Amplifier mains hum problem and computer noise

Hello, new here.
I had, for years, a Marantz tuner/amp which I used to play audio from my Dell laptop via 3.5mm stereo jack into Aux in
No issues, until the amplifier right channel recently suddenly started putting DC into the right speaker. I will repair this later.

In the meantime, I replaced the amp with a Technics unit. Using CD in and also tried Tape Playback in. I am using the same cable, same laptop, same power points, same location.

Immediately, there was heaps of noise present and I discovered this comes from the laptop power supply. I eventually solved it by running a wire from the one Input gnd ring to the amp chassis.
No idea why it was present as the setup is identical to the old amp. Neither amp is mains earthed and adding an earth does not fix the issue.

It now works with my wire fix but I can hear a slight mains humm. It is volume dependant so comes from the input. With input cable disconnecred, it is gone Again, it was not there on the Marantz amp.

How can I solve this?

I had no idea that a simple swap of amp would produce all these issues, just like that.

Best regards
Bernt

 

So what can be so different between 2 amplifier types that t=such problem can even exist?
I really thought the would use common design principles for grounding and input circuitry?