Transistor audio mixer (CB8759E)

Found in a 90s Popular Electronics, this mixer can be expanded to more channels. Transistors can be modern general-purpose equivalents.

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Telegraph Practice Oscillator (CB6474E)
Found in a 1965 Radio Shack documentation this oscillator can be used with modern general purpose...
Voltage Frequency Converter with the 4047 (CB12736E)
In the figure we have a voltage / frequency converter circuit that can be used as a transducer interface...
Headphone Amplifier LM3900 (CB13929E)
This small stereo amplifier for headphones was found in a documentation on integrated circuits from...