Build a party-line Intercom

cbenjes

Member
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. Last edited: Tex Active Member This is just like the system used at the theatre where I was a resident company member for several years. It was decidedly low-tech, but worked great. Thanks for this. I love a good hack. I've been in a couple of theatres that had no com system, but did have unused aux speaker lines. This system would be great for that application. Studio Active Member Sounds like the system in my schools old theater except you could "call" (flash a light) on the other boxes. Les Well-Known Member cbenjes Member The intercom system in my old high school (circa 1974-1978) had indicator lamps on each of the intercom wall plates that were wired in series. Press a button on the wall plate and all of the indicator lights flashed, and you hoped the right person would pick up a headset to see what you wanted. This feature requires a separate power supply and cable that were totally separate from the voice circuit. For the system I just installed, I didn't have the luxury or ability to run a separate circuit and install indicator lights and push-buttons. Wish I did, but heck, I'll take what we have over crappy walkie-talkies ANYDAY. Studio Active Member The intercom system in my old high school (circa 1974-1978) had indicator lamps on each of the intercom wall plates that were wired in series. Press a button on the wall plate and all of the indicator lights flashed, and you hoped the right person would pick up a headset to see what you wanted. This feature requires a separate power supply and cable that were totally separate from the voice circuit. For the system I just installed, I didn't have the luxury or ability to run a separate circuit and install indicator lights and push-buttons. Wish I did, but heck, I'll take what we have over crappy walkie-talkies ANYDAY. Thats what I ment. Pics: erosing The Royal Renaissance Man Looks fun, good to have in the back of my head for emergencies/no funding. museav CBMod CB Mods Departed Member I moved this thread to the Sound forum since that is where most other Clear-Com and production communication system discussions live. A very creative solution to your situation and an interesting story, I just want people to realize that it may not be a practical approach for all applications. Did you consider adding push-to-talk at the headsets and handsets? Having all sets live all the time would seem to potentially make communication difficult and be undesirable in many applications. And not having volume control for each unit also seems a potential issue in many applications. cbenjes Member Thanks for the move. This topic fits better here. Believe it or not, we really haven't had problems with people talking over each other, but push-to-talk might be a good solution to eliminate chatty stage managers that don't think about moving the mic away from their mouth when they are screaming at someone NOT on the headset system. I did modify one of the telephone handsets over to push-to-talk because one particular intercom jack is very close to our ETC Sensor+ racks, and their cooling fans create a lot of background noise that gets picked up by an idle handset. It was simple enough to install a red push-button switch from Radio Shack on the handset and wire it to the mouthpiece. The trick is getting novice users to push (AND HOLD) the button to talk. Chris15 CBMod CB Mods Premium Member Departed Member Sounds like a solution to a problem has been found. However the use of TRS connectors for the connections is a cause of slight concern. As they are connected, a momentary short is created across the contacts. This will eventually kill the power supply and if someone if particularly slow about plugging in, it may kill it then and there... The risk will be lessened if a current limiting resistor is installed in series with the power supply connection... cbenjes Member Very true, you do get a distinct click when you plug-in and plug-out, so there may be a momentary short, but I think a good power supply will be able to compensate for it, though. The design for this intercom system came from my high school's theater. Their original intercom system was installed in the late 1960's and was used through the 2002 school year. Through those years, the system only ever had 1 power supply - a Bogen PS-1 power supply that was manufactured in the mid-1960's and was powered continuously for over 30 years. In 2002, the school moved to a wireless system and disabled the original wired intercom, but left the power supply unplugged in the catwalks until I picked it up in October 2009. I used it as a test power supply during the install phase of my current project. I now have that power supply in my shop and it still works great for testing and repairing headsets and handsets. They just don't build stuff like they used to. rervin Member Hi have a similar system and am trying to replace it with a newer version. Have you found any source for this party line type of system? I just can't see spending 3-5K for the off-the-shelf systems that usual supplier are trying to get me to purchase. Was wondering also is you have tried to update the headsets. We have lite headsets made by eartech from a wireless system that no longer works( never did work that well but dropping the master in a mop bucket during a show finished them off :-( ) We have tried to hook them up in place of the old style telephone headsets but seem to need an amplifier???? rob cbenjes Member Rervin, this is the only type of intercom system I am familiar with, so I really can't speak to any type of modern system. From doing research, I do know that if your theater is wired with 4-wire cable, you should be able to install some type of modern system that incorporates beltpacks, but you're still talking lots of dollars per station once you get the headsets, beltpacks, master stations, power supplies, etc. The beauty of this simple party-line set is that it is: simple - plug them in and they work. durable - I have headsets that are 50 years old that still work! cheap - headsets are$85 new at Audiosears , a power supply is \$35, and all you need is a 2-wire cable strung throughout the system terminated with TRS connectors.

Yeah, the old telephone operators headsets look like pictures from a Life magazine article from the 1960's, but hey, I don't have any budget to speak of, but my kids have a working intercom. Believe me, that's better then the cheap walkie-talkies they had.

I think only these headsets work on this type of system because of the make-up of the elements in the mic/ear piece, and that you can't use newer types of headsets without amplification. These are carbon elements and it's a design that's been around since Alexander called Watson and told him that he needed him quickly.

cbenjes

Member
Update: The school just acquired a real Clearcom headset/beltpack system. No more 1950's technology! So nice to have modern technology. I think I'll donate the WE52 headsets to the Smithsonian.

macsound

Well-Known Member
Dredging up an old thread, but thinking about the viability of making a wired telephone party line using regular phones and possibly headsets.
Is it as simple as just having a power supply and people can pickup phones anywhere and they would just have an open line?
Thanks

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TimMc

Well-Known Member
Yes, that was pretty common. A DC supply and some kind of twisted pair wire to each station. Wire the stations in parallel. Experiment with putting a pair of 300 ohm, 1 Watt resistors between the line and the power supply.
This is where we need Granny's crank magneto wall phone... {evil laugh}

FMEng

Well-Known Member
Fight Leukemia
This is where we need Granny's crank magneto wall phone... {evil laugh}
And like most things built by Western Electric, that phone would still work perfectly.