Any Audio Engineers? - no left channel

[Flying Canuck]

I built my audio system around a PMA8000B audio panel and have been having issues with my left channel being very faint (barely there) in both the pilot and copilot headsets. As the left channel is all using separate wires for both my sources and outputs, I presumed that something internal to the audio panel had failed. I replaced the audio panel recently with a known working same model, the problem persists.

I've attached the audio harness wiring diagram. Is it possible that a short or a dropped pin on one of the left channel inputs or outputs would cause the whole left channel to fade out? I can only think of the music jack and the headsets that would be unswitched, so probably one of those. Or a short anywhere else that might give the same outcomes.

I'm seriously considering rebuilding my entire harness (I do have a spare connector kit for the PMA) or even designing an entire new audio system around a DIY mixer (biggest benefit is that I would understand it). I'm prone to rabbit holes so I really should stay away from the latter, especially since I already own 2 audio panels that likely have nothing wrong with them.

I hope someone can spot something. I can live with this issue, but I'd really like it fixed.

 

[blaplante]

What headsets are you using?

Unsure if this applies....

"The pilot and copilot positions work with stereo or mono headsets. All passenger headsets are connected
in parallel. Therefore, if a monaural headset is plugged in to a PMA8000B Stereo installation, one channel will be shorted. "

 

[Flying Canuck]

What headsets are you using?

Unsure if this applies....

"The pilot and copilot positions work with stereo or mono headsets. All passenger headsets are connected
in parallel. Therefore, if a monaural headset is plugged in to a PMA8000B Stereo installation, one channel will be shorted. "

I've tried multiple. All of them stereo. I currently use KORE Aviation KA-1 headsets.

 

[blaplante]

Well.... per the wiring diagram all of the inputs from the COM radios etc. are MONO, and somehow it makes stereo output out of that. Does this panel support something like "3D audio" maybe not the right term... where COM1 sounds like it is on your left and COM2 sound like it is on your right? This helps with hearing two COM radios concurrently. But I'd imagine it would be odd if only listening to one radio.

Do you have the low audio problem when feeding a stereo source into the panel (via music input)?

Are the headset jacks insulated from the panel? Should be fiber/plastic washers there.

MAYBE... the headset jacks aren't actually STEREO.. There are 1/4" MONO w/Switch headset jacks that have 3 terminals... which could be mistaken for a stereo jack. See one here https://www.stewmac.com/electronics...nK1hzfcChFF1ojRnhD0Kt7pP_Js4faTMaAhkOEALw_wcB

 

[Flying Canuck]

Nothing like 3d audio that I can find in the manual.

Music input doesn't work at all. Not sure if it ever has.

I'm fairly sure the insulating washers are there, but haven't looked in a while.

You had me worried there for a second, but this is what is installed.

SWITCHCRAFT STEREO HEADPHONE JACK SWC-12B

This problem started some time after installation, it did work in both channels initially.

 

[blaplante]

Well now the puzzle deepens...

It DID work for a while, now doesn't, and you put a known good audio panel in, and it exhibits the same troubles.

Sure sounds like a short somewhere.

With everything off, headsets disconnected, measure resistance from left to common and from right to common. If no anomaly noted, try again with headset(s) plugged in.

I've seen cases where the headset jacks are so tight to other metal that they get rotated (as a result of plugging in/out) and while they didn't originally touch metal --- they do now.

 

[Avanza]

GND mixed up ?

Lower left hand corner of your diagram.
Connector M2F #1 (GND) goes to D44F #10 and 11. This is correct.
However is also seems connected to M4F #4 (music shield). Not correct ?

Good luck

 

[John Tierney]

Check all of your shield ground connections. I used solder sleeves and one of mine didn't melt completely and could be pulled out of the sleeve. With the daisy chained shield ground the loose connection caused me to have audio issues with my PM3000.

 

[Flying Canuck]

Lower left hand corner of your diagram.
Connector M2F #1 (GND) goes to D44F #10 and 11. This is correct.
However is also seems connected to M4F #4 (music shield). Not correct ?

Good luck

Possibly an issue. the PMA end of the shields all terminate to the metal PMA connector back shells. The ship ground in the bottom left also connects to the J2 back shell (but not J1). This might be wrong, it would definitely ground the shields for the phones, music and co-pilot mic circuits.

I'll dig into the manual and see if can explain that decision.

 

[Flying Canuck]

Check all of your shield ground connections. I used solder sleeves and one of mine didn't melt completely and could be pulled out of the sleeve. With the daisy chained shield ground the loose connection caused me to have audio issues with my PM3000.

Good idea, this would explain why it worked initially if a ground worked its way out. Also makes sense with me being able to temporarily resolve this issue last year with a reseat of the PMA (different thread)

 

[BobTurner]

Look very carefully at the harness plugs. Push the pins in gently, none should move. What you’re looking for are pins that have not snapped into place, and can slightly back out, not making contact.

 

[RV6_flyer]

Look very carefully at the harness plugs. Push the pins in gently, none should move. What you’re looking for are pins that have not snapped into place, and can slightly back out, not making contact.

In addition to the issue that Bob talks about, I connected a stereo music source (car radio) to the music input of an aircraft intercom and had issues. The music source would not work when the speaker output / headphone audio input was connected to ground. To get things working, I had to use a audio isolation transformer between the music source and intercom.

Only sharing this as something I ran across in the past and took some time troubleshooting to find the solution. I did not use this isolation transformer but it is similar in size to what I used. Not saying that this is your problem but want to point out that sometimes different equipment does not always play well together.

 

[Flying Canuck]

I had a look at the PMA8000 manual again, My ground connections are mostly correct. Here are a few statements from that manual.

The shields can be daisy-chained together, and then connected to a ground lug mounted on the corner of the back plate.

This is what I've done.

If a music jack instead of a music source is installed for Music 1 or 2, we recommend grounding the jack to airframe ground.

I haven't done this.

By way of diagram for J2 connector:
Pins 10 and 11 are connected to all shields, the ground lug and aircraft ground.
All shields should be grounded at audio panel only, other end remains floating.

This is what I've done.

Ultimately, apart from the music jack not being grounded, I think I've followed the guidance. I'm fairly sure there is a wiring fault of some kind that I'll need to track down. Thanks for the tips for where to look. Might be able to get to this for a while tomorrow.

 

[rapid_ascent]

Let me see if I can clarify your problem.

1) You have all mono sources with the exception of your music input.
2) You have stereo headsets.
3) Your audio panel should be providing the mono inputs to both ears.
4) The problem exists in both the pilot and co-pilot headsets.
5) Your stereo music input has never worked.

Your thought that it had to be the audio panel is a good one. On the input side you only have mono so you can't loose right or left at that point.

What about trying to disconnect the music input connector. If you wired this connector backwards you would have shorted out the left input by connecting it to the shield.

 

[blaplante]

What we keep forgetting is that the OP stated that the system DID WORK for a while. [as in Com output to L&R equally]

So a wiring mistake seems unlikely. How does a wiring mistake work, and then not work?

I really think a wire has rubbed through somewhere causing a short... or my suggestion above that the phone jack has rotated and is now touching something it shouldn't.

 

[RV6_flyer]

What we keep forgetting is that the OP stated that the system DID WORK for a while. [as in Com output to L&R equally]

So a wiring mistake seems unlikely. How does a wiring mistake work, and then not work?

I really think a wire has rubbed through somewhere causing a short... or my suggestion above that the phone jack has rotated and is now touching something it shouldn't.

I have had an INTERMITTENT alternator issue that took months to find. Like Bob Turner suggested above, I had a bad contact that sometimes made contact and sometimes did not. Have also seen a similar issue with a bad shield on a Garmin Can Bus that would come and go.

 

[BobTurner]

What we keep forgetting is that the OP stated that the system DID WORK for a while. [as in Com output to L&R equally]

So a wiring mistake seems unlikely. How does a wiring mistake work, and then not work?

I really think a wire has rubbed through somewhere causing a short... or my suggestion above that the phone jack has rotated and is now touching something it shouldn't.

I tend to agree, except: I cannot think of a single failure that would cause this issue in both pilot and copilot headphones simultaneously. Unless there is another issue - E.g. if both headsets were wired to the ‘passenger’ outputs.

BTW, if testing the rotating jack theory, be sure to look at the back side of the jack(s) with a plug inserted. The plug will cause the spring contact to move out (closer to whatever metal is nearby).

 

[Flying Canuck]

I typed this all up earlier but I must have been distracted before actually posting it. To easy to do right now - just got a new 3d printer.


I went out and tested everything today and it's not all that bad. I do have left channel audio, but it is quite faint. Same for both COM sources and both headsets (different headsets in different jacks). I didn't have enough skyview annunciations to say for sure, it's possible that it is coming though fine on the left. I did confirm before testing that the areas around the jacks are clear, with and without a plug in place, also all 5 jacks have the fiber washer installed.

I brought the appropriate cable to test the music input and it works wonderfully (stereo). It also responds properly to priority audio to the panel.

The intercom seems to work but that's hard to test on my own, I think it does have the low level in the left channel though.

I am left with an issue that is pretty minor, I do think the replacement audio panel is working better than my first one. I hadn't tested the music jack on the new one, but I know that it didn't work on the old one.

For now I'll just not worry about it. If I happen across the reason for the lower left channel level, then I'll address it.

Thanks for all of the input on my output issue.

 

[BobTurner]

The intercom seems to work but that's hard to test on my own, I think it does have the low level in the left channel though.

The intercom is easy to test. Put on the copilot headset. Rotate the mike back over your head. Pick up the pilot headset, place your lips close to the pilot mike, talk. How does it sound? Swap headsets and repeat.

 

[Flying Canuck]

One more set of tests. Unswitched (Skyview annunciations) is balanced stereo. Intercom is faint left channel for the co-pilot headset and faint but better for the pilot headset.

All in all it's quite acceptable.