Gebalanceerd Netfilter

Some details on the components

F2 on the schematic is the temperature fuse that is integrated in the 8N330 transformer. It must be wired in series with the mains supply. Not all transformers have one, you can ignore it if your transformer hasn't got one.

The VR1 varistor S20K250 is suitable for 230V mains voltage only. If you have a different mains voltage, you have to use the appropriate varistor value. For example, for 240V mains, it's better to use the S20K275 to avoid accidental tripping.

L2 is a 68uH / 6.2A ferrite inductor from C&D Technologies / Murata, type 1468362C. I got it from Farnell, their order number is 1077059. If you use an alternative part then that's fine, as long as it has the same specs.

R1 and R2 are two large 5 ohm NTC resistors. They are required to limit the inrush current from the transformer and are also placed in series with the mains supply. I used the S237 type of 15mm and they work fine.

For C1 and C2, you can't use capacitors with a mounting screw. The can or housing needs to be completely isolated from ground. That's because these capacitors will carry mains voltage and the internal isolation is not calculated to carry such a high voltage. It's best to use electrolytics that are completely isolated. You'll have to use two additional clamps to mount them in the case.

Most of the X2 and Y2 capacitors are rated 300V nowadays, this if fine. Just make sure they they all can handle 240VAC and are all X2 and Y2 types, as listed in the schematic. The ones between ground and mains MUST be Y2. This is for safety reasons because they are directly connected to mains voltage.

Tweaking the output voltage

Beware that the output voltage can be a bit higher than the input voltage. That's because the transformer will not be fully loaded in most cases. It depends how much audio equipment you have connected. Let assume the output voltage of the filter is 238V under load, with 230V input voltage. This is a bit too high, so time for some tweaking!

You can lower the output voltage by putting a new winding on the transformer's toroidal core. This winding is then placed in series with the transformer's primairy winding.

Start with 10 turns of wire and place them around the core. Use 0.75mm2 wire with insulation that is suitable for mains voltages. Then connect this new winding in series with the transformer's primary winding. Measure the output voltage, without any load, just to be safe. Has the output become even higher? Then you must reverse the polarity of the new winding. The output voltage should now be lower. Then, measure again with the actual load and write down the value.

Then comes the math... Let's say that the voltage has now become 236V under load, so it's 2V lower with 10 turns on the winding. That means you need 40 turns in total to get 8V lower to reach 230V. Measure the length of the wire that is needed for the ten turns and multiply by 4.5x to get an idea of how much wire you need for the final winding.

The enclosure is from Retex I think, but i'm not sure... It's a steel instrument case with aluminium front and back panels. It measures 30 x 19 x 12cm. (width x depth x heigth).