Old topic but 250-2 new transformer hum

I have been running a 250-2 for a few months- no sum from it at all. Two nights ago Mrs Bruss says what’s that and describes a hum loud enough for her to think an insect has got in to the room. I hear nothing but turn the music down and can then hear it. Mrs Bruss says it suddenly clicked in. I put tit down to a fridge in the other room and the door being open. Close room door and carry on. Last night after switching music off to go to bed I can now hear a loud hum and investigate. It is the 250 and is I guess a transformer hum.
This morning I have disconnected everything, turned off everything electrical in the house, and finally moved the 250 to a separate room and circuit - I have two circuits and dist boxes serving the house.

The hum is still there.

What could be causing a hum to suddenly develop?

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Have you eliminated all possible culprits within your house? Either by switching off individual appliances or whole circuits to narrow it down? Also, have you added any new network equipment to your LAN that might be causing this hum?
If you have eliminated all possible sources within your house, there’s the possibility that the issue lies outside. Maybe a nearby neighbour, or some new equipment on a farm or industrial unit, but of course that becomes more difficult to track down.

Bloody annoying TX hums. My cd5si is noisey. My Ndx2, connected to the same socket is silent.

In the house, yes. Everything off for testing. Also tested on a separate dist box ie a separate circuit. Power here comes in 3 phase and is split in to the two 3 phase feeds in to the dist. boxes and then balanced out to separate circuits. I guess it could be something downline but in a small hamlet I can’t see anyone having changed anything obvious. The power supply comes from a distance away so local conditions could have changed though. It just seemed strange how it suddenly appeared after a few months of no hum at all. I wondered if there was anything electronic inside associated with smoothing the transformer itself, or even a mechanical mounting that could be checked.

Separate but related.

Are DC blockers any good in a home environment, and is anyone using anything in particular with success against transformer hum?

If it’s been fine up until recently then it’s very likely something external that has started to cause the problem. Only trouble is, it could be from a neighbouring property. As it’s getting colder it could be something using a heating element (prime suspect for causing hum issues)…

Yes that could make sense. I’m just wondering how that works with 3 phase supply and my property being a long way from the nearest- probably about 1/4 mile away. Is it that once a hum is introduced somewhere on the line it then remains until filtered out?
Does a DC blocker do anything to filter that?

Thanks for any info.

Can do, yes. My system used to all start humming like crazy at certain times. Turned out it was the smithy using his mig welder about half a km away. More recently a neighbour’s cheap/faulty electric blanket (yes, we figured it out - they have replaced it!) caused hum from my NAP250DR and 552PS at the same time each evening.

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Thanks Richard, and Chris. Any views on DC blockers and amplifier hum?

I’ve never used one, so couldn’t comment.

I use an Isol-8 Powerline Axis DC Blocker. Eliminated my hum issues and improved fidelity.

Not cheap though.


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That’s good to hear.

I was looking at the MCRU Excelsior DC blocker.

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Come on Richard, confess, don’t blame it on the neighbours, it was your electric blanket wasn’t it ? We know all southerners are soft, wear fluffy slippers, have electric blankets and have Horlicks at bedtime.



Funnily enough I have a real aversion to electric blankets. In fact I don’t like my feet getting too hot, so I like the sheets to be cool even in winter.


Weird things can happen indeed, reminds me of this recent one, “Old TV caused village broadband outages for 18 months”, https://www.bbc.com/news/uk-wales-54239180

For me a new fridge reduced the hum significantly.

Keeping the cheese a bit fresher then?


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Much to Mrs Bruss’s distress I have this morning turned everything off, including the fridges and freezer. Not popular may be a slight understatement. lol

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I would second @GraemeH comments re Isol-8 Powerline Axis DC Blocker.

I installed this and it immediately stopped my 250 humming.
It was a relief not to sit in a quiet room (with system off) waiting for the humming to start.

Despite much investigation I could not identify source, the humming does become a bit like an ear worm, once heard hard to ignore !



I bought an MCRU Ultimate in line DC blocker to deal with a similar issue on a 250.2 and it worked for me