Sorry for snapping 
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I wish I had such a fine collection of Naim 500 series unitsā¦
Perhaps other 500 series unit owners might be able to share their experiencesā¦?
Hereās hopingā¦
The point I was trying to make here was that the fault lay with the electric blanket. Remove that and all was well.
Iām not sure how accurate these things are but I downloaded a sound meter app it measured the hum in our quiet room at 32 DB near and 26 DB in the seating position.
Thank you Richard, your point was taken, sorry for my slightly sarcy reply. My point is that I have no items on except the Naim items on their dedicated mains circuit, there is no interference at least from this household and itās non friendly items.
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No problem. I well understand the frustration.
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Hi @IanRobertM, that seems like a good idea.
I have checked the Electrical Certificate that was issued after the dedicated mains circuit was fitted but I cannot see anything regarding the level of incoming mains voltage. If I was to contact our energy supplier what would I say. Warning moderate level of sarcasm incoming:
Dear Sirs, I have hifi that exhibits some humming in a non musical manner, I believe this is an issue with your supply can you help
Have any other member contacted their energy supplier with any success?
Try plugging the noisest box into your non dedicated mains. If you can try somebody elseās mains as well that may help pinpoint the problem.
I have had hum in the past that switched on at 8am and off about 30 minutes later. 
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@0.0 - There are numerous previous Forum Posts about this. You ask your Supplier to check your incoming Voltage. Higher voltages can cause hum, I believe.
Yes, other Members have done this - and their Electricity Supplier lowered/corrected their incoming Voltage.
YMMV, as alwaysā¦
PS. Good suggestions from @Guinnless above. Tryā¦!
Hi @Guinnless, the hum is constant here phasing in and out as described above. Guess Iāll just have to play my music constantly too 
What is the level on the phone when everything is off? I measured with my phone due to the other, parallel thread on this. I hear a hum only when my ear is within 10 or 20 cm from the PSes, and there is no outside noise in the room, but the phone still shows 25 dB wherever in the room I am. (There are some appliances in the room that are part of life, like a fridge in the open kitchen, so itās never 100% quiet). So itās difficult to know how your values relate to an all-off baseline in your room and on your phone.
The fact that your hum pulsates and is even there with everything in the house switched off seems to point to something going on on the outside. Your supplier may or may not be able to help, asking does not cost anything.
And I would not worry about asking them regarding hi-fi. Their job is the supply of clean power within spec to electronic equipment, nothing odd about using such equipment. And if the equipment emits unusual noises, they may well be interested in case it does come from an issue on the grid. Or not.
You can ask your supplier to check that the voltage is within the tolerances specified.
Given the relatively small cost of an ATL dc blocker I would just buy one and try it. Your system is too good to not do so.
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Hi @Suedkiez, good question.
Although the room appears very quiet with everything switched off I just re-measured it at 25 DB, the same as you experienced yourself.
After taking the leads out the back of the power supply to check they were seated correctly and turning things back on the humming in the seated position is measuring 27 DB with the close to power supply reading 35 DB.
Subtracting the room noise makes the seated position 2 DB and the close to power supply 10 DB.
On the basis of your recent post about switching off other appliances, either your 552PS transformer has recently gone off (when did the humming first start to occur?) or the source is external to your home.
Asking your electricity supplier to measure the voltage because you are concerned that it may be too high is one path to follow. Another is to take a look on the Airlink Transformers website for a BPS7500MP. This will isolate any ādc offsetā on your dedicated radial. It also allows you to drop the voltage by circa 10V if this is necessary.
We have a BPS5000MP installed in our garage, just below the mains distribution box and it has been excellent. Our hi-fi sees a dc-free 230V on the dedicated radial and none of the Naim boxes hum anymore.
Best regards, BF
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Hi @Guinnless, I should have done that when I had things unplugged !
I may get the chance later today to take it around to my mother laws (
hippopotamus trotting images).
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Hi @Bluesfan, the Airlink Transformer (BPS7500MP) is definitely a possibility although space under our stairs where the fuse box resides is a bit sparse. If the power supply is well behaved at Iāll know I have to contact our energy supplier. If they canāt help itās a matter of shoehorning in the transformer.
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Re: Electricity Supplier - please re-read the initial article where it describes my experience of contacting the Electricity supply, and confirming I did have an over-voltage situation. I contacted them on the basis of my HiFi, and to be fare they never dismissed that. I was also able to measure the high voltage beforehand and could give them that detail. Also bear in mind that they had no idea what DC Offset was, so if you are experiencing that problem from something external to your house, you may struggle identifying it, and even if you could, what would you do. So in that case a DC blocker may be your best bet, but see if you can demo one first.
Regarding getting an electrician in to test your mains, in my experience having qualified as an electrician (only for hobby reasons) there is nothing in the courses on mains quality. I also asked a few other electricians, and none of them had a clue about DC Offset. You may find some electricians that are also electronics enthusiasts or Radio enthusiasts that may be able to help, but they may be rare to find. So I would say once you have tried and eliminated all the other areas suggested in the initial first post, then you are looking at some sort of mains conditioner which may or may not help.
Actually one other things - can you describe your dedicated mains? E.G what cable was used, and do you have a dedicated Consumer Unit?
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Hi @GadgetMan, there was a fairly substantial thread regarding this matter, one in which Iām sure you participated. I followed the details as set out in the thread and remember thanking @anon4489532 for his help.
We do have a dedicated supply here from a separate consumer unit to an un switched socket, more than that (10mm core, 50amp just numbers to me) I cannot recall, I did post picture.
Iāll try our power supply in a different house in an hour or so and if itās quiet the energy supplier will be getting a call tomorrow to see if they can check the incoming voltage. If itās still humming away on the supply in a different home itās possible that home also has issues or our power supply is just noisy. At that point I have the option of returning to Naim or getting the mains conditioner mentioned above by @Bluesfan.
I must say weāve tried exceptionally hard to make sure our Naim equipment has a good home. Dedicated mains, double Naim Fraim etc Iām not sure it should require yet more pampering.
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Sorry I hadnāt linked the threads up. Sounds like you have done the right things to date. The Electricity supplier should certainly be happy to come and confirm if you have Over Voltage, and they may even put a meter on your supply for a few days. They will also be able to see who else your sub is connected to, e.g. any local businesses that may be mechanically reliant on high loads. I have also heard of issues where a problem was caused by the heater of a swimming pool in a neighbours house.
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