This interesting circuit uses a sodion valve that we have already described in an article on the website. The circuit uses several batteries for power. The phone is of high impedance.
The load controlled by this circuit depends on the triac used. The circuit is from Electronics Now magazine...
This circuit was obtained from a Philips documentation from 1970. It can be built on modern...
We found this preamplifier with 70 dB gain for microphones in American documentation from the...