On Wed, 23 Jun 2004 23:33:10 -0500, George Skoubis wrote:
>I shouldn't bring this up again for someone will blame it on the "Pin 1
>problem". The fact is there's typically 4 connections for microphone
>audio.
>There is the Audio Hi or +, Audio Lo or -, Audio Ground, and Audio
>Shield.
I suspect that the problems here may be related to the difference between
balanced and
unbalanced audio wiring.
Virtually all ham gear uses UN-balanced audio -- as I understand it (I'm not
flush enough to
own one), the Orion uses a balanced mic input, which is a nice step up, but it
can lead to
problems when you interface an unbalanced mic. A balanced mic uses only + and -
for
audio -- the shield is ONLY a shield.
An unbalanced mic uses the shield both as a shield AND as "audio -", so if you
want to
use an unbalanced mic with a balanced input, you must connect the shield to
both chassis
and "audio -". The chassis connection terminates the shield, and the "audio -"
connection
completes the audio circuit.
There's another big advantage to having a balanced audio input -- you can plug
a pro mic
directly into it!
The Standard wiring (AES and IEC) for a cable-mounted XLR connector is
1 - Shield
2 - Audio high
3 - Audio low
Shell - no connection (there are VERY good reasons why this is the Standard,
and they
involve the prevention of ground loops in installed systems).
The correct wiring for a chassis-mounted XLR connector carrying balanced audio
is
1 - Chassis
2 - Audio high
3 - Audio low
The correct wiring for balanced audio is:
Shield to chassis
Audio High to Audio High
Audio Low to Audio Low
Much equipment, including all of the pre-Orion Ten Tec gear I have seen, is
built
improperly, in that it takes the shield to the circuit board and not to the
chassis. There are
two fixes.
1) Rewire the equipment connector to disconnect the shield (pin 1) from the
circuit board
and connect it to the chassis. (Best) Unfortunately, lots of equipment use
connectors that
are mounted to the circuit board, and it is difficult or impossible to rewire
them correctly.
2) Rewire the cable-mounted connector to connect the shield to the shell.
(Second best,
but it fixes all but the most severe problems).
Hope this helps.
Jim Brown K9YC
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