The circuit is placed parallel with the exit of power amplifier and us gives the level of signal ,from output. Changing resistance R1, in the input circuit, we adapt the indication of power, in the resistance of loudspeaker, that we use.
R1=10Kohm for 4ohm R3=390ohm D1-10=LED
R1=18ohm for 8ohm R4=2.7Kohm S1=mini switch
R2=10Kohm C1=2.2uF 25V IC1=LM3915