[TenTec] Heil ProSet adapter with Orion -- hum

[Jim Brown]

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