This circuit protects the speaker from dangerous direct currents that could damage it. The circuit is from an Electronics Experimenter Handbook 1993 edition.
Found in a 90's EDN, this circuit generates signals in the frequency range given by the graph. The...
The figure shows a circuit that shows how to control an optodiac to trigger a triac from the...
This circuit, from an American documentation from 1975, can be assembled with modern transistors such as...