[Elecraft] Re: Elecraft Digest, Vol 53, Issue 9

[Larry Phipps]

Thanks for your typically thorough testing and analysis, Jack. I would 
add one comment to your tests as it relates to my testing during the 
design of LP-PAN. You assume a 10K load Z for a typical sound card. I 
have now characterized half a dozen "professional" sound cards suitable 
for use with broadband "softrock" type receivers. In my testing I have 
found substantial variation of input Z vs. frequency, ranging from under 
500 ohms to almost 100K ohms depending on card and frequency. It should 
be noted that my test equipment leaves something to be desired when 
measuring Z at very low audio frequencies. I don't know that such loads 
would affect the distortion of the circuit being tested, but I suspect 
it could at certain frequencies.

I add a resistive load across the xfmr secondary to moderate the effects 
of the varying loads, and set a lower limit for cards with extremely 
high input Z. The optimum value changes from card to card, but I found 
approx. 2K ohms to be a reasonable choice with LP-PAN (using SP70 
xfmrs)  where flat response out to 100 kHz is a primary design goal. 
This, of course, would not be the case for a K3 audio output where 5 kHz 
would be adequate response. The added load could probably be lower in 
that case, but that may be counter productive if the sound card has a 
low Z in the 400-800 Hz range used for CW. These things are never 
simple, are they?

Larry N8LP



> Date: Fri, 05 Sep 2008 21:22:33 -0400
> From: Jack Smith <jack.smith at cliftonlaboratories.com>
> Subject: [Elecraft] Harmonic Distortion and Audio Intermodulation in
> 	K3's	LIN OUT Port
> To: elecraft at mailman.qth.net
> Message-ID: <48C1DB59.6030401 at cliftonlaboratories.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> I've completed a series of detailed measurements trying to understand 
> why my K3's LIN OUT port exhibits greater distortion and anomalies than 
> found in the headphone and speaker ports.
>
> Details are at 
> http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm
>
> What I found is that the 604 ohm resistor between U29's output amplifier 
> stage and the TTC-108 transformer primary winding forces the TTC-108 
> isolation transformer to operate in a regime where the performance 
> improving effects of a zero ohm driving source are not available. Hence, 
> the TTC-108's 0.5% THD performance significantly limits the LIN OUT 
> ports performance.
>
> I've also provided a  theoretical parts value change that should improve 
> the LIN OUT distortion and intermodulation. I have not implemented this 
> change in my K3, but the external circuit mockup I used for testing 
> strongly supports the change.
>
> I've supplied my measurements to the gurus at Elecraft for their 
> consideration. If my data and  recommendations make sense, I would 
> expect confirmation from Aptos, after they have time to think it over 
> and consider possible side effects.
>
> Jack K8ZOA
> www.cliftonlaboratories.com
>