This high impedance circuit was found in 1967 documentation by Ferranti. The original operational amplifier is from that time. The circuit can be implemented with modern equivalents.
This power supply, found in an Argentine publication from the 1970s, uses a wire potentiometer of 25 W or more...
This circuit was obtained from an English magazine from 1979, but can be easily assembled today...
The oscillator shown in the figure generates a 1 kHz tone that is reproduced on the speaker with...