Converting a monitor from 625 to 405

[Mark Hennessy]

...must get around to mounting some in boxes for this sort of job...

Some years ago I picked up a s/h ISB-060W medical isolation transformer for about 40 quid

Ah, but that's a mains isolation transformer. I'm OK for those. I was talking about signal isolation. I've got loads of really nice ones - all different types, but all full-bandwidth and designed for professional audio. The one I was thinking about has a bandwidth of many 100s of kHz, and can take huge levels. The isolation is tested to 4kV IIRC, but obviously I'd have to double-check that before deciding how to rate the finished thing.

I already have various audio transformers in boxes for studio/stage use, and have used them for this sort of thing in the past, but it's a lot of faffing around with adaptors to make the required connections. So I'd probably have BNCs on both sides, as this is for the bench. If I ever find the bench again

Of course, the modern way of dealing with all this is something like this: https://www.eevblog.com/product/hvp70/

But quite "spendy"...