[Elecraft] Electret microphone wiring

[Jim Brown]

On Fri, 23 Apr 2010 06:04:26 -0400, Tom W8JI wrote:

>Pin  7 and 8 are the same connection point.

The ONLY proper connection point for a cable shield is the shielding 
enclosure. Doing anything else creates what is known in the audio world 
as a "pin 1 problem," whereby shield current (RF or baseband power-
related buzz) can flow on interior "ground buss" wiring, couple into 
gain stages, and be detected and/or amplified. 

Since the AES charges non-members for Standards, I've put technical 
discussions of that Standard, for which I led the writing group, on my 
website. http://audiosystemsgroup.com/publish.htm

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. 

For many years, I didn't own a dedicated ham mic other than a PTT that 
came with a radio, so I grabbed an RE16 from my stock of recording mics 
and used that. It worked fine with suitable EQ as long as I made sure 
that the cable shield went to the chassis, but a rig with a pin 1 
problem could experience RFI. Several years ago, I acquired an 
FT1000MP, and got reports of RF feedback on 75M and 15M. I ran the pin 
1 susceptibility test that's described in one of the AES papers that's 
on my website on that mic connector, and found strong susceptibility 
with two peaks -- one a broad one that peaked just below 5 MHz and the 
second that peaked just below 21 MHz. Seven turns of the mic cable 
around a #31 2.4-in o.d. toroid killed the RFI. 

MOST ham rigs come with pin 1 problems -- the MP and K3 are not alone. 
It has taken radio circuit designers a long time to realize that 
someone outside their own limited world might know something more about 
RFI than they do, or to even read beyond their world. The guys who 
worked with me on AES EMC Standards all have serious RF backgrounds. 

To get back to the original question, the K3 has EXCELLENT good support 
for unbalanced electret mics. All that is needed is to connect the mic 
output to the mic input and turn on the bias via the menu. Inside the 
radio there's a 5.6K resistor from DC+ to the mic output. And there's 
enough TXEQ to accommodate the frequency response of any decent mic. 

73,

Jim Brown K9YC