This is a simple and low-cost audio booster circuit design. The circuit is based on general purpose amplifier and switches, 2N3392 transistor packed in TO-92. This circuit should be installed before power amplifier module.
To get the maximum audio performance, use metalfilm resistors, MKM and tantalum capacitors which have small tolerance of its value. Also, use stabilized and regulated 9V power supply for better result. You can simply use LM7809 IC regulator on 12V power supply to get stabilized 9V output. The 100K variable resistor is used to adjust the volume, you may remove this component and use main volume in your audio system.