This voice scrambler and descrambler circuit was found in a Radio Electronics from July 1963. Transformers are common types at the time with impedances according to telephones and line.
This headphone amplifier using a pickup coil can be used as an extension for devices such as TVs,...
This rapid response amplifier uses an integrated one that is not common today but can be...
This low noise microphone preamplifier circuit was found in American documentation from the 90s....