by gammaxy » Wed Apr 29, 2020 5:42 pm
You'll definitely need an amplifier since an intercom will require 3-5ish watts and the radio alone can't deliver that much power to a speaker.
You could probably connect the amplifier to the headphone wire. The input of an audio amplifier is normally relatively high impedance (like >10,000 ohm) so won't appreciably affect the signal to the headphone. Since it's available, though, I'd probably use the output intended for the recording device.
Any audio amplifier intended for that approximate power level is going to be able to drive 4, 8, etc., ohm speakers. Adding a 600 ohm resistor would be counterproductive--you'd end up with an intercom quieter than your headphones.