[Elecraft] K3 "hum" problem - not the usual?

Jim Brown jim at audiosystemsgroup.com
Mon Sep 28 15:06:22 EDT 2009


On Mon, 28 Sep 2009 13:29:54 -0400, Brian Machesney wrote:

>The problem only occurs in an
>SSB mode when the nearby PA is both on and enabled (i.e., not on "standby")
>and when the K3 is both within about 8-in of the PA and parallel or at right
>angles to it; rotating the rig about 15 degrees around the z-axis eliminated
>the hum. All in the usual vein, so far.

Magnetic field coupling from the power supply to the K3's input circuitry, 
probably due to the use of unshielded audio transformers in the K3. This is a 
well known problem. One good band-aid is to go to the TXEQ1 settings and fully 
cut the three lowest octave bands. There's little communications value to the 
speech below 300 Hz anyway, and it burns TX power, so it's a win-win. Also, 
the acoustic stuff in that lower frequency range includes breath pops and room 
noise. 

It's also possible to have magnetic coupling on input wiring if you've managed 
to form it into a big loop, but the transformers are the more likely cause. I 
ran into the loop problem once when I jury-rigged a headphone amp for SO2R. No 
problem in TX audio, but it was in my phones. One of my neighbors had so much 
magnetically coupled hum into the Line In transformer that it regenerated to 
full output power when he tried to run RTTY in AFSK mode. 

73,

Jim Brown K9YC




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