This is a simple audio amplifier with (when properly adjusted) has a pretty good gain to take a small signal to a crystal type earphone (about 10K ohms impedance) to an 8 ohm speaker at audible levels.
The amp in the photo is wired slightly differently than the drawn schematic, and I don't remember why I used the capacitor, unless to DC isolate the audio input fully from the battery, though I don't think it is absolutely necessary. I might have used it to shunt out crackly noise from the carbon contacts—I made few of these in different versions two or three years ago and do not remember.
The graphite leads are resting gently on the conductive part of the earphone (the metal diaphragm) and as the diaphragm vibrates with sound, the pressure variation on the graphite sticks varies the resistance in series with the battery and output transformer (and a larger power) in step with the audio. In this way it is somewhat similar to the old carbon granule transmitters (microphones) used for decades in telephones before transistors and such came along. The air pressure from your voice changed the resistance of the granules' contact with each other and controlled enough current to send your voice hundreds of miles down telephone wires without any further amplification. When early phone line amplification was used, early telephone repeaters used the same principle but were driven by an electromagnetic coil to vibrate the granules. Later on, vacuum tubes did a better job of it.
Anyway, don't expect high fidelity, but it can be fairly clear and quite listenable. Just thought someone might be interested. I like experimenting with homemade amplification devices!
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Rob Tracy is the operator of Rob's Radio-Active, LLC. Parts and vintage stereo, radio, audio, and test equipment sales and restoration services.