I have had the pleasure of working with Mr Laurier Gendron of Burnaby in British Columbia, Canada to make this circuit. Please make sure you visit Lauriers web site, Handy Dandy Little Circuits.
In our doorphone circuit, an 8 ohm speaker is used both as a microphone and also an output device. The BC109C stage amplifies in common base mode, giving good voltage gain , while at the same time matching the low input impedance of the speaker. Self bias is used allowing for small variations in transistor tolerance.
An LM386 is used to boost voltage gain and drive an 8 ohm speaker. The 10k potentiometer acts as the volume control, and overall gain may be adjusted using the 5k preset.
A double pole double throw switch is used to enable talk and listen, (at the house end).