Found in an 1868 Mullard documentation this circuit has a dynamic range of 10 Hz to 25,000 Hz. Modern equivalent transistors can be used.
This circuit is from an American magazine that no longer exists, Modern Electronics, in a February...
Found in a 90s Popular Electronics, this circuit aims to find the voltage of a zener diode up to...
Found in a Popular Electronics from the 90s, this circuit is suitable for testing and tuning...