This circuit is from American documentation from 1970. It can be designed with a BC558 and for the operational, we can use equivalents. The FET can be the BF245.
This filter rejects 1 kHz signals, being suggested by the manufacturer of the operational...
This is a simple variac type source found in the July 1962 Radio Electronics magazine. Voltage and...
The current of this source is 200 mA at the most, and the output depends on the zener that may...