This little circuit was originally designed to have an electret microphone insert connected at the input to be amplified to a speaker; such as in a megaphone (or bullhorn). However, this can be used to amplify virtually anything by omitting R1, the 22k resistor. This will present DC to the "IN" pin on the PCB and is not a good idea to connect any other audio device to it without omitting R1.

The gain is set by R2 to around 50, the 1k resistor. By increasing the resistance, we reduce the gain; decrease the resistance, we increase the gain. If a shorting link was used in place of R2, then the gain would be 200. Refer to the datasheet for the LM386N for calculating the resistor value for your desired gain requirements.

The power supply requirements for this are 9 to 12V DC with a current capability of at least 300mA. Do not exceed 12V, as the capacitors chosen have a maximum working voltage of 16V. Exceeding the working voltage will cause capacitors to release the magic smoke, and the LM386N-1 to LM386N-3 may also be destroyed as the maximum supply voltage is 12V. Only the LM386N-4 can go to a supply voltage of 18V max, but the absolute maximum should be 12V to keep the IC happy. I've destroyed many LM386's over the years!

C3, the 10uF capacitor connected to pin 7 of U1, bypasses the supply lines and keeps U1 stable, preventing it from oscillating and howling (which can be quite common) at high gains. Ignore the 25-turn 100k pot in place of RV1 (10k LOG 9mm PCB-mount); I was using this for testing as I didn't have the correct pot at the time.

Video on assembly of the project: