The title could also be how amplifiers work.
What can be better than a U.S. Army training film?
Caution: Lethal voltages are normally involved. Take all safety precautions. Your own risk. For lower battery voltage operation see Pleiades bias and amplifiers. Even then all safety precautions should be exercised. For example a fuse in series with a battery etc.
So here is Electronics "Basic Amplifiers" 1963 U.S. Army training film:
https://m.youtube.com/watch?v=EAxB1yXPsxw
It is not to be found strange that electron tubes, vacuum tubes, or electronic valves (called in UK) are shown. They have produced some of the best sound recording on the planet. Such as When I Fall in Love - Nat King Cole. And they are still used in the best recording studios on earth as well as inside tye best microphones still prefered by top producers. An important reason why is the [Hamm] paper.
In the beginning of the film you will see a microphone membrane moving only from centre to the right. This is simplification . The actual movement is swinging between right left right etc passing through the equilibrium point.
For experimenting and making a brilliant low noise battery powered microphone booster amplifier external negative bias is not needed as the anode or plate voltage is low. In fact electron tubes are already negative biased when the cathode is at high temperature. It emits electrons and becomes positive with respect to grid. Or the grid negative wrt cathode. An external pull up resistor, the Pleiades bias resistor connected from anode to grid compensates for this. The bias is still negative but less so. The electrons are free to travel to the anode with a small anode voltage supplied by a battery. See Pleiades V6 schematic.
References:
Tubes vs Transistors, is there a audible difference? - Russel O. Hamm - Journal of Audio Engineering Society
The BBC research department report the Neumann U47 microphone http://downloads.bbc.co.uk/rd/pubs/reports/1954-23.pdf
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