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796 Pirep report on audio noise

Brantel

Brantel

Well Known Member
Since I got the opportunity to fly behind my new 796 for ~ 7.5hrs last week, there is one issue that I would like to report....maybe two but I am not sure of the design intent on the second one.

The 796 when plugged into ships power produces some audio noise on the audio output leads. The sound it makes in the headsets sounds like the charging circuit for the battery. The noise is quite strong and you can hear it over the engine running. I use Lightspeed Zulu headsets.

I have my 796 wired to the music input of my PSE PM3000 intercom using the Garmin 796 flying lead cable. The cable bundles the audio leads in with the power leads and the audio leads are not separately shielded. This may be part of the problem since the audio lines are not shielded and run the full length of the cable laying right beside the power leads.

I have isolated the issue to the 796 since if I remove the GPS, the issue goes away. Also when the PM3000 mutes the music input, the noise is muted as well so I know it is coming in on the PM3000's music input.

I have ordered a ground loop isolator and will try that but I am not sure it will help this issue since the noise does not sound like ground loop noise.

The second possible issue is that the software selection for the audio output that selects either line out or headphone out does not appear to effect the flying lead audio outputs whatsoever. Not sure if it is suppose to or not. It may be that that switch only controls the cable with the built in 3.5mm stereo jack and not the flying lead cable? I will ask Garmin about that.

Neither of these issues are bad enough to change my opinion of the 796. It is one awesome piece of technology! I am amazed at how far technology for pilots has come in just a few years time!
 
Same problem

Brantel said:
Since I got the opportunity to fly behind my new 796 for ~ 7.5hrs last week, there is one issue that I would like to report....maybe two but I am not sure of the design intent on the second one.

The 796 when plugged into ships power produces some audio noise on the audio output leads. The sound it makes in the headsets sounds like the charging circuit for the battery. The noise is quite strong and you can hear it over the engine running. I use Lightspeed Zulu headsets.

I have my 796 wired to the music input of my PSE PM3000 intercom using the Garmin 796 flying lead cable. The cable bundles the audio leads in with the power leads and the audio leads are not separately shielded. This may be part of the problem since the audio lines are not shielded and run the full length of the cable laying right beside the power leads.

I have isolated the issue to the 796 since if I remove the GPS, the issue goes away. Also when the PM3000 mutes the music input, the noise is muted as well so I know it is coming in on the PM3000's music input.

I have ordered a ground loop isolator and will try that but I am not sure it will help this issue since the noise does not sound like ground loop noise.

The second possible issue is that the software selection for the audio output that selects either line out or headphone out does not appear to effect the flying lead audio outputs whatsoever. Not sure if it is suppose to or not. It may be that that switch only controls the cable with the built in 3.5mm stereo jack and not the flying lead cable? I will ask Garmin about that.

Neither of these issues are bad enough to change my opinion of the 796. It is one awesome piece of technology! I am amazed at how far technology for pilots has come in just a few years time!
Click to expand...

I'm having the same problem but i'm not going through anything but right into the Bose headset jack. I have tried this isolator thttp://www.hifisoundconnection.com/Peripheral-PGLI35-3.5mm-Ground-Loop-Isolator-PGLI35-SNI-1-3.5 that does take care of all noise but also makes the music sound like s****
 
RS232 lines are +-12V and a resistor will properly imedance match the input and output. If you dont believe me disconnect your blue data wire and see what happens to your noise...:)
 
Brantel said:
I have my 796 wired to the music input of my PSE PM3000 intercom using the Garmin 796 flying lead cable. The cable bundles the audio leads in with the power leads and the audio leads are not separately shielded. This may be part of the problem since the audio lines are not shielded and run the full length of the cable laying right beside the power leads.

The second possible issue is that the software selection for the audio output that selects either line out or headphone out does not appear to effect the flying lead audio outputs whatsoever. Not sure if it is suppose to or not. It may be that that switch only controls the cable with the built in 3.5mm stereo jack and not the flying lead cable? I will ask Garmin about that.
Click to expand...

On the 696 they eliminated the audio leads in the accessory cable, on the 496 they were desigated for "speaker" use and should not be used for headphones, the 796 menu selections probably do not change the audio on these "speaker" wires which would be similar to how the 496 worked.

I would try using the 3.5mm audio out plug and see how it works.

edit: Brantel: thanks for the clarification, did not know the 796 was such a different animal!
 
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There are not any speaker level outputs on the Aera 500 series or the 796.

This is not the same as the 496. The 296-396-496 had a cable that had an actual speaker in it that worked in the auto mode for giving turn directions. It did not do anything in the aviation mode.

The Aera 500 series Bare wire cable has the same wires as the 796 has. They are intended to be wired into an audio panel or intercom. There is no 3.5mm jack on the Aera 500 series. There is in the yoke mount cable but not the bare lead cable.

There is also no 3.5mm audio jack on the 796. There is one on the yoke mount cable but not on the bare lead cable.

The 796 has little in common with the 696. The connector and cables are more like the Aera 500 series on steroids.
 
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I wired my audio wires to a 3.5mm (1/8) stereo jack. My music/audio
to my intercom is thru a 3.5mm jack. Before installing the 796 in my panel
i used the aviation harness (with the DC acc power plug with the
stereo jack) and had no issues listening to XM or the audio output of the 796.
However after the panel install and using the bare wire harness with the audio
wires to a 3.5 mm jack, i had the same issues you are. A groung loop filter from
Crutchfield, inline from the 796 to the intercom audio input, completely eliminated the noise.

http://www.crutchfield.com/p_127SNI135/PAC-SNI-1-3-5-Noise-Filter.html
 
Tom,

Good news because that is the same unit I bought off Amazon. It will be here in a few days.

Thanks!
 
No issues with noise and very clear audio from my panel mounted 795 (no XM in Australia). When I installed the unit I cut a fair bit of length off the bare wire cable harness and maybe this helped reduce any possible interference occurring from the power to the audio cables in the harness??

Fin
9A
 
As RocketBob mentioned in a previous post, I have my RS 232 connected
to both my EFIS and autopilot with no resistor to dampen the noise. If i would
have know this, i might have tried his fix. When using the Aviation harness,
the RS 232 from the 796 wasn't connected to anything (only available from
the bare wire harness). But when using the bare wire harness with the RS 232
from the 796 connected, there was audio noise.
 
We can test Bob's theory by just setting both serial ports to "none".

If the noise goes away then it is communication noise.
 
I'll second the data line. I've installed a few using the bare wire cradle, disconnect the data line and the noise is gone. Only connect the data line that you require, such as if you are using something like a 430 or 650 only hook up the 232 tx line from the radio and the 232 rx on the 796. Opposite for something like a sl 30 or 40 where you're putting frequencies in the standby. Cap and stow the unused wire and the noise will dissapear.
 
Chino Tom;672057 completely eliminated the noise. [url said:
http://www.crutchfield.com/p_127SNI135/PAC-SNI-1-3-5-Noise-Filter.html[/url]
Click to expand...

Did the sound get degraded with that ground loop?
 
Brantel said:
The 796 when plugged into ships power produces some audio noise on the audio output leads...... I have my 796 wired to the music input of my PSE PM3000 intercom using the Garmin 796 flying lead cable.
Click to expand...

The PM3000 has two unswitched audio inputs. Is it possible that connecting through one of these instead of through the music input might solve the problem.
 
Captain Avgas said:
The PM3000 has two unswitched audio inputs. Is it possible that connecting through one of these instead of through the music input might solve the problem.
Click to expand...

Yes it does and that is how I had my older Aera 510 and it did the same thing just not as loud. The downside to way of connecting it is #1 you lose the stereo for the music and #2 the auto mute function does not work on those inputs.
 
I can confirm that at least part of the noise I have been having is coming from the serial data lines. I turned off the serial ports and the noise went away......

There are still some noises coming from the unit. I will let you know if the ground loop isolator takes care of em...
 
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Noise Noise Noise...

Well I got my Ground Loop Isolator:

416ZLyfET8L._SL500_AA300_.jpg


In the mail yesterday and went over to the airport to try it out.

Bad news is that it actually makes the noise much louder. I suspect that there is an issue with the impedance. I have asked Garmin for the impedance of their audio output and the support rep stated it was 8 ohms. I have asked the makers of this ground loop isolator for the impedance values in and out of their box and have gotten no response. I have asked the maker of my intercom (PSE) the impedance of their music input and got a response within 5 minutes! The intercom's music input is 4.7k ohms.

I investigated further the source of the noise and it seems to be made up of different components.

#1. Noise from the capacitive coupling of the data lines to the audio wires since they are in the same bundle and not shielded. This can be confirmed by turning off the serial ports. Turn em off and this noise component goes away.

#2. Noise from the battery charging circuit inside the 796. Once the battery gets fully charged, this noise goes away.

#3. Noise from the backlight driver circuit. The more you dim the screen, the more noise you get. Turn the backlight wide open and most if not all of this noise component goes away.

So if I have a fully charged battery, turn off the serial ports and turn the screen backlight all the way up, I get no significant noise.
 
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Brantel said:
The 796 when plugged into ships power produces some audio noise on the audio output leads. The sound it makes in the headsets sounds like the charging circuit for the battery. The noise is quite strong and you can hear it over the engine running. I use Lightspeed Zulu headsets.
Click to expand...
I have the exact same problem with my 696.

TODR
 
4.7k resistor

rocketbob said:
4.7K resistor in series on the data line will take care of the noise.
Click to expand...

Well, I put a 4.7K resistor in line on my blue data line from my Garmin to the auto pilot and still have all of the noise? Any other ideas out there?
 
celstar said:
Well, I put a 4.7K resistor in line on my blue data line from my Garmin to the auto pilot and still have all of the noise? Any other ideas out there?
Click to expand...

Try larger values of resistors. A friend of mine did this and it worked.
 
Noise found!

The noise I had coming over the headset was because I did not have the Audio common wire grounded to the shielding on my 3.5 plug. Connected them together and no noise.
 
Any updates on this noise issue

Just started wiring in my 796 and missed the fact that the bare wire cable does not have the line out jack for audio which I used with the 496. Planned to hook up the audio/music the same way as the 496 into the music #1 input of the PM3000. I can cut the cable length down significantly as the 796 will be in the panel, will this help? I plan on wiring a 3.5mm socket into the cable so that I can just plug in the existing audio cable.
Need the RS-232 as it drives the A/P, SL-30 and Dynon.
 
I'm not totally certain that the 796 uses the same interface as the Garmin Aera, but here is what I found with the Aera:

When the Aera was switched OFF or removed from the panel, I got substantial serial port noise in my system (PM3000, Aera on music inputs). I confirmed this by turning serial ports off. With the Aera ON, this noise disappeared.

Since the Aera has 8-ohm outputs (speaker drive capability), I terminated the audio lines with 10 ohm resistors to audio ground. This eliminated all of the noise when the Aera was off and seemed to help reduce system noise with it on.

The termination lowers the impedance of the floating and unshielded audio wires in the Aera cable, making them less susceptible to radiated and induced noise sources. It would probably work with a higher value of resistance, but I haven't spent the time to find out how high it can go.
 
Noise from backlight dimming on Garmin 796

Brantel noted a few months ago the various noise sources generated by the 796 which manifest themselves as audio noise. I have been doing some night flying recently and the backlight dimming issue was very apparent and annoying. Just as Brantel stated "3. Noise from the backlight driver circuit. The more you dim the screen, the more noise you get. Turn the backlight wide open and most if not all of this noise component goes away."

Unfortunately at night there is no way to fly with the backlight intensity anywhere above 4 or 5. I think I needed 3 to be comfortable from a light level perspective. However that also produced more noise. I addressed the issue with Garmin with little result. I was instructed to change audio from headset to line - noise level improved but no where near gone. It is unreasonable to accept that I can't use this unit to fly at night. Sounds like enough of us are experiencing noise related issues with the 796 that Garmin should address it.
Don't get me wrong, almost every other aspect of the product is wonderful, just the noise!!!

Comments?
 
It is important to note that not all installs will experience this issue. It depends on to what device and how someone connects their 796 to the audio system.

I for one experienced an issue using the 796 with the bare wire cable while connected to the music input of the current design of the PSE PM3000 intercom. It appears that the high gain of the music input coupled with the capacitive noise coupling of the 796's bare wire cable and possible impedance issues all together help the noise to manifest itself.

To minimize the issue, I changed from the music input of the PM3000 to one of the non-switched auxiliary inputs and this reduced the noise to the point that it does not bother me in flight. I suspect that people that use a different intercom or a full blown audio panel may never experience the issue. I also believe that even if the issue shows up, certain techniques like Vern mentioned above can be tried until the right fix is found for a particular setup to make the noise a non-issue. It might take some experimentation however.

The 796 is an amazing device. It is currently at the pinnacle of modern portable aviation GPS's and is the result of many generations of technology advancements. The more I use mine the better I like it and the GDL-39 has even expanded its capabilities beyond my expectations. It is my opinion that we have not seen the last of the enhancements that Garmin has for the 796 either.
 
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We did some testing today with an Aera 796 connected to a GMA240 audio panel to determine the optimum settings for this installation. We realize that not everyone has a GMA240, but perhaps the setup that works well with this audio panel can work with others.

We connected the stereo audio output from the Aera 796 to the Music 2 left, right and low input pins on the GMA240.

We set the audio volume settings (music, alerts) and master audio settings to maximum (or close to maximum was also fine) on the Aera 796. We selected the Line Out setting for the audio output.

Next, we selected the Music 2 source on the GMA240 and used the GMA240 music volume knob to set the audio to a good level.

The results were excellent for all audio including keytones, alerts, and XM music with any backlight setting.

Key to this success, other than good wiring techniques as explained in the GMA240 installation manual, is setting the volumes high on the Aera 796 to keep the useful audio signal much higher than any audio signal induced by internal systems like the backlight. These high settings cause an excellent signal-to-noise ratio and also forces a lowish music volume setting on the GMA240 with corresponding low gain for the music input audio amplifier.

The GMA240 is the perfect platform for this connection as it utilizes relatively low impedance audio inputs in combination with a dedicated volume knob which allows you to set the volume level high on your audio source (in this case the Aera 796) and then adjust the volume to the perfect level in flight without ever having to visit the audio volume page on the Aera 796.

Not sure how many intercoms and audio panels will work as well as the GMA240, but we tested with what we had and sell for experimental aircraft and hope this information is useful.

Thanks,
Steve
 
Steve,

Your testing and post above make perfect sense and also confirms what I have experienced as well.

Unfortunately the PSE PM3000's music input is fixed gain and it is very hot. This causes us to need to run the 796 at low volume. The exact opposite of what you recommended on the GMA240.

When I changed to the unswitched AUX input, this input is much lower gain and therefore there is much less to no issue on that input.

I have been messing around with live audio for years and everything you said above is correct. The same principles apply even in large audience live audio systems.

One test that may be worth trying with the PM3000 is to PAD down the 796's output right before the PM3000's music input to allow us to run the 796's volume at a much higher level to increase the signal to noise ratio.
 
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Thanks Steve

My set-up is virtually identical to Brantel (796 & PM3000). I will play some more with the levels, but really don't see a way to clean up the backlit dimming noise. Maybe just don't fly at night ;) If I recall, when I pulled the breaker feeding the 796 and let it run on battery power noise level was fine. This tells me the noise is getting coupled thru the dc wiring to the 796.

Brantel, my unswitched audio inputs to the PM3000 are already populated by 1) my afs 4500 EFIS and 2) sl30 nav audio. The 796 is feed into the music input.

I'll keep experimenting. Thanks.
 
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MS19087 said:
My set-up is virtually identical to Brantel (796 & PM3000). I will play some more with the levels, but really don't see a way to clean up the backlit dimming noise. Maybe just don't fly at night ;) If I recall, when I pulled the breaker feeding the 796 and let it run on battery power noise level was fine. This tells me the noise is coming getting coupled thru the dc wiring to the 796.

Brantel, my unhitched audio inputs to the PM3000 are already populated by 1) my afs 4500 EFIS and 2) sl30 nav audio. The 796 is feed into the music input.

I'll keep experimenting. Thanks.
Click to expand...

Mark,

I would try a PAD on the bare wire cable right before it goes into the music input. Since it is stereo, you will need two of em for a full test.

I recommend you start with a 20dB pad and try that and if that does not work, keep increasing the pad's attenuation level until it gets you there without lowering the 796's volume too much in the headset.

Like Steve said, we want to be able to turn the 796's volume way up so the pad should allow you to do that....

Here is some examples of a pad, you should not need the decoupling capacitor:

http://www.epanorama.net/circuits/line_to_mic.html
 
Thanks Steve - my setup is nearly identical to what you described. However, being a 795 coupled to a GDL 39, my GPS audio will only be alerts. Still, you're configuration advice is appreciated!
 
Thanks Brian . . .

Brantel said:
Mark,

I would try a PAD on the bare wire cable right before it goes into the music input. Since it is stereo, you will need two of em for a full test.

I recommend you start with a 20dB pad and try that and if that does not work, keep increasing the pad's attenuation level until it gets you there without lowering the 796's volume too much in the headset.

Like Steve said, we want to be able to turn the 796's volume way up so the pad should allow you to do that....

Here is some examples of a pad, you should not need the decoupling capacitor:

http://www.epanorama.net/circuits/line_to_mic.html
Click to expand...

I'll try that path . . .
 
MS19087 said:
I'll try that path . . .
Click to expand...

Let us know how it goes. I will do the same if I get a chance to test before ripping my panel apart for the upgrades.

Mark from PSE told me once that he cranked up the gain on the PM3000's music input because people were trying to use low powered MP3 players that did not have enough oompth to get enough volume.
 
Brantel said:
Steve,

One test that may be worth trying with the PM3000 is to PAD down the 796's output right before the PM3000's music input to allow us to run the 796's volume at a much higher level to increase the signal to noise ratio.
Click to expand...

Brian,

Thanks for your input. You may be onto something. We have also considered whether there might be a way to use an inline attenuator between the Aera 796 and any device like this without any means to adjust the audio input.

In addition to the front panel volume knob, the GMA240 has top cover access holes to adjust the volumes independently on Alert 1, Alert 2, Alert 3, Rear TEL input, and Front TEL input audio inputs.

If all intercoms and audio panels had these types of adjustments available to the installer it would be much easier to tune these installations and receive similar results to what we received with the GMA240.

Thanks,
Steve
 
Add another very noisy 796 bare wire. All shielded and shields grounded one end. Installation into a PMA 7000 music input.
Wired in parallel with my music input jack which has been a quiet jack for many years..
I just got it installed. Ill try a few of these items and report back.
Its VERY noisy.
Will report back
 
Mike,

I tried a loop isolator and it did not help. The noise does not appear to be ground loop noise.

What seems promising would be to pad the output of the 796 down to the point where you have to crank up the 796 volume almost wide open to get the volume you want. This will increase the signal to noise ratio out of the 796. Do this with a homemade pad or a store bought attenuation cable.

This assumes the PMA7000 has a fixed gain setting on the input you connected to the 796 and that input is pretty hot gain wise.

How low were you having to run the 796 volume to keep it from being too loud? If it was low what I propose above should help a ton. If it was already cranked up high, the above most likely won't help.
 
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I realize this is a 7 year old thread, but it?s completely relevant to my situation.

Anyone come up with a fix for this?

I also have a 796 that is hard wired to a PMA8000BT music input. I have always had noise in the music line from at least three sources: 796 battery charging, RS232 and screen dimmer. When the 796 battery is charging, it?s noisy, but that goes away by the time I?m doing the run up. The screen dimmer is obnoxiously loud anytime it?s not at full-bright. Fortunately, I rarely fly at night so this isn?t usually an issue. The 232 noise persists unless I turn it off in the 796, but I use it many times each flight to send freqs to the SL30.

Here?s what I?ve tried:
1. Shortened the bare wire cable as much as possible. No change.
2. Terminated the L & R audio lines to audio ground using 10 ohm resistors. This quieted down the 232 noise, but also lowered the overall volume and killed the music and warning audio quality. I?m going to pull the resistors next time I?m in there.
3. Maxed out the 796 volume and lowered the volume of the music in the PMA8000. This gets rid of some (not all) of the 232 noise.
4. I spoke with PS Engineering, who initially tried to help (they really were great), but in the end pointed their finger at the unshielded bare wire cable. Unfortunately, I can?t see a clean way to rewire the 796 dock with shielded wire.

I really like the PMA8000BT, but this overly hot music input seems to be a common thread among PSE intercoms. I?m at my wits end with the noise and am ready to dump it for a Garmin GMA 345 and cross my fingers.
 
Electrical Shielding Ground Loop?

We had a 796 with the audio wired directly to a Bose A20 LEMO plug that was also directly wired to a Garmin GTR 225 comm radio (Air Tractor). Since this was a single-seat there was no ICS. We had a terrible noise whenever the PTT was pressed. The GTR 225 was swapped out with a new one and same problem. The LEMO headset jack was confirmed to be electrically isolated from the panel. After a year of issues we cut the headset sheilding drain wire connected to the radio rack chasis and everything cleared up. Apparently there was an electrical ground loop in the audio shielding somewhere. Since there wasn't an intercom or audio panel the headset wires, 796 audio wires, and com radio wires were all interconnected at the back side of the radio rack.

Common rules of thumb for shielded wiring: If it's a data signal line like RS232, CAN bus, ARINC 429, etc then bond the shielding at both ends, to the chasis of each device using a shield drain or soldered jumper wire. lf it's an audio circuit then bond the shileding at one end only, preferably at the upstream audio panel or intercom. This is also why headphone and mic jacks are installed with plastic shoulder washers so they don't ground to their mounting surface which was commonly done back in the good old days before digital radios.

Jim
 
Thanks, Jim.

One of the problems is that the 796 bare wire harness runs the 232 lines, audio, power, everything in the same bundle with no shields. There?s a connection for the XM and GPS antennas at about mid-cable as well, further complicating any effort to rewire it with shielded wires.
 
Whenever I have cut into a 796 bare wire cable it does have shielding. So do the OEM bare wire cables for the other aera models. Garmin just terminares the braided shield before the free wire ends and solders it to the black wire (ground). This is hidden by the piece of heat shrink at the free end. They also wrap the two larger gauge power and ground wires together with foil (black and red) for bus-generated EMI.

Jim
 
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I had a lot of noise until I grounded the 796 audio ground. You probably did that but just throwing it out.
 
Did you simply ground the audio low line, so it was connected to both the audio panel as well as ground? Or did you disconnect it from the audio panel and run it straight to ground?
 
jliltd said:
Whenever I have cut into a 796 bare wire cable it does have shielding. So do the OEM bare wire cables for the other aera models. Garmin just terminares the braided shield before the free wire ends and solders it to the black wire (ground). This is hidden by the piece of heat shrink at the free end. They also wrap the two larger gauge power and ground wires together with foil (black and red) for bus-generated EMI.

Jim
Click to expand...

You?re right. I forgot that there was a foil shield in there. I guess my issue was that unless I?m mis-remembering things, the RS232 lines were within the same shield as the audio high and low lines. Thus allowing the 232 lines to generate noise in the audio lines.
 
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