This circuit usually used for amplifier that do not require additional device in one box amplifiers. Enhancements such as Equalizer , Tone Control , Mixer , etc. Only by requiring a potentiometer mono or stereo if stereo power amplifier , and few cables you can make this circuit easily.
See picture below :
First connect the cable from the power amplifier input to pin2 potentio , and connect ground to pin 1 , then connect the output of tuner or other media player to pin 3. Try turning to the right then the sound will be high , and if played left back then the sound will below.