On the other hand, the upper PCB is the "clean side" which focuses all critical and high sensitivity components. This circuit is isolated from the second PCB and Pi. It can (and must) be powered separately.
If the prospect of a double power supply for one interface can seem restrictive, the return on investment is considerable:
The "clean" power is extremely flexible. Since it only bear sensitive audio components, the technical specifications that it has to meet are less heterogeneous. It is much easier to achieve satisfactory operating conditions with this type of configuration. The circuit will operate without failures with voltages from 5 to 6V as long as it is provided with at least 100mA current. Whether a battery, a linear power supply or even a good switching power supply is up to you.