This circuit with two operating modes was found in the September 1989 issue of Radio Electronics magazine. Other operational amplifiers can be used. The power supply must be dual.
This is an old test circuit for integrated TTL counters, feeding a Nixie tube, common at the time....
This old oscillator using germanium transistors has a high impedance headphone as inductor to...
This preamp with several inputs using germanium transistors from the time was found in Japanese...