Hey great responses - thanks.

Power supply is from common strip - have changed that, tried plugging in on another mains circuit - still get the loud hum.

I have tried changing leads, swapping units, changing locations, all sorts.

And whatever I do I get the same hum - and it always resolves when I depower the same monitor.

Eddie - I wish it simply were just some hidden AC pickup - but I get the same thing in any location. BTW what do you think of the speaker soundwise?

Key observations:

1 I dont get hum if I dont plug the TRS in.
2 I get hum if I plug the TRS into any input source.
3 No hum if I depower one of the units - always the same one.
4 I removed the earth/ground connection from mains lead on the suspect unit - hey presto - no hum - all works perfectly - except risk of getting fried if goes faulty.
5 When I first got the monitors, I didnt have this problem. Same cables, same set up. It just started out of the blue.

I dont have the luxury of a balanced OP from the laptop. Anyone know how to wire unblanced OP to balanced IP?

Thanks again

Tom