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

[AD6XY]

I think this plot explains why. 

http://n2.nabble.com/file/n1078026/capt0809091710.jpg capt0809091710.jpg 

It is a sequence from my GB3RAL 70MHz beacon sending JT65B. The high
frequency alias is quite clear. It is inverted which might be why it sounds
so strange. So for example 900Hz is aliasing to 11.1kHz. It is a long way
down, 80-100 dB. You might not think it to be a problem but perhaps if the
headphones are more sensitive at HF it could be.

Here is another where I moved the VFO to show the effect of different
frequencies over the passband.
http://n2.nabble.com/file/n1078026/capt0809091717.jpg capt0809091717.jpg 

The amplitude appears fairly consistent. I think a simple low pass filter is
all that is needed and a cut off of maybe 7 kHz would be appropriate. Maybe
even lower as this 4kHz ESSB plot with the FM filter shows, the audio
response is limited above 2.5kHz.

http://n2.nabble.com/file/n1078026/capt0809091722.jpg capt0809091722.jpg 

Mike



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.
> 
> 73 Paul.
> 

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