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

[Berni G0IDA]

I've been sharing my findings with M1PAF and we have now both put a RC 
Low Pass Filter across the headphone transducer of our Heil headsets but 
only in the right headphone so that the left one can still hear the 
artifacts for comparison.
I have chosen a LPF with values of R=260 Ohms & C=0.2uF This has a 3dB 
point at 3060KHz.
The Impedance of the Heil Headset is 200 Ohms per transducer.

There is no doubt that the RC filter works. I can hear a nice clean CW 
note in my right ear and artifacts in the left. This is almost true for SSB.
I do have to adjust my volume to a higher level, if the volume knob was 
originally at 9 o'clock position for normal use with the RC filter 
fitted I turn it up to 10 o'clock position..... not a lot of difference.

Although these tests are none scientific and don't involve expensive 
test gear, one is for sure I can't hear the artifacts anymore on CW and 
less so on SSB and my test set are my ears, which I'm afraid, won't give 
you any fancy dB signal levels or a spectrum analysis !!

Perhaps someone out there with test equipment might like to look into a 
Low Pass Filter design which could be incorporated into the K3 as 
standard or an improvement over my simple RC LPF in a headset?

All the best,

Berni
G0IDA



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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