Transformer hum…. Found the culprit

[Deltadel]

I used to have the same thing happen with my old Ion Nexus power amp, when my wife used her hairdryer ! The transformer used to hum very loudly when there was a heavy load on the mains.
I just paused the music until the humming stopped.
Not so bad now with my NAP250DR, thankfully !

[Duncs]

Same issue here Brendan but a bit more dramatic. System had been sounding rubbish for couple of days and we had a mains trip out on Saturday in the middle of the Dua Lipa concert at Glastonbury. Did the usual switching off all the usual culprits and managed to get most things back up. Tracked it down on Sunday to my Linn Ikemi Cd Player (don’t use it much as all CD’s ripped via Melco to hard Drive and played back through NDX2). Think I’ll try HungryHalibut’s approach and go for dedicated mains

[feeling_zen]

@robpearson

dc current in the AC

DC offset is not DC current on AC mains. It simply means an asymmetrical sine wave for an AC signal. The fact you can detect it (but not very accurately) with a multimeter set to DC is just misleading.

For example, a symmetrical wave might be +115 and -115v on every oscillation. A 5v offset might make that +120/-110v .

[robpearson]

thanks for the accurate information (too technical for me). But it is unwanted DC in the AC causing the hum, correct?

[feeling_zen]

The asymmetrical AC sinewave which is referred to as DC offset, yes. That is one of the causes of hum and from the device you identified, certainly the root cause in your case.

[GadgetMan]

Here is a simple example, although I understand there are a lot more complicated examples. I would imagine a hand held oscilloscope would show it, and if that’s true, I have wondered why a HiFi shop would not buy one, and use on their clients home setup in order to sell them more DC Offset gear. Better still, why cant someone like iFi develop a black box that measures it

[feeling_zen]

I’m not sure I’d trust that every HiFi shop would remember to use isolating probes and end up frying themselves.

I think a plugin wiring, voltage and ground tester would be a massive step though.

[GadgetMan]

I’ll take 5% for the idea, if you want to take that forward and develop the idea. We can then sell the patent to iFi, and but a Statement each

[feeling_zen]

I’m just simply not smart enough nor an electrical engineer. But there are electrical engineers with product design experience on the forum. I was thinking it would be good to add to those $15 plug in units that already test the wiring is correct, and measure voltage between all poles to make sure nothing runs over safety ground. Most of the time, hum won’t be DC offset, it will be something else that’s easy to test for. But an affordable all in one with that offset feature added… Yeah it would save me getting my scope out.