This inductance bridge uses an external audio signal. The circuit is from a Radio Electronics from January 1966. The instrument can be a common multimeter in the lowest current range.
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....