[Elecraft] RFI in audio chain

[Joe Subich, W4TV]

Jan,

On an XLR pin 1 is shield/ground, pin 2 is mic hot, pin 3 is mic
return.  The proper connections (as I described in another forum
a couple days ago) are:

> For *any* microphone with a 3pin XLR one needs a cable with XLR
> on one end (for the "professional" mic) and a Foster plug for
> the rig.  Connections are:
>
> MIC (XLR)                Radio (Foster)            signal
>                Yaesu   Kenwood/Elecraft   Icom
> Pin   1        shell         shell        shell    Shield
> Pin   2          8             1            1       Mic +
> Pin   3          7             7            7       Mic -
>
> Note: Install a 10 uF capacitor between pin 2 of the XLR and
> pin 1 of the Foster for use with Icom (+ to the Foster) to
> block DC on the mic line.
>
> Note 2: *NEVER* connect the shield (Pin 1 of the XLR) to the
> Mic - or "mic ground" and *never* connect "mic ground" to the
> PTT Ground/shell or chassis of the transceiver.

As pointed out in private e-mail, professional installations have
migrated to *three wire* plus shield cables with pin 1 connected
to circuit return (PTT ground in the case of amateur transceivers)
and shield connected to the shell of the connector (chassis) on
both ends.  However, most analog work still uses two wire plus
shield with an XLR plug that ties pin1 to the shell internally.

> I see several alternatives:
>
> 1. Remove the shield connection to pin 3 at the XLR; leave pin 2
> connected.

If your cable is a "coaxial" one - a single wire with shield, the best
short term "fix" is to connect the center wire from pin 2 of the XLR
to pin 1 of the Kenwood/Elecraft Foster plug and connect the shield
from pin 3 of the XLR to pin 7 of the Kenwood/Elecraft Foster plug
*with no connection to pin 1 of the XLR plug*.  In this case the cable
is acting as a simple two wire cable.

> 4. Get a new cable with two or three conductors inside a shield and
>     still use only pins 3 and 2 at the XLR, using only conductors, not
>     shield.

If I were using two conductor with shield, I would wire pins 2 and 3 as
above and connect the shield between pin 1 on the XLR and the *shell*
of the Foster plug.  If the transceiver properly bonds the Foster jack
to a metallic chassis, the shield is properly terminated on the chassis
as it should be.  If the transceiver mounts the mic jack to a circuit
board on an insulated panel, the shield is floating (isolated) where it
should cause no problem.  *Under no circumstances* would I connect the
shell of the Foster plug to either the mic return (e.g. Pin 7 as done
by Heil) or the PTT ground (e.g. Pin 8) as that "circuit' can wind its
way many places in the transceiver before it is bonded to the actual
chassis (and power supply) return.

If replacing the cable, look for shielded, *twisted pair* cable.  The
twisted pair will help balance out any common mode RF and hum resulting
in lower noise even when the shield is floating on one end.


73,

   ... Joe, W4TV


On 11/23/2015 2:20 PM, Jan Ditzian wrote:
> Joe and Bob,
>
> I decided to chop things up to see what was inside.  I found a
> surprising result:
>
> The microphone wire is a two-conductor wire, consisting of a single
> insulated conductor and a shield.  At the microphone XLR end the single
> wire is connected to pin 2.  At the rig end, this same wire is connected
> to pin 1 of the eight pin connector.  At the XLR end, the shield is
> connected to pin3 and pin 1, while at the rig end, this shield goes to
> pin 8.  I know that audio gets to the rig with this setup, but it
> clearly is not what you guys are talking about.
>
> I see several alternatives:
>
> 1. Remove the shield connection to pin 3 at the XLR; leave pin 2 connected.
> 2. Remove the shield connection to pin 1 at the XLR; leave pin 2 connected.
> 3. Get a new cable with two or three conductors inside a shield and
>     still use only pins 1 and 2 at the XLR, using only conductors, not
>     shield.
> 4. Get a new cable with two or three conductors inside a shield and
>     still use only pins 3 and 2 at the XLR, using only conductors, not
>     shield.
>
> Alternatives 3 and 4 leave me the choice of connecting the shield only
> at the microphone end, or not, depending upon what works best at
> reducing RFI.
>
> I think that this exhausts the alternatives, so if one or both of you
> can give me a shove in a single direction, I may be able to row out the
> rest of the way by myself.
>
> I gently patched things back together so I can meet my 2000Z sched, but
> I await further instructions, O Masters of the Universe.
>
> Your obdt. svt,
> Jan, KX2A
>
>
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