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A new comms PSU query

Pete Alcock

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Hi B-R,


I've had to build a replacement PSU for a standard Canford/Tecpro intercom system after the PSU went missing. I've got the 24V OK, but it's the terminating impedance I can't get a unified view on.


Some say 4k7 Audio to Gnd, plus 220R in series with 10uF Audio to GND. (Bluecom project)

Others say 2k2 Audio to GND plus 220R in series with 1uF. (Another post on B-R

Others say just 2k2 audio to GND. (Crowley intercom)


Does anybody know what's used inside a genuine Tecpro PSU?


Thanks as always,



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According to the MS741 manual recommended termination is 200ohms in the audio band and 5K at D.C.


4k7 in parallel with (220R and 10uF in series) is a pretty good approximation using preferred values


Edit for spellcheck intervention!

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