This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
This circuit is from an old publication from 1977 and can be implemented with more modern...
This basic buffer circuit can be implemented with other operational amplifiers. The circuit is...
This circuit can be applied to other 3-terminal regulators and equivalent transistors can be used....