In the figure we have one of the many options found in this section for an intercom using ordinary phones not in use. This was obtained in a Popular Electronics from the 90s.
The Texas Instruments TPS7A92 integrated circuit consists of a low-voltage (LDO) linear voltage...
Found in a 1969 EDN, this high current circuit, not indicated in the documentation, uses...
This circuit was obtained in a 1991 Electronics Handbook. The publication no longer exists, but...