Audio Hum

[johnsteichen]

After hooking up my audio outputs to my PMA4000 audio panel, I noticed annoying hum. I did the direct chassis ground hook up with no improvement. I used a dedicated unswitched input into the audio panel. Summing the audio with the audio from the Garmin GTN 750 reduced the total volume  just like Dynon said it would. Do not do that!
The good news, after futzing around a bit, I noticed that the hum is probably related to the voltage control on the Dynon screen. When the screen was on full bright, the hum disappeared completely. I can tolerate the hum the few times I need the screen dimmed. 

 

[dynonsupport]

John,

Do you have a metal panel or a non-metal panel?

When you say "I did the direct chassis ground hook up..", does that mean you connected the SkyView to the audio panel through a direct wire from the D37 Pin 30 to the ground on the audio panel?

The Installation Guide says: "To minimize noise, ensure that your SkyView [audio] ground and intercom or audio panel ground are directly connected together, even though they nominally share a common ground via other aircraft wiring (audio ground is pin 30 on the D37)." I added the extra [audio] in there to make it clear. You cannot depend on the main SkyView ground and the audio ground just sharing a common point. There should be a dedicated connection between the two.

 

[lindsayj]

I apparently have the same problem John has. I did the initial audio installation to pin number T on a PMA 6000 M with the ground from pin number 30 on the SV 37 pin connector going to the airframe and noticed the hum -- very mild but noticeable with the radio quiet and Zulu headset. As John stated, the hum was related to screen brightness both in volume and to a degree pitch. Almost silent at the extremes very bright and very dim. So today I put the ground wire on pin 1 (Ground lug) on the PMA 6000. No change in the hum. Which leads me to believe it's always been there but I hadn't noticed it before until I read John's post. So tomorrow, I'll put everything back the way it was, and let you know what I find. Reminds me of the screen flicker problem I brought up last summer (I think) whereby the intensity varied with screen brightness.
I must say the hum is very mild in background and will not be noticeable with the engine running. It's also masked by any activity on the radio or intercom. 
Otherwise the 3.3 update is a thing of beauty so far on the ground. Fly tomorrow hopefully. Thanks bunches.
Jack L

 

[preid]

JackL- I discovered a Hum after installing the 3.3 upgrade. I flew briefly to test the various changes 3.3 added, audio alerts, etc. hum is not all that obvious with the background engine noise. I did a short flight around the pattern and honestly can't say if that was enough so I'll go get out and do a longer flight and see if the issue remains. I can only think it has something to do with the audio alerts as it was not an issue at all with 3.2.

 

[lindsayj]

JackL- I discovered a Hum after installing the 3.3 upgrade. I flew briefly to test the various changes 3.3 added, audio alerts, etc. hum is not all that obvious with the background engine noise. I did a short flight around the pattern and honestly can't say if that was enough so I'll go get out and do a longer flight and see if the issue remains. I can only think it has something to do with the audio alerts as it was not an issue at all with 3.2.

Thanks for that. I flew last night for about an hour and could not detect the hum with the engine running and it had no effect on communication with ATC. After shutdown in the dark with a very dim screen, I could not detect the hum. I'll probably disconnect the 13/31 input to the audio panel just to see for sure if it's a 3.3 problem, but it's no longer a priority after flying with it. Alerts work great if you want them. Impressive. Thanks Dynon. Hope you can couple up to glideslope as well...soon.
Jack L.

 

[jc2da]

I confirmed that my hum does indeed vary with the screen brightness. Full brightness and the hum goes away. I should have tested on low brightness.

In flight, it is a non-issue. However, I did notice it while taxing after landing with engine at idle.

SV volume is set to 75%.

Again, not a big deal, but we certainly dont want the hum getting any louder.

Jae

 

[JohnH]

I have the same problem. I connected the SV audio output to a PS Engineering PM3000 intercom music input. The hum is directly related to the screen brightness. Only very faintly audible when turned up very bright. I double checked the grounding and I take the audio ground directly to the PM3000 ground. I was very careful with the installation not to create any ground loops with the shielded wires. The PM300 uses a floating ground at the headphone and mic jack ends.

This does not seem to be the case in all installations, but is there any fix for this?

 

[johnsteichen]

Same problem. Hum associated with dim screen . Not an inflight issue. I do not fly at night much, so my screen is usually on full bright.
Was careful with grounding issue.

 

[airguy]

I'm curious if you guys that hear the hum are using noise-cancelling headsets or standard passive headsets?

 

[dynonsupport]

You can prove to yourself that this is a ground issue easily. Grab a medium sized wire, say 16 gauge. Touch it to the case of SkyView and to the case of the intercom at the same time, and the hum will go away. Traditionally this is an issue in non-conductive panels, but can be an issue if paint also prevents any connection between the screen and the intercom.

JohnH, You really shouldn't use the music input on an intercom for the EFIS outputs. Music inputs mute when radio or intercom is active, which could cause you to miss critical alerts.

 

[JohnH]

Thanks for the information, I have more information since yesterday. The SV output volume is around 35 to 40%, so it seems like the solution is listed under the subject, "noise" on this forum.
I do see the point of not using the music inputs as they will mute, but I don't see where else to input the audio from the SV. There is a music 2 input on the PM3000, but that only outputs unmuted audio to the passenger headsets, of which there are none in a Kitfox?

 

[JohnH]

The auto-pilot also creates noise on the audio output when the auto-pilot servos are engaged. The dimming function on fully bright and fully dim seem to eliminate the almost all of the noise.
My next step is to check if the music input jack is causing the noise and I will try the idea of grounding the SV body to the Intercom body.

 

[dynonsupport]

Matters what PM3000 you have. The 4 place units have "unswitched audio 1" and "2". The 6 place units don't.

 

[JohnH]

By the way, the headsets are Lightspeed ZULUS.

 

[JohnH]

I tried removing the music input jack, no chnage, but when I connected the SV body to the intercom body, a large reduction in the noise.

I have the 6 place intercom, I thought it was the 4 place with unswitched audio inputs, does anyone know ifI parallel the Dynon audio out with the VHF audio out, if that would work?