safety. The kit includes a cleverly-designed PC board which mates with a snap-together (“clamshell” type) plastic case. No mounting screws or bolts are needed to hold the board in the case – once the board is mounted in the case (it will only go one way due to various cut-outs on the board) the case is closed and fully encloses the bitey bits. Just a word of caution before we go any further: yes, this device operates directly from the 240V mains and could therefore be lethal if used outside of the case. Never work on a live board either outside the case or with the case not snapped closed. dissipate around 0.05W). Two 1nF capacitors, each rated at 3kV, connect between the Active and Neutral lines and the Earth line. These are responsible for getting rid of any high-frequency garbage which might be on the line. Following a second pair of inductors in series with the Active and Neutral lines, there is another 100nF X2 capacitor forming a second LC circuit, along with a voltage-dependent resistor (VDR – also known as a MOV) . The VDR offers a high resistance at the normal mains voltage (240V) but when the voltage rises significantly above this (as it could with a spike on the mains), its resistance drops, effectively shunting the spike away from the output. The circuit One area where we had to make changes to the circuit involves the earth lead. While the documentation which came with the kit went to great pains about not using the mains earth but instead running a separate earth lead to a “ground stake”, we The filter is a traditional LC type. The graph of Fig.2 shows the attenuation achieved (in dB) vs frequency. This was measured using a 75Ω input and 75Ω output HF generator and AC millivoltmeter. There are four inductors in the circuit, all 1.8mH. The inductors are supplied pre-wound and potted. A 100nF capacitor is connected across the Active/Neutral after the inductors. The “X2” after the value doesn’t mean there are two of them – it means that the capacitor is an X2-type. These have ratings sufficient enough to cope with this role. Immediately following the first capacitor and in parallel with it is a 1MΩ resistor. This is to bleed off any charge on the capacitors when not in use – to save you getting a not particularly dangerous but nevertheless annoying bite, if you touched the 3-pin plug Active and Neutral pins. This resistor is a 1W type – not because it must dissipate that amount of power but to get an acceptable voltage rating. (The resistor would in fact only The earth lead Fig.2: this graph shows the filter’s attenuation in dB vs frequency. www.siliconchip.com.au February 2003  37 The completed PC board, ready for insertion in the case. Here’s what the kit of parts looks like. This shot also gives a good idea how the PC board slots into the clamshell case. The arrows on the PC board go from mains in to mains out. Here’s how to cut and strip the mains cable between the plug and powerboard. Just where you cut it is up to you – you could even make it a longer cord if you wished. believe this would be illegal under Australian wiring rules. (Their rationale, by the way, is that the mains earth is often noisy in itself. This may or may not be true, depending mainly on how well the earth wire is actually earthed). But many overseas countries do not have the “multiple earth neutral” or MEN system used in Australia, where every building must have its own earth and the earth is bonded to the neutral at the switchboard. Unless a device is manufactured – and specifically labelled – as “double insulated, do not earth” an earth wire must be supplied to that device – hence the Australian 3-wire mains lead (Active, Neutral and Earth). Sometimes people get into trouble with earth loops, causing hum, because of this multiple earth system and cut the earth lead to one or more of their hifi devices. Sure it works – until something goes wrong and a tiny signal earth lead is required to carry fault current. It fails, and the device is live. This can kill (and indeed has done so in the past). Therefore, we made some mechanical changes to the circuit to ensure that the earth lead was not only used but stayed intact from input to output. problem. Despite trying several times, we could not get the mains cords (input and output) to grip properly with the anchorage and terminal blocks supplied. Our test was simple: two people pulled on the cords to see if they would separate. Unfortunately, they invariably did. Not happy, Jan! Fortunately, the answer was really simple: scrap the mains terminal blocks, drill out the holes they were supposed to sit in so they would accommodate the mains wires and solder them in. This also gave us more “meat” on the outer cable for the cord clamps to grip to. Cord anchorage This was the other area of concern – and it was a real What about the earth wire? On the input side, we drilled a new 2.5mm diameter hole in the space between the two now unused holes, right in the centre. We passed the earth wire (including insulation) right through this hole, then snaked it across the back of the board to the point marked “true earth connection”. We soldered the input earth wire to this point. Then we repeated this for the output side – exactly the same process, with the “output” earth wire also soldered to the “true earth connection” point. Now when we repeated our tug-of-war test, no problems. Even when tugging much harder than we would reasonably You need to make a couple of modifications to the PC board – drilling out four holes and drilling two new ones, to accommodate the different mains cord attachment method we use. This back-of-board diagram is approximately 1:1 scale. 38  Silicon Chip www.siliconchip.com.au expect, or applying short, sharp tugs, the mains wiring never moved. Happy, Jan! Construction We’ve already detailed the process for the mains lead connections – just follow the diagrams for the cable cuts and lengths. Where you make the cut on the cable depends on your circumstances – it can go nearer the 3-pin plug, nearer the 4-way powerboard, or in the middle. If you wish, you could even use a new piece of 3-wire mains cord with a 3-pin plug and extend the length of your powerboard/filter assembly to save using extension cords. The rest of the construction is pretty straightforward, especially if you refer to our photos and PC board overlay diagram. As usual, start with the low-profile components (resistors, 1nF capacitors) Don’t mix up the VDR with one of those capacitors. Next fit the two 100nF capacitors and finally the two potted chokes. The chokes are identical and symmetrical – they can go in either way and in either position. It’s impossible to get them wrong! Check out your soldering and component placement very carefully (even though it’s hard to make a mistake on this board, solder bridges could prove briefly very spectacular!). Now place the assembled board in the plastic case – it can go either way up but logically the components should face upwards. Make sure the three small notches on the edge of the board line up with the three pins in the case. Fold the top of the case over and squeeze very tightly – the case will snap closed – and that’s all there is to it. Note that once the case is closed, it is (deliberately) quite difficult to open up again. It can be done, but . . . Using it All of your hifi devices (amplifier, tuner, cassette, CD, DVD, etc) should be plugged into the filter, otherwise mains-borne noise may get in elsewhere. It may be even better to attempt a cure at the source of the noise rather than at the hifi end. In the past, we have used filters which made little or no difference at the amplifier end but stopped noise from a refrigerator completely when the ’fridge was operated via the filter. It’s basically a matter of experimenting for the best results. SC Where from, how much? The complete kit of parts, (minus the powerboard of course), is available from Soundlabs Group, PO Box 307, Surry Hills, NSW 2010, for $99 inc GST (not including p&p). Please note this is a special price, exclusively for SILICON CHIP readers – normal retail price is $125.00. You can order the kit direct from the Soundlabs website – www.soundlabsgroup.com.au; by phone – (02) 9660 1228; or by fax – (02) 9660 1778. Don’t forget to mention that you are a SILICON CHIP reader to get that special price! We have been advised that Soundlabs Group intend to modify future kits according to this article, so the PC board drilling may already be done for you. www.siliconchip.com.au Fig.3: the complete wiring diagram of the filter, from the beginning (the three-pin-plug) to the end (the power board). February 2003  39