> I suspect that the crucial thing with interference from alternator
> whine etc. is the screening of the Mike and Headset leads.
>These should be screened from the Intercom end and left open
>at the jack socket end. Think that's right. Check with Bob Nuckolls.
>
>
Actually, MOST alternator whine gets into audio systems via ground
loops. The most common loop mode occurs when headset jacks and
microphone jacks get grounded to airframe where they mount. Mount
the jacks on a thin piece of fiberglas or better yet, use insulating
extruded washers to keep the jacks from getting grounded to their
metal mounting surface. I have a few of these washers at $1.00 a
set (one flat and one extruded washer for one jack). I'm looking
for a lower cost source and will feature them in our catalog which
will publish later this year.
Shielding can be useful for microphone and headset leads but
for the most part, twisted pairs will suffice. If the jacks are
NOT insulated, no amount of shielding of wires will make any
difference. You can use single twisted pairs for both headset and
microphone jacks. Carry push to talk (plug tip) and mic audio hi
(plug ring) on the two wires . . . use shield as mic audio lo and
push to talk return (plug sleeve). Connect shield to jack common
at jack end; to mic audio lo at radio or intercomm end. I'd use
22AWG twisted pair for headsets . . . no shielding needed.
On the topic of shielding in general . . . use shielded wire
only where manufacture recommends and/or supplies it with their
product. Strobes and audio systems commonly have some form of
shielded wire. Magneto p-leads should be shielded as a matter of
standard practice. I don't think the ignition control leads from
a rotax system carry any sharp, high voltage spikes . . . has anyone
looked at these leads with a scope? I suspect they don't require
shielding. Except for audio, strobes and an occasional mag, other
forms of shielding are generally a waste of time, money and in
some cases, creates noises that wouldn't have been there otherwise.
Wait til you have a noise problem before taking action. (1) identify
source (2) identify victim (3) identify propogation mode. Then either
(1) filter source (2) filter victim or (3) break propogation mode.
Advance shielding of most wires prior to doing the "homework" is
wasted effort.
Bob . . .
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