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.
Found in a Popular Electronics from the 90s, this circuit uses components that are still current....
This circuit uses an operational that is not currently common. We found it in an old American...
This circuit was obtained from an Electronics World of 1968. The transistor may be general purpose and...