[Elecraft] Electret microphone wiring

[Tom W8JI]

<<<IF the cable shield is separate from both signal 
conductors (that is,
hot and return, as in a balanced mic), it's fine to separate 
return and
shield. But if there's only the shield, it MUST go to the 
chassis. >>>

If one "ground" pin was dedicated to the chassis or cabinet 
ground, the other pin could be run through RF bypasses to 
the "cold" side of the audio input PC board. Then, in the 
case of a mic with an isolated ground return inside the 
shield, or even a balanced mic or unbalanced mic, the audio 
return could go to that pin. The shield could always go to 
the cabinet or chassis with the lowest possible impedance. 
No chokes, no bypasses. This is the same place the mic 
connector shell should go.

If the mic had only the shield for an audio return, the 
shield and audio return pins could be bonded at the 
connector. This would be a simple solution that would cover 
all bases.

For some reason this is an age old problem with Ham gear. 
Early tube gear used the case as a ground, but for some 
reason when the digital engineers got involved grounds 
started getting messed up. This is true for lightning 
grounds also.

For example I did an antenna switching controller and made 
the chassis the common point for all grounds. The fellow 
writing the firmware did a whole new layout on his own where 
he isolated the digital and signal grounds. The result was a 
unit that failed CE testing for ESD between ports, and had 
significantly more RF out on leads from the clock. Strong RF 
would just kill the thing.

I had a similar problem with a VOMAX speech processor. The 
RF power supply decoupling was on a single toroid and both 
negative and positive supply leads passed through it as a 
bifilar winding. While this provided reasonable common mode 
isolation, it offered virtually no differential isolation. 
The foil trace from the PTT lead (that had no RF isolation 
at the jacks) ran right along side the audio trace. 
Bypassing to the case as a common ground, and splitting the 
power supply leads into two isolated chokes, totally cured 
the VOMAX.

73 Tom