[Elecraft] Re: K3 Audio

[Tom W8JI]

> It's a 600 ohm transformer, rated for 7dBm max (about 1.7 
> Vrms across 600 ohms).
>
> Bob NW8L

But that doesn't mean it distorts or "saturates". It is 
simply a rating.

My K3 for example on a good Agilent  selective level meter 
starts to distort at -5dB reference to 1 milliwatt into 600 
ohms. That's a front panel setting of about 50 on the line 
output level adjustment.  At -5dB the second harmonic is 
down 58 dB (absolute level of  -63 dB mW ). At lower 
settings it gets better.

While I have not looked inside the radio (and won't bother) 
the distortion has all the characteristics of a mismatched 
output device because it clips at about the same level 
regardless of frequency. If it was that commonly spoke of 
but elusive to find "core saturation" the point of 
distortion should change with frequency. That's because the 
flux level should change with frequency for a constant 
voltage across the primary as the reactance changes with 
frequency.

When I reduce line output setting to 25 (whatever that 
means) the distortion is too close to noise floor to measure 
accurately.

I would think -10dB below 1 mW, while a bit low from 
commercial standards, is good enough for any properly 
adjusted ham radio sound card or audio system. I'm pretty 
fussy about things that matter, and I think the audio 
distortion shortfall (at least from what  measure) is 
greatly over rated. It isn't out of line with anything else 
I use, and is a lot better than some things I won't use.

This is a sample of one unit, but it looks good to me when 
the level is kept about -10 dB 1 mW into 600 ohms. I can't 
see a problem in a single tone harmonic distortion test.

73 Tom