[TenTec] Re: Heil ProSet adapter with Orion -- hum

[Jim Brown]

Based on the correct info from those who have the schematic for the radio, it seems 
clear that the Orion has an UNbalanced input. In other words, I was mistaken about 
that.  If you want to use a balanced mic with any ham gear, you need to tie the "audio 
-" line to "mic ground" at the radio. The shield is still only a shield, and should go to the 
chassis.

There's another potential issue with many rigs -- they may not have enough gain to 
work with a pro mic, which is 150 ohms impedance. When I want to use a pro mic with 
my ham gear (I sometimes do), I use a step-up transformer  between the mic and the 
radio.  The turns ratio is not critical -- something between 1:2 and 1:4 will work, but it 
needs to be a decent audio transformer. The original Omni has enough gain that it 
doesn't need the transformer.  I do need the transformer with my Omni V and Elecraft 
K2. 

Jim

On Thu, 24 Jun 2004 15:08:45 GMT, Ron Martin wrote:

>Jim,
>
>Thanks for the information. 
>
>I believe that I will rewire the Heil (and the one I built) transition cable 
>to do what you suggest.  I do have some 'old time' balance mikes that I 
>would like to try on the Orion. 
>
>
>Thanks 
>
>
>Ron 
>
>Jim Brown writes: 
>
>> 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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>