Analog telephone speech interface

How it works

This circuit handles the audio conversation functions of an analog telephone. It is intended as a speech interface rather than a complete telephone with dialing and all line-management functions.

The archive suggests replacing an older telephone’s talking circuit. Compatibility depends on line impedance, bias current, protection, and the exact telephone arrangement; it should not be assumed universal.

On the bench

Use an isolated telephone-line simulator for experiments. Public telephone lines can carry significant ringing voltage and require approved isolation and protection interfaces.

Resources & provenance

Rewritten from the original circuits/tel/nu4p01x_tel001.php page. Component values and historical performance claims have not been independently bench-tested. The circuit figures have had their watermarks removed.