[Elecraft] K3 Hum on Audio...

[Phil Hystad]

I just did one more test.  I hooked up my little 4-cell Li nano-phosphate battery to the input.  Turned on the K3 and did the test again with the K3 in TEST mode.  Same hum occurs.  With this experiment, the only thing plugged into the K3 is the MH2 microphone itself and all AC in the room is switched off at the breaker.  So, K3 on battery and it still has this hum.  This is a bit perplexing.  I am writing here on my laptop which is battery powered too.

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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