Audio Hi-Fi Mains Filter DIY

This is a full mains filter build that can be made with or without the chokes if you wish. The first picture shows the unit I made in a simple ABS case. This filter is a complete DIY solution where you make the circuit yourself, either on a PCB, vero-board or tag strip.

The sonic benefits of any mains filter is subjective at the best of times. Some people swear by them and others dislike them. I have always preferred my system with some kind of filtering and cannot see the purpose of using high grade audio mains leads without a filter circuit before them. The filter isolates the audio grade mains lead from the yards of twin and earth grey cable under the plaster and the miles of cable to the local sub station on your phase.

I find a filter really cleans up the sound and seems to remove a grittiness from the mid band and treble. The sound becomes smoother with more inner detail and space around the mix. You can choose to filter just your source components and pre-amp if you wish but try it with your power amp since in some systems it does sound better. I personally find Tube amps sound better when filtered but some higher power solid state amps may benefit from a choke-less filter build. A mains filter also protects your expensive equipment from spikes which can cause damage over time.


Advise on Building the Unit

X-Class capacitors are specially designed to be connected across the mains supply and Y-Class between the supply lines and earth. Y-Class types are double insulated. The resistor at the end in the circuit discharges the X-Class capacitors when the unit is switched off. The exact values of the capacitors in this circuit are not critical but care must be used as not to use too high values. There are safety limits on what can be connected across the mains and even more so between the lines and earth, otherwise the leakage can be too high.

This circuit has been built and fully tested by me and a few others with excellent results so you can go ahead and build this. It is also possible to wind the chokes yourself if you cannot find a supplier of ready made chokes or want to keep costs to bare minimum. Each winding on the choke should be in opposite directions to each other. That is the neutral winding runs the opposite way to the live winding. The live winding is made on one side of the choke and the neutral on the other side. The chokes should be around 1 to 2mH inductance. A medium to large ferrite ring can be used as the choke former and these are commonly available or salvaged out of old switch-mode power supplies. An inductance capable multi meter is useful here to measure the value of the choke if you are making your own. RS sell a ready made common mode choke (Part 188-9040) rated at 10A 2mH which is a very well made potted choke.

It is then a matter of mounting a mains sockets for an output into a box and feed normal 'dirty' mains in. If the case is metal then make sure it is earthed but ABS is easier to work with. My Black case was from Maplin. You can use dual tag strip to assemble the circuit which is what I did, reserving some central tags for the earth connections. The varistor's need to be rated at 250V AC RMS or slightly higher and ideal is to use more than one varistor in parallel at the input. Remember that most HiFi does not pull as much from the mains as people imagine so a choke that can only handle a few amps is sometimes enough. My old 300B mono block's pull 50W each and my CD Player 40W. A typical pre-amp will pull less than 40W so total is just under 200W. In the end you should have a filter that easily competes with the those on the commercial market at a fraction of the cost.


PCB Layout

I must thank Will B and Chris R for sending me this PCB layout they designed for this project and the completed pictures. Will has drawn out a PCB that can be used to build this project for those of you that prefer PCB builds instead of veroboard or tag strip. The tracks on the pcb are fairly wide but they can be thickened with solder or copper wire soldered along the tracks if it is likely to carry a high current. I think adding solder along the tracks is worth doing anyway for the little extra time involved. The layout show the PCB as if you are looking through from the top. If you have a printer, a laser type is ideal for this task, you can print this onto printable transparent film sheets. This film can then be used in a UV box to expose against UV sensitive PCB prior to etching.

Regarding the use of transparent film for UV based PCB etching, this is the film commonly used for feeding through laser printers or photo copiers which toner can adhere and fuse to. If you print 2 or more copies at a high toner density setting they can be over-laid on top of each other and taped together at the edges to increase the darkness of the tracks. The objective is to block UV reaching the PCB surface where you want tracks to form so the darker and denser the blacks on the film then the better. UV sensitive PCB or Pre-Sensitised Copper-Clad Board is available from RS, Farnell and Maplin along with the developer solution and etching fluid. There is far more prep work and cost involved when making a PCB but this method is ideal if you plan to make quite a few.


** Scaling the Filter PCB Image to 1:1 **

Once you save the PCB image to your computer you will need to print it at the correct scale before using it to make a real PCB. This may vary slighty between printer and the paint package you use to print it. The easiest way to get the correct scale is to print the PCB on plain A4 paper first and see if the component legs line up with the holes in the PCB without the need to bend them out or in too much to fit. If it is too big then scale the image down and re-try until you get the correct scale. In my setup I use Linux and a program called The Gimp for image processing and printing. I found that the image needs to be scaled down to 60% size. That is I printed the image at 60% and that was perfect. Any half decent paint program has a scale option and the ability to print out at different sizes. Gimp is also available for Windows users and is Open Source (Free).

The PCB files are in PDF and PNG format as an atttachment. I also have included this full article as a PDF for download and offline reference.


Caps and Chokes

The choosen value of the choke and caps is such to minimise any dynamic compresion that some complain of when using in-line mains filters with the power amplifier. The choke in particular is a high current low mH design and the low value X-Class caps operate on the higher noise band. You should hear a smoother sound, but hopefully no compression. You can  experiment with the X-Class cap size and increase them if you wish by adding more in parallel. I added a 0.1uF cap at the 3 stages, input, middle and output across L-N.

** Main circuit inside zip file download below **

Audio Hi-Fi Mains Filter
Audio Hi-Fi Mains Filter