I have had a problem with humming Naim power supplies (supercap, 552, 300, 555 x2) for many years.
My solution was to put these behind a wall and under a staircase, so they were not in my music room.
This worked fine but it remained in the back of my mind to solve the root cause one day.
I always thought the problem was dc offset so I recently bought a ifi dc blocker which I found made no difference to the hum.
So I measured the mains ac voltage which I found to be 253V. I borrowed a friends oscilloscope and confirmed the voltage and that there was no dc offset and that the waveform was clean.
On investigation I found to my surprise that high voltages are not uncommon in the UK.
As a result I talked to Airlink transformers and they built me a balanced power supply which now feeds my hydra to the above naim kit. The result negligible hum so I’m very happy.
I would love to say there is an improvement in SQ but feel that needs more time to convince me this is not a placebo effect.
I hope that this may help others who suffer from humming Naim kit.
253 Volts is right on the upper limit which is allowable on a UK mains supply. If you can measure it at 254 even just for a moment your DNO would be obliged to adjust it down, which might bring further improvements.
We also had above 253 Volts, which was probably like that for 20 years, until I reported it a couple of years back. The local substation was set on the wrong tapping. For a short period they dropped it to 240 volts, and humming nearly fully disappeared. Unfortunately they couldn’t leave it like that because the house at the furthest point on the line would then be under voltage, so it’s now settled to around 248. I do get some humming at this rate
I recall Warren Stolmack (Naim’s original distributor for Australia) telling me that excessive voltage could be quite a common issue in Australia, especially in the remoter areas. He regularly measured voltage exceeding 250V, but aside from sometimes making the transformers buzz in the Naim amps and supplies, they otherwise dealt with it without any obvious issues.
Typically people tend to put the transformer near the consumer unit (at the beginning of their dedicated circuit), as DC Offset can cause them to hum. Is yours quiet, indicating no/minimal DC offset?
Although “DC offset” is used like a broken record, over voltage is by far a more common cause of transformer hum.
I’m not familiar with how the mains is supplied in Australia but in many countries, the power company can tune your voltage slightly up or down at the local transformer which is often on the utility pole on your road or a small substation cabinet at the end of the road.