rjquillin
Joined: 13 May 2007
Posts: 123
Location: KSEE
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 Posted: Sat Jul 14, 2007 5:23 am Post subject: Connecting a Garmin (Apollo) SL40 COM to a PS Engineering |
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At 20:28 7/13/2007, you wrote:
| Quote: | What I'd like is for some to confirm that they match up (are connected ) as follows:
Headphone - 14 to Com 1 Audio HI - Top -19
Audio Ground - 13 to Com 1 Audio LO - Top - 7
Mic 1 - 8 to Com Mic Audio HI - Bottom - 21
Mic Ground - 7 to Com 1 Audio LO - Bottom 9
TxKey - 4 to Com 1 PTT - Bottom - 22
Thanks for suffering thought these details with me. It seems that PS Engineering refers to the ground as "LO" consistently.
Jeff Davidson |
Should work fine Jeff, as you've indicated.
-Back in the days-, before all our new solid state electronics, many audio systems were truly 'balanced'; that is for increased noise rejections capability neither signal was ground referenced, but referenced to one another. A ground referenced shield covered the pair. Low signal level signals, microphone lines for example, are still, in professional systems, done this way. However, the higher level 'line' level signals, that used to be 500 or 600 ohm balanced lines, have for the most part become single-ended; with a high signal and a ground return completing the path. You will note most of our com radios have a ~500 ohm output and most intercoms and audio panels have their input impedance spec'd as ~500 ohms despite the fact generally in newer equipment neither really is. It's a carry over for compatibility with as MS would name, legacy equipment. But still a good idea when properly implemented and interconnected.
Problem here with 'single ended (not balanced) systems' is that the signal will "see" any stray currents and voltage drops through the resistance of the real world as an additional voltage. That additional voltage will appear as unwanted noise, or whatever, with the signal we desire; an especially bothersome issue with the low level signals our microphones provide. With the higher level line signals it's less a problem, but is just ignored by many manufacturers. PS, to their credit, didn't ignore the real or old world and provisioned their equipment with both hi and low inputs for those wishing to use shielded twisted pair cable to reduce noise in all interconnects. While their "lo" may still be very similar to ground, it is still provided and by following their IM one can end up with an audio system having greater immunity to externally generated noise; this by running the 'lo' along with the 'hi' and not using the airframe as the signal return.
It's very important to keep the microphone jack mounting bushing, and to a lessor degree the headphone as well, insulated from the airframe and rely on the LO wire to provide that connection. To not do so negates the advantage of what PS has provided us.
Ron Q.
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