[Elecraft] K3 related - non-linear audio transformer behavior

[Jack Smith]

Indeed, similar harmonic and IMD distortion are created inside the LINE 
IN transformer. The question is their level and importance.

The LINE IN transformer, however, is fed by a 10:1 voltage divider, 604R 
in series with 62R shunted across the transformer primary. This 
arrangement provides a lower drive impedance to the transformer, around 
56 ohms as a matter of fact. (Assumes the LINE IN port is driven by a 
low impedance source, which I believe is the case for the typical sound 
card output.) 56 Ohms is low enough to see significant THD improvement 
over a straight 604 ohm series resistor according to my measurements.

It should be possible to measure harmonics generated by the input 
transformer, perhaps using the monitor function. I'm too far behind on 
other projects to work on it at the moment, however.

Jack


Paul Fletcher wrote:
> Jack,
>
> Thanks for the research - very interesting and it looks like a simple fix to
> improve the performance of the K3 significantly. You've got me wondering now
> though about DATA-A modes. As the K3 uses the same transformer for line in
> your work suggests that the transmit performance will be affected by the
> sound card output impedance - worth considering at least.
>
> Cheers,
> Paul M1PAF
>