The 12 KHz artifact appears to be the audio output DAC clock, with the
demodulated audio forming upper and lower sidebands with it. It's down
considerably, but still is apparently causing issues with those who have
good high frequency hearing response.
The K3's audio seems to have way too much high frequency response, with
white noise visible up to 20 KHz and beyond in the headphone and speaker
output ports. Most communications receivers roll off the audio response
at a few KHz.
A bit of roll off in the K3 would be beneficial, I think, either RC or LC.
Jack K8ZOA
Dave G4AON wrote:
Using Spectrum Lab with my "on motherboard" sound card shows a
persistent artefact at 12 KHz with sometimes one either side of it.
This doesn't appear when listening to typical speech on SSB but tuning
to CW shows it, the signals either side of the 12 KHz one are +/- the
tone of the CW signal, i.e. typically +/- 700 Hz.
Audibly, there is more hiss and high frequency "roughness" when
listening to CW signals with the K3 than listening to the same signal
on a parallel connected K2 with it's AF DSP filtering, I would have
hoped the RX equaliser would cut the high frequency signals but it
doesn't.
Looking at the circuit tends to suggest the place for a low pass
filter would be the output of the DAC prior to the audio amplfier at
"LHPOUT/RHPOUT" on the K3 DSP IF: DAC Audio board. Looking at the
audio spectrum from my K3 (without FM) shows nothing above 4 KHz, so a
low pass filter to cut everything above perhaps 5 KHz would do no
harm. I am reluctant to modify my K3 unless it's an official Elecraft
modification, but there is certainly some high frequency audio present
that could do with removing.
73 Dave, G4AON
K3/100 #80
--------------------
It suggests that perhaps you were hearing the artifacts > 4 KHz, such
as the 12 KHz with modulation sidebands or the one at 3.9 KHz. Maybe you
were not hearing the artifacts directly but beats amongst them?
Jack K8ZOA
Paul Fletcher wrote:
>/ In order to try and eliminate the objectionable "noise" (careful
avoidance of
/>/ distortion, harmonics etc deliberate) in my headset I've fitted an
LC filter
/>/ with a 4kHz cut off. This would still pass odd order harmonics for
lower
/>/ audio frequencies. This filter has totally eliminated the annoying
"noise" I
/>/ was hearing with the downside that I have to run more AF gain to
compensate.
/>/ This kind of backs up previous measurements made by others that the
/>/ harmonics are well down, and not noticeable. It's a bit
disappointing that I
/>/ had to do this but the headphone audio is now very nice indeed.
/>/ /
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