This simple circuit was found in a Japanese magazine in 1967. The circuit is listening on a headset and the coil is 50 + 50 turns of wire 28 or thinner on a ferrite rod.
In this amplifier, the supply voltage is 35 V and the input resistance is 2k2. Equivalent...
This high speed laser diode driver was found in a 1980s documentation. Power FETs can be replaced...
This instrumentation amplifier with differential inputs was found in documentation from the 90s....