One way to make an old broadcast receiver more useful, is to connect a bluetooth audio adapter to one of the radio’s audio inputs, e.g. the Tape input. This works well as long as the radio and the bluetooth are driven from separate power supplies. You may however want to eliminate the need for an extra mains adapter or battery pack.
When driving both the radio/amplifier and the bluetooth adapter from the same DC power source, the music will most likely be accompanied by annoying noises.
In this example, both the amplifier and the bluetooth adapter are powered from the same battery pack, while the 7805 drops the voltage to 5V.
The bluetooth adapter draws current in bursts/pulses, which causes noise on the power supply lines (both 5V and GND), even though these are bypassed by capacitors.
The input signal of the amplifier stage uses the common ground as reference, and when GND is noisy, the audio output gets noisy.
Powering the bluetooth adapter via an isolated DC-converter like the B0505S breaks up the ground loop, leaving the GND virtually noise-free. Now the amplifier delivers clear audio signal with minimal noise.
Retronics 
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