Strange Hum when powering on Sub?

[rna]

So I have a Dynaudio BM12S and I have no audio connections connected to it. When I turn it on I hear mains cycle hum on my main speakers. They are powered by the same socket. The sub is not near to any of my stereo connections or Bryston 4B amp. I did a quick test by lifting the earth from the mains plug which made no difference. I even wired a fancy IEC input with filter in line to the power input. But that did not help.
I am guessing the PSU in the BM12S has some common mode issues and polluting my mains?
Any ideas before I waste my money on something like this https://www.amazon.co.uk/iFi-SilentPower-GND-Defender-Intelligent/dp/B08XWJNS9T/ ?

 

[user 130681]

If you have mains hum with no inputs connected and not affected by any level/volume controls, there cannot be any other reason for it but a faulty BM12S.
You have already lifted the earth wire, so to purchase an over priced connector that is doing precisely the same thing, will change nothing but your pocket.

 

[rna]

If you have mains hum with no inputs connected and not affected by any level/volume controls, there cannot be any other reason for it but a faulty BM12S.
You have already lifted the earth wire, so to purchase an over priced connector that is doing precisely the same thing, will change nothing but your pocket.

Thanks for the sanity check. Yes I am sure the BM12S is faulty but it itself does not make any buzzing sounds, input or no input.
The really odd thing is I just set aside a day to do more investigations, turned on the studio and no buzzing at all.
Maybe it appears when the sub has been on longer. I will update this post if it comes back.
Crazy but I was looking forward to pulling the psu out and having a look.

 

[user 130681]

It is doubtful the psu will give you mains hum, if the main smoothing has failed, it will be twice mains frequency hum due to bridge rectifier rules and frequency doubling but, if it has a switch mode psu, it will be different again and only produce hum under load.

 

[sodderboy]

The true test is to have the inputs shorted- dead patched or L—R with a cable or pins2-3 with 100 ohms or less. Floating open is an invitation to possible emi. Hum with the inputs shorted and only a ac cable connected indicates the unit itself, or power as an outlier issue.
I just had a Gen 7071a with inputs shorted hum, may 2010 d/c (+1 for the recap column) but it was a loose interconnect inside. Dollop of heat glue, next!
Mike