Powerbank for clean side of digione signature

[Chriswal]

Hello

I got my digione signature yesterday. I tried to be smart and bought a power bank to power the clean side and used my Raspberry power supply for the dirty side. At first the green LED was shining bright and I heard some music for a few seconds. Then the shining of the LED got very weak and no music. Changing power supply to clean side and power bank to dirty side everything works. Any idea why? 

Thanks in advance

Kind regards

Chriswal