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  #1  
Old 03-28-2018, 09:05 AM
RobinHou RobinHou is offline
 
Join Date: Mar 2009
Location: Pasadena, California
Posts: 214
Default Head Phone vs. Low Level Audio

I am helping a friend wiring his very old Narco COM 120 to a SPA 400 intercom. The only documentation on the Narco is a pinout from AeroElectric.com

http://www.aeroelectric.com/Installa...120-Pinout.pdf

There are a few pins that are very confusing to me:
3 head phone
5 speaker
7 aux audio in
8 low level audio out

Which COM 120 audio output (3, 5 or 8) should I use as com audio input on the SPA 400?

I have seen installations when using a speaker output to drive the SPA 400 com audio input a resister was used in between. But why is there an inter connection between 7 & 8 with a resister?

If I use 8 as input to SPA 400, do I need the inter connection between 7&8 with a resister?

Thanks in advance for your help.
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  #2  
Old 03-28-2018, 10:30 AM
RobinHou RobinHou is offline
 
Join Date: Mar 2009
Location: Pasadena, California
Posts: 214
Default

I may have just found the answer to my own question:

https://www.sigtronics.com/_pdf/air_...m-120_Inst.pdf

However, I appreciate very much if anyone can explain what is the purpose of the interconnection between pins 7&8.
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  #3  
Old 03-28-2018, 12:37 PM
BobTurner BobTurner is offline
 
Join Date: Dec 2011
Location: Livermore, CA
Posts: 6,797
Default

Quote:
Originally Posted by RobinHou View Post
I may have just found the answer to my own question:

[url]http
However, I appreciate very much if anyone can explain what is the purpose of the interconnection between pins 7&8.
Just a guess: This looks like a post-production ?fix? to me. It may be that an unterminated line could cause feedback, etc. Note the note to remove the resistor if the line is connected to an audiopanel.
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