The LM386 is an amplifier that is only powered on a battery.
Its output works with an 8 Ohm HP and can deliver under 9V, 700 mW.
The gain: output voltage / input voltage is set at 20.
To change this gain, a resistor must be connected in series with a capacitor between terminals 1 and 8.
A single capacitor between 1 and 8 allows a gain of 200.
