This circuit controls the passage of the signal from one speaker to another. The potentiometers must be of wire 50 to 100 ohms with dissipation according to the power of the sound system.
Using an operational that is not common today, but which admits a modern equivalent, we found this...
This simple source of up to 1 A can supply voltages according to the secondary of the transformer, in this...
This interference rejection circuit for telegraphy was found in a 1980s documentation for radio...