A circuit for backup audio over POTS

Conversations around originating a webcast for conference, including cameras and mixers.
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A circuit for backup audio over POTS

Postby rmrichesjr » Wed Dec 24, 2014 6:40 pm

The attached schematic and this description are posted in case they might be useful for someone else. Recently, after a webcast communicator overheated, we tried using the crab boxes for backup audio. That failed, because the crab boxes have too much attenuation, even with the volume control set to max. This circuit goes between POTS (landline telephone) and line-level audio. The same circuit works in either direction. A pair of them worked pretty well going from a small CD player into POTS, then from another POTS line into the chapel PA system's line-level input. (Apologies for the poor graphical representation--it's the best I could do with a plain graphics editor. At least it's better than hand-drawn.)

When setting up a call, I recommend keeping the receiving side's SW2 off until the call is connected, both sides have SW1 turned on, and both telephone handsets have been hung up. At the end of the call, I recommend turning the receiving side's SW2 off before doing anything else to terminate the call.

Circuit details: I used a dual RJ-11 jack plate and a terminal barrier strip for the POTS jacks--the terminal strip in case there are bare wires to work with (broken RJ-11 plug, for example). The MOV protects the rest of the circuitry from any large/nasty spikes on the phone line. C1 blocks DC from driving the transformer core toward saturation. L1 and SW1 release the line to on-hook state or hold the line off-hook (with minimal attenuation of the audio). The L1 I used had a DC resistance close to 600 Ohms, which is the desired DC resistance to hold the line off-hook. If you have a choke with less DC resistance, add a series resistor. SW2 mutes the line-level side to avoid loud pops when switching things on the POTS side. The two zener diodes limit the amplitude of transients in case you forget to turn SW2 off when you should. R1 prevents switching transients when moving SW2.
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