Transformer Hum, how to reduce this

I think I am safe in saying the NC range use the same or similar transformers… :thinking:

Both my NAP 250s hummed, which you could hear from the listening position when playing music.

I fitted two IFI DC Blockers, which killed most of the hum. To hear it now you have to put your ear right next to the 250.

It didn’t appear to have any effect on the SQ which was a good bonus.


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Talking of root causes, I was doing a house inspection today and saw the worst of three worlds. Bloody awful. Several sockets wired with earth and neutral reversed. 20v flowing over all earth. A 20v DC offset. And 45% over voltage on a third of the sockets whilst a 25% under voltage on others.

This was a brand new proprty with a “licensed” electrician. I’d never seen anything like it.

If that is where the standards are going then it seems rather pointless to have a quality HiFi system. Maybe this should also part of the assessments recommended by dealers as a part of home demos and assessments. To be fair Naim does mention in their documentation that in some cases that they will not work, but this is in the case of RFI. Maybe this should be extended to poor household supplies as well

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To be honest, I was less concerned about how the mains would sound and more worried with it being unsafe and, given we now “know” about it, any fires wouldn’t be covered by insurance. Insurance is based largely on things being correct to the “best of your knowledge”.

A lot of the requirements for any decent hifi are nearly impossible to meet in rental accommodation or apartments. Not sure I’d spend money above a certain level without building a house and doing electrics from scratch. In fact I stand by that statement so much, when we sold our last home and moved into a rental flat while building a house, I put the big Naim system in storage and bought a new hifi at 1/7th the cost. Because it just seemed pointless to even bother with the Naim and the rubbish electrics.

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Yes with our renovation and new extension work, the basement and living room extension has new electrics plus the earthing is all new under the foundation. The extension has its own consumer unit and the village recently put in an isolation transformer for a few houses up the hill which includes us. My Naim boxes here (ok only a small transformer in the NAP110) are deadly silent. I did not do any measurements yet but I think the mains is very good here. In our appartment (drei Familien Haus) in Germany the Supercap hums quite a lot and it is a house from 1970s. Old consumer unit but a dedicated spur for the HiFi. I have the impression it sings on a warm day when everyone has gone out😂

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Now having heard the new 200 series I’m surprised at the level of transformer hum, I thought Naim would have tried to reduce it with the newer models.


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It is may be the electricity supply at fault, rather than Naim’s fault… (One day they might change to SMPS supplies, which would avoid the issue.

Wow, there are definitely some wiring monstrosities out there.

That sounds lethal without the fire risk. On the dc offset thing. I understood it to be a natural but normally frequently short and corrected occurrence as phases become unbalanced and dc is induced. What I don’t understand is how the unbalance can be perpetuated long enough to cause a period of hum?

Well thinks like hair dryers, boilers, aircon etc can leave an asymmetric leak back on the mains. And if you share mains with several other housholds it can leave an offset on the whole phase line coming into your house.

My issue aside, other the rural residences where villages have old appliances, persistent offset is pretty rare. Over and under voltage though, are quite common.

I’ve had some terse exchanges with the sparky in the last 24hrs.


I’m not sure that applies to UK.
UK has a very steady supply, also its been reduced in the last 2 years (as I understand) a national policy thats not been publicly admitted to reduce voltage.
My area used to be close to 250v, but I’ve moniored my house supply at between 238 to 240v over a long period.

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IMHO I think Naim should have introduced a much quieter power supply’s by now. Other manufacturers seem to do it. For example for son has a Macintosh front end with room correction and a pair of active ATC 150’s all plugged in to the normal ring main and dead quiet. He might be lucky but I don’t know.



I think it’s stable ish in towns and cities but over voltage in small villages in England and Wales is quite prevalent.

Also, unrelated to location, over voltage on a sunny day with solar panels is something all of my friends with panels contend with. Not clear how common that really is. 100% of several friends hitting 270v at midday is, admittedly, not statistically relevant.

That does not match my experience, I live in a small village .
As for solar panels, that a whole other story, nothing to do with the national supply.

My house averages around 253V, and thats mostly because it is close to the substation, and they need to also consider the voltage of the longest legs of the runs from that substation, and so will not reduce it any further.

I’ve certainly not noticed it go any lower recently, however what I didn’t appreciate until I spoke to an SP Energy recently is that lots of the substations and legs are inter connected and so it’s a balancing act that when you change one, it might have a much bigger effect that you might originally think. They are doing more work to have better monitoring of all their devices, so perhaps this will allow them to better fine tune.

That’s a but of a coincidence! Officially it’s supposed to be 230V, but the range of tolerance allowed is +10% to -6%, which just happens to put the upper limit at 253V.
If you are seeing this as an average, that presumably means that you are regularly getting more than that, which might be useful ammunition for you to use to pressure SP into doing something about it.

It’s not your proximity to a substation, but 253v is not right even tho it’s the absolute maximum for UK. I would check it against another voltmeter, and if it’s the same, raise a concern/question with your ‘lecky board.
My local substation is less than 100m walking distance, I suspect cable distance is the same.
I just checked and my voltage is showing 237 to 240v

Yes you’re right, in fact for 20 years it was around 260V because the substation was on the wrong tapping. Changed thanks to me (and slightly reduced my humming). It does creep up sometimes, but I suspect the tolerances are an average over a day, a bit like the frequency is.

They have put a meter on twice before, and haven’t found anything consistently too high. I suspect it makes sense to have the highest voltage nearest the substation, especially if it’s a long leg to the furthest house. Of course my measurements are not against a regularly calibrated multi-meter, so 1% out would mean 2.5V variation

Hi Mike, the only real test would be to use a calibrated device. Also depending on where you do the test in the house, as you know, you can get a voltage drop to sockets furthest from the CU. I tend to test quite near the consumer unit. I did a rough test against a calibrated meter a while back which showed it wasn’t far off.

It may well worth be taking some more measurements throughout the day to see what it gets to, but ultimately you need one of the full time Voltage Generating devices that track the whole day over short time periods.