We found this circuit in a documentation from the 70s. The circuit uses amplifiers from the time, but it can be implemented with modern equivalents.
This old voice circuit suitable for telephones of the time was found in a Radio Electronics from...
This circuit was obtained from a Popular Electronics of the 90s. The SCR is of any type and the...
Other operational amplifiers of the same the series, such as the MC34072, MC34074, MC35071,...