We found this precision circuit in a manual from the 80's. The operational amplifier is of a common type at the time, and equivalents can be tested. Transistors are general purpose types.
This Siliconix circuit was found in a 1980s documentation. The coil consists of 18 turns of 22...
This circuit was found in an old Poptronics but is still current as transistors can be replaced...
This input stage has unit gain but a high input impedance and low output impedance. The circuit is...