[Elecraft] Re: Heil Proset & K3 Audio fixed, sort of.

[Dave G4AON]

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.
/>/ /