Because I am ‘electronically challenged’ I prefer to hand them to someone who knows what they are doing. Fortunately I have been able to take the to Alan (Firebottle) over on AoS, who is close to me. The smart thing would be to take the notes above and let him spend some more time with them.
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Looks like it, at least on the 110/120V version (clearly a two-core mains lead). Of course they might not even be double insulated - just not earthed. I’ve seen quite a bit of Japanese kit like that, even these days.
I can’t tell whether this 220/240V one has a three-core lead or, indeed, whether that lead is even original
But if these are 220/240V (I can’t quite read the print) then they were two-core too
I suppose it might be possible to squeeze an audio ground lifter in and that that might help. But, as always with hum, it’s very difficult to predict what will work and what won’t. In the end it always comes down to trying it.
VB
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Not solved it yet, thinking of giving one of these a try? Seem to deal with mains related nasties, not sure if the upshot would be solving the ground loop/humming…
https://airlinktransformers.com/product/conditioning-balanced-power-supply-cbs1500
I have one of Sodder’s, that you are welcome to try.
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That’s very kind, of I am over your way in the next week or two will give you a shout
If Gregg can get it to Nick’s on Saturday, I can get it the rest of the way?
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More than happy to meet up somewhere if this is at all possible, much appreciated!
I will take it to Nick’s. Hopefully the taxi service will get it to you.
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Or I can get to nicks at some point, wherever nick is, thank you @catcando
Northampton
Should be, Gregg and I will both be at the bakeoff on Saturday.
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So the magic ground loop isolator arrived from @gthang, works a blooming treat. No hum at the speakers at all. Mystery solved, suppose in the long term I should have the grounding of the amps looked at, but for now I can’t detect any loss in quality of sound. Even with Lionel Richie.
Big thank you to @gthang, I will make the equivalent donation to the AA coffers if someone lets me know how.
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No need for the Sodder’s toy?
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Not really sure, is it a DC Blocker of sorts? I wouldn’t mind trying to see if it has the same effect, but no worries if you don’t fancy lugging it around.
If I’ve understood what’s happening correctly, the ground loop is being created by the amps, so they would need attention rather than the mains?
Hoping someone more knowledgeable like @sjs @Valvebloke may be able to clarify?
It will be there tomorrow.
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Result!
Can I have them back now? 
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Well, it’s a loop, so it’s ‘created’ by every part that’s in it. In principle you can break it anywhere. Graham’s ground-loop isolator breaks it in the interconnects which, effectively, join each amp back to mains ground via the source. The fact that you can also eliminate the hum by using an extension lead to a remote mains socket suggests that is also creating a break in the ground circuit. That would scare me. Mains sockets and extension leads are NOT meant to have breaks in their ground wiring.
VB
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Thank you @catcando, will help me understand what’s going on a bit better I’m sure.
Enjoy the bake off too!