This amplifier uses an integrated circuit from the 70s, which is no longer common on the market. The circuit was used at the time as a sound output stage for televisions.
This FET voltmeter is from an old 1974 amateur radio publication. The circuit can use MPF102 or...
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
The circuit below shows how to use an LM139 comparator or series equivalent as an amplifier. The...