[Elecraft] K3 Hum on Audio...

[Jim Brown]

On 1/13/2012 9:20 AM, Phil Hystad wrote:
> Unfortunately, it is in both mics but not as strong in the Yamaha CM500 mic as in the MH2 but it is present.

Clarification question.  Is it HUM (pure 60 Hz), or BUZZ (mostly 
harmonics of 60 Hz)?  If it's HUM, I would suspect magnetic field 
coupling into the audio, either from a big power transformer (like the 
one in a power amp or a big linear power supply) or from a AC power 
wiring fault called a double-bonded neutral.

The K3 has unshielded audio transformers at all the audio inputs and 
outputs, and an unshielded transformer is a sitting duck for magnetic 
fields.

The good news is that the K3 has excellent audio equalization (TXEQ) 
that allows us to remove that 60 Hz hum by filtering.  The lower audio 
frequencies in the human voice make NO useful contribution to speech 
intelligibility, but they do waste transmit power. So it is ALWAYS a 
good thing to set the TXEQ for maximum cut of the lowest two bands, and 
at least some cut of the third band. This is true with virtually ALL 
mics and ALL voices.  AND it will reduce that hum enough that you may no 
longer hear it.

There are several solutions to magnetic field coupling.  1) Rotate the 
noise source or the victim circuit to put the fields at right angles to 
the victim.  2) Move the noise source further from the victim. 3) If the 
hum field is produced by that AC power wiring error, fix the error to 
eliminate the field.

73, Jim K9YC