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This is a simple low cost device that allows an external audio feed to be "injected" into a Techpro intercomm system.

This is a DIY project, so be sure to read our disclaimer.

Schematic

The big item here is obviously the amplifer module. I used an item from Maplin Electronics modular audio mixer kit, the Line Amp Module, kit part reference LK87U or ready assembled YM21X. The requirements are line level input, low impedence line level output and happy to run on 24V DC.

The output capacitor is a 1uF non-electrolytic.