Here’s a great success story that may help someone out with an intercom requirement and minimal funds. Recently, I have become the technical advisor for my son’s high school theater. First duties were to get the equipment fully functional. In the process, I realized how much we truly needed a technician intercom system for communication between the catwalks, backstage and the booth. Walkie-talkies simply were not desirable. How can you work a follow spot and still communicate with a hand-held walkie-talkie? I was fortunate to discover that the theater, built in 1978, had the makings of an old intercom system that was probably never used. There were a total of 7 intercom stations, three in the catwalks, one in the booth, one immediately outside the booth, and one in each of the wings backstage. Each intercom station consisted of a 1/4" TRS (Tip-Ring-Sleeve) jack (stereo headphones jack) for plugging in a headset. The stations were connected using a 2-wire shielded cable with the red lead connecting the tips and the black lead connecting the rings. After a little troubleshooting and some very basic repairs, I managed to get the stations solidly connected with no short circuits. I could loop back a signal from one end of the theater to the other proving continuity of the circuits. Now I had the basis of an intercom. After some research, I devised and implemented the following set-up: First things first, we needed a power supply to drive the voice signal. I acquired and connected a filtered, low-voltage DC power supply to a centrally-located intercom station. Positive went to the ring, negative went to tip. The power supply provided approximately 11 volts (with no load) at 70mA of clean DC power filtered through a 1000mF capacitor inside the power supply. (The power supply needs to be carefully selected to match the headsets and to provide clean power. Voltage anywhere from 6-14 v DC should be OK.) Filtered (minimal ripple), regulated power is the key. Without the filtering effect of the power supply's capacitor, the system would have had too much noise to be usable. Communications are full duplex utilizing the "party-line" style of simple telephone circuitry. Extension cords were run from the intercom station in the first catwalk to each of the follow spot operator positions. Headsets in use are Western Electric WE-52 vintage telephone operator's headsets fitted with a 1/4" TRS plug instead of the traditional 2-prong telephone switchboard plug. Most of the ones I have were manufactured back in the 1950's and 1960's and still work great. Just make sure they are wired correctly. They just don't build them like this anymore.... Interestingly enough, a traditional Western Electric (or AT&T) G3 telephone handset can also be used as a cheap communications device (I bought one on e-bay for 99 cents, you can probably find one at a yard-sale cheap) and is a good choice for backstage areas since these handsets are virtually indestructible. The coiled cord can be refitted with a TRS plug which will connect right into the intercom station. In order to make a traditional telephone handset fully compatible, the resistor on the earpiece speaker must be removed, otherwise users of the phone are heard clearly, but the volume in the earpiece for phone users is very low. So now we have a basic full-duplex party-line intercom system that works great. You can build your own cheap! The tough part is running the cable to the stations, but if you are lucky like we were, it may already be done, or can be done easily. Parts needed are a DC regulated filtered power supply, some headsets (refurbished WE-52 headsets are about $40, new ones are about $85), some TRS-jacks, 1000’ of shielded 2-wire cable (about $55), and misc TRS jacks, plugs and extension cords (nominal). Hope this info can help someone out. It was a lot of fun to build, and even more fun to use.