Historic Transistor Construction Project

Germanium Power Transistor Audio MiniAmp





The schematic for the updated amplifier is shown above.  Only the final two transistors are needed, since the input signal (from a CD player) is quite large and obviates the need for any preamp transistors.   A resistor voltage divider has been added at the input jack to convert stereo signals (non-existent in 1956) into mono for the single audio output.  Resistor values were hand-selected for this updated amp, and varied significantly depending on the individual transistors used.  An additional bias resistor (220 ohms) has been added at the base of the power transistor to limit output – otherwise the transistor tends to quickly overheat. Even with the added resistor, the heatsink of the 2N68 is warm to the touch.   The original 1956 amp used PNP transistors, and that is how the updated schematic has been drawn.  For the actual unit built in this project, NPN equivalent transistors were used, with appropriate reversing of the polarities (capacitors and battery).   A 2N35 transistor (NPN) was substituted for the CK722 (PNP) and a 2N95 (NPN) was substituted for the 2N68 (PNP).  These transistors are all dated from the mid 1950s and still work quite well.  Finally, an audio output jack has been added in parallel to the internal speaker.   This allows a modern user to connect earphones to the MiniAmp, and enjoy personal listening – tone quality is excellent.  For a modern germanium “surround sound” system, connect a pair of powered computer speakers to the MiniAmp output jack, and adjust volumes so that all three speakers are functioning. 



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