See also Schematic Diagram
External Communication Wiring
The circuit uses standard 3-pin microphone connectors and cable to interconnect the intercom boxes. Pin 1 is connected to the outside shield, and pins 2 and 3 are the inner pair of shielded conductors. In the intercom system, pin 1 is ground, pin 2 is the power supply, and pin 3 is the audio/signal circuit. "Belt packs" are normally fitted with parallel-connected male and female 3-pin XLR-type connectors so the intercom line can be extended, "daisy-chain" style, to many intercom boxes. The number of intercom circuits which can be connected in parallel is limited only by how beefy the power supply is, and the length (or actually line resistance/loss) of the cabling.
Audio and Signaling
The intercom audio signal and the call signal share the same wire. This is done by AC-coupling all the audio parts of the circuit to isolate them from whether the signal signal is present or not. The audio signal appears to be nominally around 250 to 500 mV. The Call signal puts the better part of the power voltage (a couple dozen volts) on the intercom bus. (Note that when the call function is activated, you can hear the DC signal voltage turning on and off as a "thump" in the audio signal.)
About 28 to 30 volts filtered DC is supplied on pin 3 of the line connector. D1 protects the intercom circuit from accidental voltage reversal (which would fry all the ICs and transistors!) R6 and C0 form a simple filter to remove any garbage that may have been collected in the cable on the way to the intercom circuit. This supplies the main power ("VCC") to all the circuits.
Because most of the intercom circuit is designed using IC op-amps, and since we have only a single-ended power supply, we must provide a "virtual ground" which is halfway between the VCC and ground. The VCC voltage is divided in half by R17 and R18 and is filtered with C9. This raw VCC/2 feeds the + input of the mic preamp IC (U1) because it is the most sensitive part of the circuit. The raw VCC/2 is sampled by U2A which is configured as a simple unity buffer which provides the main VCC/2 supply to the rest of the circuit.
U1 amplifies the microphone signal to a level high enough to apply to the intercom bus. There is some fancy frequency shaping done in the feedback circuit (C4, C8, C11, R9, R10, R11). A microphone mute feature is implemented by shorting out most of the feedback circuit (with SW1) thereby reducing the gain of the microphone amplifier essentially to 1 (which would effectively feed nothing to the intercom bus.) The mic preamp is AC-coupled to the intercom bus by C3 which isolates the circuit from the DC component of the call signal.
U2B accepts the intercom audio signal from the intercom bus, does some frequency shaping, and provides a self-nulling "duplex hybrid" functionality. The intercom bus signal is fed into the non-inverting ("+") input of the op-amp, and the local microphone signal is fed into the inverting ("-") input. By adjusting the level of the microphone signal with RV1, you can adjust how much of your own voice is subtracted from the intercom bus signal. This effectively adjusts the "Sidetone". This circuit is AC-coupled to the intercom bus by C15, C16, and C25 which prevent the DC call signal from screwing up the function of the circuit.
U3 forms the output circuit which drives the earphone of the intercom headset. U3A provides several db of voltage gain, and U3B is configured as a unity buffer to drive low-impedance earphone(s). Note that this amplifier could also drive a small speaker in a wall-box. But use an external amplifier unless you are in a quiet environment.
You can send a "call" signal by pressing SW2. This causes base current to flow through the PNP Darlington transistor Q1 to ground. When this happens, Q1 sends the filtered VCC voltage onto the intercom bus. This is the indication to all the intercom modules on the bus to activate their signal circuits. Note that the Call circuit is DC-coupled to the intercom bus, isolated only by the low-impedance R14. C1 and R2 form an R/C time constant which limits the Dv/Dt voltage rise on the intercom bus, and C7/R9 form a similar R/C time constant which limits the voltage decay on the bus. This is done to limit the magnitude of the "thump" which is heard by all the people on the bus as the DC "call" signal is applied and removed.
When a high DC voltage is detected on the intercom bus (i.e. when one of the people on the bus pushes their "call" button), the voltage raises to a significant fraction of the power supply voltage (depending on how many circuits are on the bus, and the resistance/loss of the wiring). In any case this is more than enough voltage to cause base current to flow into Q2. This causes Q2 to turn on and allow VCC to flow though the signal lamp LP1. R4 and C2 form a filter to keep the signal lamp from activating on brief noise spikes on the intercom bus.