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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