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.
This is the circuit of a digital clock published in the January 1975 issue of Radio Electronics magazine....
This output stage can directly excite a small speaker. The circuit is from a 1960 Mullard...
In the figure we have one of the many options found in this section for an intercom using ordinary...