Found in a 90s Elektor, this mixer makes use of integrated circuits of the time. The supply must be symmetrical at 15 V.
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...