[Elecraft] Microphone preamp and Heils

[Jim Brown]

On Thu, 15 Feb 2007 10:17:57 -0500, Don Wilhelm wrote:

>The stock K2 works best with a high output mic. 

Yes. But that's really a nice way of saying that the audio 
circuitry of the K2 lacks sufficient gain to work well with a real 
mic. There are several mods to improve this situation, and all 
that I've seen posted look pretty solid. 

One simple mod that I've done to my K2's increases the gain by 6 
dB and raises the low-cut filter to about 250 Hz. The importance 
of the low cut change is that the lows don't contribute to 
communications, but they do suck power, and they also cause the 
compressor to reduce the mic gain. 

That mod is:

Change R14 to 2K.  Change C34 to 0.22 uF.  Add 2.2 uF in series 
with R15. 

If you have a source for good audio transformers, you can get 
another 6 dB by adding a step-up transformer with a turns ratio of 
about 1:3. I found a vintage UTC Ouncer in my junk box that did 
the job. It's wired with an XLR on one end (that plugs into my EV 
RE11) and the Elecraft connector on the other. 

The radio mods will work for any decent mic. The transformer will 
only work with dynamic mics. It won't work with electret mics 
(because they need a bit of current from the radio to run the 
capsule and it's FET output stage).  

73,

Jim Brown K9YC