[Elecraft] K3 Audio out artifacts

[Mike Scott]

I have been investigating audio artifacts in connection with the K3 AGC hold
experimental firmware. As a reminder, I live in a high noise area.

As part of the investigation I have been looking at both Line Out and
headphone audio using Spectrumlab on my PC. 

I am finding substantial 60-hz contamination on headphone audio that I do
not see on Line Out.

On Line Out I see what looks like fairly clean audio with donor lines at
2400 Hz and 3600 Hz outside the rx filter pass band (1 KHz DSP filtering). I
presume these are the 40th and 60th harmonics of 60 Hz. They are 50 dB below
noise background noise.

With only a change of where I plug the PC audio cable (moving from Line Out
jack to rear headphone jack) the audio looks quite different.

Now I see a whole array of 60 Hz artifacts. The 3rd and 5th harmonics are
about equal to comfortable listening level audio.  As I look higher in
frequency the harmonics generally reduce in level but only to about 20 dB
below comfortable audio listening level. There is also a broad artifact (300
Hz wide) at roughly 3300 Hz that is also 25 dB down.

The artifacts do not go away with reduced volume control settings but remain
a constant feature of headphone audio out at any setting.

Is there something about my test set up that I do not understand? 
Does anybody else see contamination of headphone audio on their K3? 
Could there be something wrong with my K3?
Could transformer coupling (Line Out) vs direct coupling (headphone) be
adding something to my test set up that I am not factoring into the problem?


I have never quite believed the audio of my K3 was like Wayne says it should
be but can make it better using slow AGC (separate post), I doubt that has
anything to do with the artifacts I see here.


Mike Scott - AE6WA
Tarzana, CA (DM04 / near LA)
NAQCC 3535
K3-100 #508 / KX1  #1311