Just popped over to Diyaudio and noticed a DC mains blocker FS as well as "DC on the mains" in a thread and was perplexed. Apparently toroidal tfmr's don't like DC & hum as a result.
When i searched online I found this article -
https://sound-au.com/articles/xfmr-dc.htm which stats that a DC offset can be present of our supplys, this is a new one on me, rubbish from SMPSU's and motors I'd heard about but not DC.
According to the article some old hairdryers and heatguns can cause this, who knew? It's probably nothing new to most of the old hands but thought it might be new to some of us.
Andy.