I saw you got a “high-dollar” answer over on Event Video Forum and that’s good, but-
I have built many “com” systems for just a few dollars.
In this system, there is no “SIGNALING RING” to alert anyone not already wearing a headset to put one on, but under normal circumstances, everyone in my crew just wears the headsets under “show” conditions.
This system is SO EASY that many will scoff… but it WORKS.
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Just buy wired headset TELEPHONES (any kind you like, ones with VOLUME controls are great).
Run phone cables to everywhere you need a headset (they “daisy-chain” perfectly.)
You are only using TWO WIRES (Red and Green) so you can get by with just about an kind of cable run.
Quick-and-dirty is to buy lots of 25′ and 50′ pre-made (with the “ends” on) phone cables, and lots of those little phone-line “splitters”… so that you can run as much or as little cable as you need, to as many locations as you need.
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The “trick” is the POWER SUPPLY:
You want between 12 – 24 volts DC (24v is better).
Connect the power supply to the Red (+) and Green (-) phone lines and turn on the phones.
NOTE: You MUST insert a “choke coil” between the AC-to-DC power supply (+) and the phone line.
Without the “choke” the “hum/ripple filter” built into the power supply will also filter out the audio on the phone line.
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Now consider THIS:
We have great luck going WIRELESS with this system, too.
Just buy cordless phones (with headsets) and hook them up in place of any of your regular phones in the system.
This is such a SIMPLE, inexpensive, and reliable system that I hardly ever unpack my CLEAR-COM sets anymore.
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If you want to just TRY this out (to see if I’m NUTS),
you CAN use three 9v batteries in series (21v) to power-up and see what you think.
(The 9v batteries won’t last a very long time under show conditions, they are just for a “test”.)