Mains line filter

An offshoot of my work on the STEPS circuit was that I started researching mains filters a bit. I kept it simple on the STEPS circuit, but decided to do a proper separate line filter PCB as well.

I’ve included something that is missing on the STEPS board, namely a differential-mode filter with an earth connection to serve as the midpoint. I’ve also put a fuse on this board as I often struggle a bit to find suitable space for a mains fuse in my builds – and obviously the fuse isn’t something that should ever be left out of a mains-powered circuit!

However, the STEPS wasn’t actually the only inspiration here: While looking for suitable common-mode chokes I discovered the Murata PLY10-series which is a hybrid containing a common-mode and a differential-mode choke winding in one part. This makes a more compact filter possible which obviously is an advantage (even if it has low-ish current capability and separate chokes are obviously more effective/efficient). The current capability of the PLY10 makes this filter suitable for preamps, headamps and similar circuits only though.

Pictures of the prototype below. To be honest, I am not sure if this is significantly better (or worse) than a normal filtered IEC socket, but it is at least a bit more versatile – and it was fun to make 🙂