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.
Found in old documentation, this circuit makes use of amplifiers of the time, uncommon today. Next...
Although quite simple, the usefulness of this circuit is great. It can even mean gains for those...
This circuit converts inductances on two scales into voltages. The circuit was obtained from a...