This is a combined circuit of tone generator and preamplifier. We found it in a 1975 documentation. Operational amplifiers of that time can be replaced by modern equivalents.
We found this circuit in a 1976 documentation. The FET can be BF245 or MPF102 and the power supply...
This circuit was obtained in a transistors manufacturer's manual from the 90s. The circuit uses an...
The instrument of this field strength meter, from a 1970s publication, can be a 200 uA...