Tip: Aux Audio Jack Insulation

As I reported on another thread, I recently found out that when I plugged an MP3 player into the Auxilliary Audio input jack in my RV-8, I got a faint but noticeable alternator whine. After a little bit of investigation, I found out that I had failed to insulate the panel jack, so it was grounding the aux input line shield at the jack end, where in fact, you should only ground the shield at the audio panel.

The fix was to make sure that the jack was insulated from the panel it was mounted to, which turned out to be a little bit of a puzzler. The larger phone and mic jacks have readily available fiber washers which make for a clean instalation, but I struck out trying to find similar washers for the 3.5 mm stero jack. You can't simply rely on a thin fiber or nylon washer, as it doesn't insulate the threaded barrel of the jack as it passes through the panel. I finally found a solution after talking to one of my EE friends at work - he suggested simply wrapping the end of the jack in teflon tape before passing it through the panel hole. This gives a nice insulating layer. A thin non-conducting washer separates the barrel nut from the panel, and voila! No more whine!

Teflon tape - a very simple solution....

Paul

Cold Flow

Hi Paul,

Teflon insulation will cold flow. This is an undesirable property where the insulation creeps out from between two pressure points, leaving you with a short. The stuff will flow at very low pressures, especially when warm.
This is not theoretical, I've seen it happen first hand.

I'd use Kapton tape, a thin, high pressure & temperature tolerant tape used in the electronics industry. Should be available in any good electronics store, i.e. not Radio S.... Or enlarge the panel hole and use heat shrink.


Ted Johns
EAA 105
RV-7 Dreamer

Paul, if that tape ever wears through and you want to replace the jack, there's a jack style called "insulated bushing" you could look at. Basically, the whole jack is embedded in a plastic carrier. You typically need to drill a little bigger hole, but it'll make this problem go away for good

Some may not like them precisely because they're plastic and not as durable as an all metal jack. I'd put one in my plane, though. They're difficult to find. Maybe we could get Stein to carry them, or perhaps he has a really good reason why we shouldn't use them.

Good point Ted...

I do have a supply of Kapton tape - I'll give that a try when the noise returns...(indicative of a short). I'm not sure it will work the same way though - the jack is really tiny, and the Teflon followed the shape well.

At least it should work until I can find an insulated jack!

Paul

Hah! I have a loose flap of skin on my finger right now because *just today* I was 'creating' an insulating washer for my music jack! The jack I bought (at - gulp - Radio Smack), has about a 25-mil-deep shoulder on it on the plastic body at the base of the threads, about 0.305" in diameter. When I ceased and desisted trying to create a shoulder washer with materiel available here in outer mongolia (and had finished dressing the wounds), I discovered this built-in shoulder (doh!), drilled the panel hole for the shoulder, and used a nylon washer under the nut - done deal. Take a look at yours - you may have a shoulder on your jack as well, which makes the whole (hole?) thing pretty easy.

It's actually nice to hear that ground loops do indeed cause problems! I've been taught from day one to avoid designing them in, so I've really never seen/heard one acting up!

Alas, No Shoulder....

Of course, the easiest thing to do woudl be to find a new jack with a shoudler like you're Alan...but you know how it is, the curent one is all soldered in, heat-shrunk, wires cut to neat length....

Agreed! If it ain't leakin' [electrons], don't fix it!

Does anyone know the P/N of the Radio Shack (or other) with the shoulder built in? Thanks!

Audio Jacks

Check out Mouser p/n 161-7300 stereo. Also similar type in mono.

John Moody
#82025

My Final Solution...

Yup, just as predicted, the teflon tape didn't work....I think it just wore through as I was installing. I looked at all my usual electronic parts sources, and didn't find an insulated jack...but I was sitting at my workbench and noticed my box of hard plastic wiring grommets fromVan's. Sure enough, the jack slid perfectly inside one of them, and stuck out enough for the nut to fit on perfectly. Drilled the hole out for the grommet, popped it in, and then, of course, had an excuse to go test fly it - worked perfect!

Another little annoyance bites the dust....

Paul

Paul,

I checked my box of hard plastic wiring grommets and came up empty handed. I guess I'll have to check the Ace Aeromotive store.

I get the same noise issue when it's grounded to the airframe instead of the return ground to the intercom.

Paul

in my cherokee I get similar funky stuff going on... I have bad strobe noise, and it goes away if I plug in a music source... UNLESS the music source is grounded to the airframe. If I use an ipod (with a floating power source), then no noise, but if I use my 496 for XM radio, and the 496 is running off ship's power, then I get noise.

made my own nylon shoulder washer

One of the things that makes having a small lathe at home - when faced with this, I just went and made my own small nylon shoulder washer. I have all the audio shields grounded at the intercom, and no where else.

Steve

RV8 finishing and FWF

Paul,

Check with Sigtronics. My 200S came with the proper audio jack and I have had no issues with it grounding to the panel.