Hum problem with Mini-I Pro 3

[John_2370]

Hi John.
Yes I downloaded the recordings.
A linear power supply composed by transformers has a specific input range, in theory, all toroidal transformers will have varying of humming noises during working, different transformer individules have different resonance points. But most of the time, it's not loud, with the input voltage increases, the noises also increases.
Your power voltage is obviously higher than the limit of the transformer input voltage range. (The limit of the transformer is 240V which is mentioned on our website and user manuals.) The transformer can work under this voltage, but may have a higher noise.
Hope you understand.

 Hi again, Wei Cao. I have tried the Mini i Pro 3 at work and the constant hum problem just is the same there. The other thing I have tried is using a step down convertor to reduce the socket voltage to 110 v, carefully resetting the voltage on the unit to the lower setting and then plugging it in. Again, there is the same hum problem. That strongly suggests it's not a voltage level issue but a problem with the transformer on this particular unit, because the lighter 110 load should eliminate any issues of my power supply. Is there anything else you suggest I try before returning the unit to the seller? Thak you again, John 

Here are the details of the step down convertor I used: 

https://www.amazon.co.uk/gp/product/B081...UTF8&psc=1 Yinleader 500VA 220V - 110V Step Up & Step Down Voltage Transformer Converter UK to US 110V & 220V outlets