Audio HiFi Chassis Mains Filter DIY



An alternative to the full DIY mains filter build in the previous article is to use a standard chassis filter make by either Roxberg or Schaffner. These are off the shelf parts that come assembled in a metal box with input and output tags for L, N,E. They can be selected based on the current rating you require and have various choke sizes.

Choose a filter with a large inductance choke (10mH) for a source component filter like CD players, DAC, pre-amp or small integrated amplifiers for example. Use a smaller inductance (higher current rating such as 10A) for large dedicated power amp filtering. A mid range inductance (5mH) is fine for a computer based filter or more general audio use. The chassis filters can be bought from RS components, CPC and even on ebay occasionally.

You then need to supply a case and output sockets which can either be mounted in the same case as shown below in the commercial version. Alternatively use a 4 to 6 way trailing mains socket as the output from the chassis filter housed in its own smaller separate box. If you cut the lead of the trailing socket, say half way, you can then use the plug side as the input to the filter and the trailing socket side as the output using 2 rubber grommets for cable in and out so just 2 holes to drill in the box used to hold the chassis filter. The chassis filter can be glued in the box with a hot glue gun for example which is ideal or any strong resin based glue.

 

Varistors

You need to add at least one or more 240V varistors to the input of the chassis filter since they do not generally come with spike protection. They are for removing RFI noise after the spike protection. I usually add 2 in parallel at the input tags and maybe 1 at the output tags.

A varistor will not trigger until it reaches the clamping voltage which is always much higher than the RMS voltage and higher than the peak sine voltage it is designed to be used on. For example the 275V ones I use clamp at 710V. You should pick a varistor voltage based on the RMS rating so for the UK you want around a 240 varistor or somewhere close to this. A 240V varistor will clamp at 650V and mains peak about half this. This means it stops more spikes and not just those reach over 1000V but the lower voltage but still damaging ones.

** Attached PDF file contains the full build article which contains additional information **

 

Audio HiFi Mains Filter
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