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

[Jack Smith]

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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