intercom power supply

[waynehoskins]

Termination for this system requires one 10K Ohm resistor across pins 1 and 3, AND in parallel with that: a 200 Ohm resistor in series with a 1uF capacitor.

Interesting. I'd always read or heard about just a single low-value resistor. But I can see the advantage to capacitor-coupling the thing. The high-impedance resistor provides constant dc-through-daylight loading of a rather modest value on the line, and the the capacitively-coupled low-value resistor loads at audio frequencies but not at DC, which I imagine improves the call circuit by not loading that nearly as heavily. Cool stuff.

 

[icewolf08]

Interesting. I'd always read or heard about just a single low-value resistor. But I can see the advantage to capacitor-coupling the thing. The high-impedance resistor provides constant dc-through-daylight loading of a rather modest value on the line, and the the capacitively-coupled low-value resistor loads at audio frequencies but not at DC, which I imagine improves the call circuit by not loading that nearly as heavily. Cool stuff.

You got it! I didn't spend a semester of school studying com systems with the man who wrote the book on "Sound Design for Theatre" for nothing! The big thing to remember is that you should only have one terminator in the line, so if your PSU has termination, you shouldn't put your own terminator on the line.