Intercom wiring

I?m installing a Flightcom 403D Stereo intercom and am not sure what wire to use. Each location (pilot & copilot) has 2 jacks and a stereo/mono switch. Each headphone jack has 3 wires, tip, ring and barrel. The headphone tip and ring are left and right audio, the ring is disconnected via the switch when a mono headset is used, and the barrel is at ground potential. Each mike jack has 3 wires going to it, tip, ring and barrel. The tip goes to the PTT button, the ring goes back to the intercom as the mike signal and the barrel is a common wire that goes back to the intercom and to all mike jacks. So, my question is what would be the best wiring scheme from the intercom to each of the jack locations (pilot and copilot), what type of wire, number of conductors and shielding would be best to insure minimum noise and best audio quality. I?m thinking a shielded 6 conductor cable, 22 awg with the shield tied to ground at the intercom end only. Does this sound OK, any suggestions?

J.C.

I think you've got it covered. I used three conductor wire from Radio Shack that used stranded wire with a ground. One of the keys to keeping noise out is to keep your intercomm wires away from any power wires as much as possible, and use the common ground that your radios/intercomm use.

Also, use insulated washers on your jacks so they're not locally grounded at the mounting points.

Jamie said:

Also, use insulated washers on your jacks so they're not locally grounded at the mounting points.

Click to expand...

Where's a good place to get the insulating washers ?

Actually, my Flightcom Intercom installation manual says to only use insulating washers on the microphone jack because the jack barrel connector must also go to the PTT switch. This is normal for most intercoms and the insulating washers are included with the intercom. I don't know why you would want to insulate the jack barrel connection on the headphone jack, my installation instructions do not specify that.

Perhaps Jamie has some additional input regarding that.

J.C.

Differing instructions....

647jc said:

Actually, my Flightcom Intercom installation manual says to only use insulating washers on the microphone jack because the jack barrel connector must also go to the PTT switch. This is normal for most intercoms and the insulating washers are included with the intercom. I don't know why you would want to insulate the jack barrel connection on the headphone jack, my installation instructions do not specify that.

Perhaps Jamie has some additional input regarding that.

J.C.

Click to expand...

Yes... but the instructions vary with manufacturers....

The PS Engineering PM501 installation instructions quote...

It is crucial that you use insulating washers
on all microphone and headphone jacks to isolate
the audio signal ground from the aircraft
ground.

And... to answer the orginal question on 6 conductor wire.... not recommended... same manual quoted...

IMPORTANT: You must use separate
shielded cable for the microphone and headphone
jacks. Combining these two wires WILL
cause loud oscillations and degrade the intercom
functions. The cause of the oscillation is due to
the fact that there is a much larger signal being
carried by the headphone wire than the microphone
wire. When these two wires are within the
same shielded cable, cross-coupling allows the
output to get back into the input, causing oscillations
to occur.


As always, it's a good idea to follow the instructions for your specific device....

What does your manual say for insulated washers?

UPDATE

The Flightcom manual is here...

http://www.flightcom.net/pdf/403-403dManual.pdf

...and states the the headphone jacks may be either insulated or not...

It has cautions on mixing grounds.... details in the manual...

gil in Tucson

B & C

Wicked Stick said:

Where's a good place to get the insulating washers ?

Click to expand...

Dave... they are somewhat elusive...

But B & C has them...

http://www.bandc.biz/cgi-bin/ez-catalog/cat_display.cgi?24X358218#s892

gil in Tucson

Yep...sorry for the slow response -- I used the ones from B&C. My intercom manual (DRE 244e) recommended isolating the jacks from the airframe.

Gil,

Thanks for the input. I have been referring to the Flightcom installation manual and followed their drilling / mounting directions for the headphone and mic jacks which end up grounding the headphone barrel connection and insulating the mic barrel. The instructions state the mic jack MUST be insulated from ground and the headphone jack may or may not be insulated and if you follow the instructions, the headphone jack will NOT be insulated. With the headphone jack barrel grounded per basic installation instructions, there is an obvious potential (no pun intended) for a ground loop so I guess I will remove and re-mount them with insulating washers. The installation kit is for a six place intercom and only included 6 insulating washers (one for each mic jack I assume) sand ince I only have a two place airplane I will use a couple of the extra washers for the headphone jacks I guess.
As far as shielding, the installation manual kind of ?cops out? in my opinion by simply stating ?Use shielded wire for microphone, headphone, and audio wiring.? So, back to my original question, if using a shielded 6 conductor cable may cause crosstalk, would it be better to use 2 shielded 3 conductor cables to each jack location only connecting the shield to ground at the intercom end?

Thanks,
J.C.

Cables

647jc said:

Gil,

Thanks for the input.

......

So, back to my original question, if using a shielded 6 conductor cable may cause crosstalk, would it be better to use 2 shielded 3 conductor cables to each jack location only connecting the shield to ground at the intercom end?

Thanks,
J.C.

Click to expand...

The shielded 2 or 3 conductor cable would be the common way to do it... A separate cable to each jack.
And yes, ground the shields at one common location, which would be the intercom in this case...

You are correct, in this case the manual does not show the shields on the wiring diagram like most other intercoms... They put it in the text, but not in the diagram...

Page 2 of this PS-Engineering document shows the shields correctly.... just use the same techniques and you will be fine....

http://www.ps-engineering.com/docs/PMA4000_DS.pdf

gil in Tucson

Thanks Gill & others..

I only wanted to know, incase my intercom didn't come with any. Will be ordering the panel avionics in a month or so.