[TenTec] Heil Pro set 5 & Orion

[Jim Brown]

On Tue, 11 May 2004 22:37:13 -0500, John Rader wrote:

>grounding the shell of the 4 pin mic connector solved this.

This is yet another example of what we in the pro audio world call "the pin 1 
problem."  That is, to be effective, the shield of a cable must be connected 
DIRECTLY to the shielding enclosure (chassis), NOT to the interior circuitry or the 
circuit board. When it goes to the circuit board first and then to the chassis, there 
will be a voltage drop across the inductance of that connection due to RF current 
flow on the shield. That voltage drop will be added to the signal at random points in 
the circuit depending on circuit board layout. 

The pin 1 problem is a MAJOR cause of RFI interference at audio interfaces, and it 
is most certainly the cause of the one you have just fixed. 

Note that "the pin 1 problem" is an equipment design error! The radios that connect 
the cable shield to any point other than their chassis are built WRONG! The fact that 
MANY radios are designed this way does not change that fact -- the laws of physics 
are not the result of a popular vote!  

BTW, it's called "the pin 1 problem" because pin 1 is the designated shield contact 
of the XLR connector, and it that contact that must be connected to the chassis 
within equipment. 

Jim Brown  K9YC