Main idea, we use a loudspeaker instead a Microphone. Knock on the front of the speaker, makes speaker cone to vibrate, causing a small electrical signal up. This signal was amplified and stronger and is converted to DC, to control the production of frequency circuit, and this frequency which it is mixed with the signal from the beginning, and then output to the amplifier.
As shown the circuit 1 is a complete circuit. When you tap the speaker causes the electrical signal, by the knocking. Through C1 and VR1, to the amplifier by IC1 pin 3. Which, VR1 controls the strength of the signal, called “sensitivity control”.
Although we will strong and light tapping. To be recognized by the control this button. For example, on tapping too hard Makes sound distortion, So reducing the sensitivity down. But if we gently tapped, a rare occurrence, we can rotate VR1 signaling through well.
The signal is amplified by IC1 to a DC voltage before. The D1 and C5 have the power to make more smooth. To control working in the frequency generators section next.
The some signal will pass IC2 that has VR3 is controller working of output signal from IC2, this signal will be controller the frequency generators at pin 9 of IC3, the sound caused the control of the VR3 will sound strange feeling.
This VR4 is controlled the signal at IC2, before send to IC3 for frequency generator to Tone treble or bass at frequencies IC3 made this (pin 4) will send the mixed signal from Q1 to Q2 first, then output at pin E of Q2 through C12 to the amplifier on, by Jack the RCA.