[Elecraft] K3 Hum on Audio...

[Phil Hystad]

Jim,

Well, I would call it mostly a buzz.  Also, the only near magnetic field of the KPA500 power transformer is off, I mean powered off and unplugged, so that does not seem to be the problem.

I just ran another test with everything disconnected, including ground and coax.  With the K3 in TEST mode, key-down on the mic still produces the hum.  I double checked the grounds.  I did this because if I were to touch anything metal on the k3, the metal part of the PL259 or the ground strap, the hum is damped quite a bit to almost insignificant.  It seems that if my rig were already well grounded this would not happen so I am curious if this is something I would normally expect.

I will experiment with the TXEQ but this is a real puzzle I would like to solve, not merely erase it.  

73, phil


On Jan 13, 2012, at 9:37 AM, Jim Brown wrote:

> 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
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