This circuit protects the speaker from dangerous direct currents that could damage it. The circuit is from an Electronics Experimenter Handbook 1993 edition.
This preamp version uses BC109 transistors that are older versions of the current BC549 in plastic...
This circuit was found in a 1980s documentation, using a comparator from the time. The...
I found This circuit is in a documentation of transistors from 1958. It uses germanium PNP...