That’s not strictly accurate. DC offset is not DC on the AC mains. It’s an asymmetric sinewave like +60v against -50v
In addition hum has several causes of which DC offset is not only one but not the most common. Over voltage beyond the transformer’s tolerance is generally no.1. Which is why getting a DC blocker before diagnosis is a bit of a gamble.
You may be right about over voltage but it was not my experience.
My voltage was running between 248 to 253, so got a step down device installed which in itself works well (now between 223 and 228), however, the gear still hummed like a b’stard.
That said I do think my mains are particularly poor but getting a power conditioner that effectively tackled DC on mains/DC offset, vibration reduction and rebalanced AC sine wave whilst improving SQ was the real deal.
As with everything YMMV.
I have fought hum for years with different DC blockers in different systems. Puritan 156 finally made my boxes and sound relax. Silence! A humming transformer is a sad transformer not performing at its best. They get saturated.
I have never understood why it is necessary to adapt the electrical system of a house to an amplification. The same thing happened to me with an XS2 I busted. tried on various sockets and in various homes always the same problem. from that day I understood that if I want to buy a Naim I must first try it at home.
Blockquote
I wonder how much they would need cut the performance to get rid of the hum.
Is that really a thing? Is that an actual tradeoff? Only since I discovered Naim hum vs performance is mentioned as a tradeoff.
In any case I am not much wiser on what is the cause and what I could do. Hum while the decivces are off? That seems weird. I can understand hum when on but not when off. I mailed Naim support and am curious what they will say.
While my 222 and 250 are quiet(er) they do hum occasionally and fairly softly. They are improved but I believe fundamentally the design philosophy has not changed - and looks like for a good reason. The below has been a good succinct reference for me and something on filtering and DC blockers in there too. I’ve not tried DC blockers but it’s my own experience with filtering mains blocks for sure.
I dont know. But the only devices I have that had hum-problems with have been made by Naim so I guess they selected some product that produced the best sound in exchange for hum-reliability. I recently had an NVA amp which also used big toroidals and it was really silent. The Aavik I use right now use SMPS and some psychedelia from Ansuz (Tesla Coils and other stuff) and is more than silent.
I tried a DC blocker on my Atom HE but the hum remained at the same high level. I have had a number of products from Linn and Rotel with big transformers that have been dead quiet. But musically Naim is totally superior to them.
I’m not sure why so many describe this as a Naim specific issue. I’ve had many amplifiers from many different brands and most would hum at times. The only amplifiers I can think of that never hummed were McIntosh models simply because they use potted EI transformers instead of toroidal.
Same here.
Puritan PSM156…….nuff said 
I guess I have lucked out (Or maybe it’s less of an issues in North America) I have only had one humming transformer and that was an un-serviced 250.2. It was fine after being Re-capped/DR’d.
Sorry to hear that. If not solved, you will ask the 3 items to be replaced or you will take something else ?
Maybe you can try this famous Puritan 156 ?
The puritan is another 1800 eu. I feel like I should not need such gear for my amp to be quiet when off.
Can someone please explain the hum while off? The transformer is not active in standby. A loud hum seems really weird to me.
Maybe try a socket tester before spending any serious money on this problem? With a socket tester you can see whether there are any problems like DC etc
Well actually power is connected to the transformer but the bits after it downstream are not active. That’s the only way it can still hum.
A linear transformer will have a nominal loss of around 2% just being plugged in with no downstream load.
The older gear had a physical power switch broke the mains circuit before the PSU. I was certainly under the impression the the SMPS controlled a physical mains AC switch to the linear transformer but if it hums while off, it must be after.
There are for sure relays that disconnect most of the gear. No humb here but I did not have it either on older gear that is with afew exceptions over some days or specific time of days. I doubt that the transformers can humb in standby on new classic, but who knows.
Claus
Hum is very rarely any fault of the amp/power supply, but caused by mains issues, though some transformers are more prone than others. As others have said, humming, especially intermittent or variable in intensity, is commonly DC on the mains but it has also been said on the forum that mains overvoltage can also cause it. It and solutions have been discussed at length on the forum! And is NOT a problem confined to Naim, so changing for something different might not necessarily stop it and could even result in louder hum, though as torroidal transformers seem to be the type most prone to audible hum, there may be more chance of avoiding hum if the choice of replacement avoids them.
In my experience humming can vary a lot from unit to unit. I’ve had a fair number of Naim components in the house over the past few years and none of them hummed significantly except for one box: an XPS power supply. This could be heard throughout the lounge when no music was playing and in quiet sections of music. It was the only thing my long-suffering wife objected to about my HiFi obsession.
I think XPSs were a bit notorious for humming and I did wonder about replacing it with a 555 on my 272, but was unconvinced that would have cured the hum for sure. In the end the hum became a factor in my jumping ship in the main system.
I did consider having a dedicated mains supply installed, but I remembered the disruption when we had our house rewired not long after moving in and thought again. It rather smacked of the tail wagging the dog. And according to some experiences related here, it’s not a guarantee of correcting the problem.
So sympathies to @Steinwerck and @ChrisSU. I hope you get the issue resolved and soon.
Roger
I have had great success with the iFi DC Blocker. Was suggested getting a DC blocker in the FAQ of one of Naim’s competitor’s websites to solve transformer hum on their integrated amp.
CJ