Recently I've stated getting hum from the transformer in my amp. It would come and go, sometimes loud, sometimes quiet and sometimes silent.

After a bit of googling I decided to try a DC blocker and came across a rather nice bare PCB from Sjostrom Audio. They also sell a populated version but it's hardly a difficult thing to build - 4 connectors, 2 diodes and 2 caps.

PCB duly ordered and parts purchased to let me house it in as small a plastic box as I could find (120mm x 95mm x 35mm). Took about an hour to make and it looks like it has done the trick, no more humming transformer!

Total price was around £35.