LEMO Wiring

Scott Hersha · #1 12-28-2012, 07:31 AM

Here's a question for those of you that have the single LEMO plug for your headset (Bose, or Lightspeed). When I upgraded my panel last summer I must have done something different because since then I have had a lot of trouble with noise in my headset. The wiring harness for the panel mounted plug has 26 awg shield wires that are supposed to be connected to existing audio shields or to an audio ground. I discovered that I had connected both ends of the LEMO shields to the mic lo and phone lo to my audio panel (PAR 100EX), which is synonymous with the barrel ground on a standard headset plug. This was wrong, so I disconnected one end of this shield. That didn't help at all. Last night I disconnected the remaining LEMO shield from the audio panel mic/phone lo and went straight to my audio ground buss bar. I haven't tried it since this change, but I thought I would ask the experts here how they wired their LEMO's.

Also, I changed my +12V power input from the main bus to the audio system bus power. I don't think that will have any effect, but that was where the power came from before the upgrade. At one point I was frustrated and disconnected the power wires in flight and went to batteries on my A20. It got very quiet and worked perfectly on battery. I know it seems like its a noisy power problem, not a grounding problem, but it seems like a grounding/shield problem because of the occasional popping noise I hear - like a static discharge. So - audio experts please help.

Thanks

rleffler · #2 12-28-2012, 07:56 AM

Scott,

I don't have a definitive answer at the moment. My first career was as an audio engineer, so I do have quite a bit of experience in tracking down ground loops in recordining studios and large sound systems.

I'm will be wiring up my lemo plugs in about two weeks after I get this cast off my foot.

I did find an old thread that isn't conclusive, but implies that some of the grounds aren't labled and you may want to ohm them out to ensure you aren't cross connecting the shields.

The other area on my to do list was to see if the lemo case is isolated from ground or not. Like the standard headphone jacks, the lemo case may need to be isolated from the panel. That would create a ground loop if the sheilds are tied to the case in the jack.

bob

CharlieWaffles · #3 12-28-2012, 10:22 AM

I hooked up four of the lemo jacks in my plane and have had zero noise issues, in fact I was delighted when it turned out I didn't have to redo any of it.

If the 26Awg wire you are talking about is the black/white stripped single conductor, that is a pigtail from the shielding on the phone and mic wires. So that should be connected to the ground lug on the back of the PAR100EX. The conductors inside are the high and low (and PTT if you wired it that way). The other conductors are +vdc and gnd.

Make sense?

Scott Hersha · #4 12-28-2012, 01:33 PM

I think I have the +vdc and gnd connected right. The shield pigtails (2 per plug) are going to the audio ground now and that's the same place the PAR100EX is grounded. After I test it in flight (works OK on the ground taxiing), if it is still noisy, I'll move the shields to the gnd lug on the PAR. First I'll check to see that the plugs are isolated from the frame like Bob suggested. It worked fine for 2 1/2 years before my panel upgrade though and I didn't change the plugs.

Thanks for the replies.

CharlieWaffles · #5 12-28-2012, 03:11 PM

The LEMO plugs are self-contained and have no conductivity so there shouldn't be any issue with local grounding unless something has gone awry.

Scott Hersha · #6 01-05-2013, 06:32 PM

As Charlie says, the plugs don't require grounding. Moving the shields to the avionics ground bus did the trick. No noise. I also moved the +12V to the avionics bus, but that shouldn't matter. Take-away, don't ground your LEMO shields through the audio panel/intercom. Just run them to ground. It's nice and quiet now.