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Hi/Lo mic and phones question

gladiator68

Active Member
Sponsor
I am currently wiring my PM3000 intercom to an Icom A210 receiver. The intercom shows a "hi" and "low" for each of the mic and phones jacks. The A210 only shows one input location(not a Hi and Low).

Is there a method that I should wire these properly? If not I may punt and get a Garmin GTR 200B.

Thanks for any assistance!
 
Most usually consider the low a ground and the hi the + In that case. This is an effort to isolate both sides of the input for better control of the noise that may get into those audio circuits.
 
Just to clarify, have you got all headset and PTTs going into the PMA3000 first, or the headsets through the PMA3000 and the PTTs through the A210?

As per the A210 installation manual pin 4 on the molex connector would be connected to "lo" from the PMA3000.
 
Rick,

I can help with that. I'm familiar with that gear and can talk you through the hook up.

I'll PM you my phone #

Ken Wilkerson
 
You are dealing with differential inputs. The idea here is that the receiver of the audio signal ONLY sees the DIFFERENCE in the voltage between the high and the low connection. Any voltage that is COMMON is rejected.
We do this to reduce coupling common mode noise from one piece of equipment to another.
In principle, if we have a true differential audio input it does not matter which line does what - they would be equivalent. However in most cases we may share a connector pin between several inputs or our differential receivers have a limitation or we condition one of the inputs in some or other way so let's treat them as different.

The big point of all of this from a wiring view: Your receiver "lo" line is signal ground - the ground of the audio transmitting side - NOT your own ground.
So wire that to the audio signal ground of the other side unless the other side has a dedicated "lo" pin dedicated to its OUTPUT.

Do NOT make the mistake of wiring your receivers "lo" to a receiver "lo" on the other side.

If your transmitting side does not have any form of audio output or signal ground - use its power supply ground pin.

Typically in our aircraft audio systems we treat the "lo" as not having an actual audio signal but something that is audio ground "from the other end of the audio connection". In a way - it measured the noise there relative to its own ground so it can subtract it from the signal.

Rainier
CEO MGL Avionics
 
Thanks!

Thanks for all the great input.

Thanks too to Ken for talking me through this helping me understand a concept new to me!
 
You are dealing with differential inputs. The idea here is that the receiver of the audio signal ONLY sees the DIFFERENCE in the voltage between the high and the low connection. Any voltage that is COMMON is rejected.
We do this to reduce coupling common mode noise from one piece of equipment to another.
In principle, if we have a true differential audio input it does not matter which line does what - they would be equivalent. However in most cases we may share a connector pin between several inputs or our differential receivers have a limitation or we condition one of the inputs in some or other way so let's treat them as different.

The big point of all of this from a wiring view: Your receiver "lo" line is signal ground - the ground of the audio transmitting side - NOT your own ground.
So wire that to the audio signal ground of the other side unless the other side has a dedicated "lo" pin dedicated to its OUTPUT.

Do NOT make the mistake of wiring your receivers "lo" to a receiver "lo" on the other side.

If your transmitting side does not have any form of audio output or signal ground - use its power supply ground pin.

Typically in our aircraft audio systems we treat the "lo" as not having an actual audio signal but something that is audio ground "from the other end of the audio connection". In a way - it measured the noise there relative to its own ground so it can subtract it from the signal.

Rainier
CEO MGL Avionics

Rainier, thanks for the info about this.

Now I know WHY this is important, lots of folks know how to wire audio stuff, but no idea why it is done that way.
 
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