Audio noise when Second HDX Added

[aitor11]

Hi!

I just finished adding a second HDX screen to my system. Back when I put together the harnesses I connected the audio outputs from both screens together and then into the Dynon Intercom/Audio Panel. I flew the airplane with only one screen for a couple of years with no issues, but now that I added the second screen I can hear a faint noise (some type of tac, tac, tac). If I turn off one of the screens off the noise goes away. I'm not sure where to look and I was wondering if anyone else has had the same type of issue in the past. I did not find anything searching the forums.

Thanks!

Aitor

 

[GKC Aviation]

 

[aitor11]

Thank you for your response!
I'm confused by the second highlighted part. The paragraph right above those notes says:

If there is more than one display on a SkyView network, you MUST connect all displays’ audio
outputs to the same audio panel input to ensure that audio is always heard. Electrically short
the respective left and right outputs together for stereo mode (i.e., left-to-left and right-to-
right) or connect all audio outputs together for mono mode. The same rule applies for audio
grounds

Does it mean that I still connect both screens together, correct? Just ground the screens themselves to the same ground point as the COM and Audio Panel. The audio will only come from one screen even though both are shorted together (my understanding is the it comes from the screen that powers on first)

Thanks agai

Aitor

 

[Rhino]

The connection between the displays GKC was referring to is chassis ground, not audio. Connecting the audio together from two screens without dual EMS modules can theoretically cause distortion, but normally doesn't, because, as you said, the first powered screen should be the only one to output audio. It's easy to test if that's a problem by temporarily disconnecting the audio from the secondary display. However, that problem doesn't occur unless both screens output the same audio at the same time. Your description indicates the problem is there all the time, so it's unlikely the screens are causing distortion by outputting audio at the same time. Try the ground wire suggestion GKC noted first.

 

[aitor11]

Ok, that is what I thought. Thank you! I’ll ground the screens and see how it goes.

Aitor

 

[Rhino]

You might also want to look at this:

Ground Loops and Noise in Aircraft Systems

Quite a few forum threads and articles have at various times addressed ground loops, and their potential to cause noise or other interference in your avionics system. Simply put, a ground loop occurs when different paths to ground are used. The difference in resistance, or impedance, on those...

forum.flydynon.com

 

[aitor11]

Thank you. That post is excellent. I read it twice yesterday and took some notes for things to check.

Unrelated to the dual screen noise. Since I installed the avionics I have had noise in the intercom (when pilot and copilot talk) that doesn’t seem to transmit through the COM and I don’t have when listening to radio transmissions. It is also related to RPMs (barely any noise on idle) It is not the normal alternator noise, and I’m wondering if it is coming from my mags. When I built the airplane I made sure that everything grounds on the same place, and it does. Avionics, lights, everything grounds to the same place.

The audio harness is the one dynon sells which grounds every shield together and then to pin one. The only thing that doesn’t ground back is one of the PTT switches on the control stick that I grounded straight to the fuselage.

Besides checking that ground I’m also thinking about the bundles of wires to and from the different avionics boxes that also run together with the audio cables. Something else to try to separate and check.

Thank you for your input, I really appreciate it. I’m doing my best to make this airplane the best it can be, and this is one of the hardest things to solve so far.

Aitor

 

[Rhino]

Your PTT switch is nothing but ground. Very little chance that is a noise source. Sounds like something isn't shielded very well, possibly the wires to the headset jacks. I've seen headsets cause that too, but it's very rare.

 

[aitor11]

Thank you! I’ve been suspicious of the helmet I wear since day one. I’ll give a try to borrowing another one and see if it makes a difference. I was looking at the wiring to the headset jacks, and I think I can separate them from the rest without too much trouble.

Aitor

 

[aitor11]

I grounded the screens per the recommendations, but nothing changed. Dynon support recommended to try an RF kit on the screens. I'm waiting on them to arrive and I'll report what happens. Regarding the noise on the intercom, I re-checked the grounds, separated the wires from the rest and there was no change. The noise is definitely not there on the COMs, just intercom (or even if I just talk to myself). I'm hoping to test a different helmet/Bose A20 this week.

Aitor