Audio amplifiers require a source of DC electricity for the operation. Usually, DC electricity is made in a power supply unit from AC electricity source, available from a wall outlet. That conversion from AC to DC generates electromagnetic fields as a by-product, and those fields can contaminate audio signal if signal amplification section is in close proximity to power supply, as found in common one box power amplifiers. A good way to reduce power supply interference is to keep DC power supply unit well away from the rest of power amplifier; indeed, that arrangement is found in some of the most sophisticated amplifier designs.
Another way to eliminate signal contamination by power supply is to use a DC source as the power supply. In such arrangement a battery is used to power the electronics. This method is quite good for preamplifiers, but very impractical for Class A power amplifiers or thermionic valve amplifiers because the batteries required to supply enough electricity would probably occupy the whole room.
Foundation # 8: DC power supply units must be located physically away from the signal processing electronics (amplifiers, etc), in order to minimize power supply interference.