Amplifier power noise

I had old semi-vintage stereo amp, Naim Nait 5i, for my speakers and it was powered by same extension strip as all this networking gear etc. It already had before some very minor background buzzing but when I plugged in the new Zyxel switch the buzzing got signficantly louder. It got to a point where I ended up also purchasing new modern amp. I chose Topping MINI300 because it seemed to have some positive reviews and I had good experience from my Topping D50iii DAC. I’m thinking that having DAC and amp from same manufacturer probably isn’t bad idea, at least there isn’t any major compatibility issues or anything like that. As side-effect the dac -> amp connection upgraded from single-ended to balanced connection. Not that I think it makes any difference here. I can confirm that the buzzing is now gone but it was annoying that I was kinda forced into this additional unplanned purchase.

Some pictures of the noise recorded with my phone:

Old amplifier without new switch plugged in:

Old amplifier after powering on the new switch:

New amplifier, also with switch powered on:

The new amplifier has clearly much better power supply filtering going on. It being class D instead of class AB might help too although I’m not expert on amplifier topologies so I’m not sure about that. What is notable is that the old amp had big chunky toroidal transformer power supply and the new one has just a separate SMPS DC power brick.

Not sure why in the last picture there is a spike at 21250 Hz, that might not even come from the speakers. I don’t think it is really audible though, some silver linings of old age I guess.

The lesson here is that power supply filtering is important. I don’t know if I should blame more on the Zyxel switch on spewing so much crap back to the power lines or the old amp not having sufficient input filtering. I’m tempted to say that both are to blame, the Zyxel switch behavior is certainly not ideal.