This simple bridge circuit can be used for impedance or resistance measurement using a high impedance earphone as an indicator. The circuit is from an American 80's documentation.
The integrated circuit of this amplifier is not very common today. The output transistors can be...
This circuit is from a1970 documentation. R1 may have 10k and P1 47k. The transistors can be BC558...
This circuit is from a 1977 documentation. It is powered by the power supply of the circuit under...