I am in the process of building a Pi based radio project. I have a 12v power supply for the amp and I was going to connect a DC-DC 3A Buck Converter Adjustable Step-Down Power Supply Module to that to power the Pi. However reading another person's construction notes I find the following note, "Do not try to tap off power from the Power supply or transformer used by the speaker’s amplifier. This won’t work (earth loops) and can cause damage to the PI and peripherals."
Can someone explain this or tell me how to avoid such problems?