Above schematic diagram is a small 325mW amplifier circuit based on LM386 with a voltage gain of 200 that can be used as a simple bench amplifier, signal tracer or used to amplify the output from personal radios, MP3 players, MP4 Players or iPods.
How this circuit works:
The gain of the LM386 can be controlled by the capacitor across pins 1 and 8. With the 10u cap shown above, voltage gain is 200, omitting this capacitor and the gain of the amplifier is 20.
The IC works from 4 to 12Volts DC, 12Volt being the maximum recommended value. Supply from USB port might be able to supply this bench amplifier circuit. The internal input impedance of the amplifier is 50K, this is shunted with a 22k log potentiometer so input impedance in this circuit will be lower at about 15k. The input is DC coupled so care must be taken not to amplify any DC from the preceeding circuit, otherwise the loudspeaker may be damaged. A coupling capacitor may included in series with the 22k control to prevent this from happening.
I am looking at building an extremely low frequency amplifier. The frequency range would be 1 hertz to 10 hertz. The input signal will range in amplitude of 0.5 mV to 5 mV. I would like to display a signal that is raised to 50 mV to view on an oscilloscope. Your page enlightens and is entertaining.