Tuesday, August 02, 2011

K2 audio switchbox

My vision and steadiness of hand have improved to the point that I am able to build projects! Here is a picture of a switchbox I made for my Elecraft K2 a couple of days ago which allows me to switch the audio input source from the microphone to the computer sound card.

One of the niggling annoyances of the K2 is that it does not have separate inputs and outputs for voice and data modes. Most users carry out one of several published mods to obtain a fixed level audio output for the computer. Mine is tapped off the KAF2 filter module and runs to an RCA phono socket on the back panel. But for transmit most people just swap the mic lead and data lead over.

I used my K2 like that for years, though it wasn't a great hassle mainly because I hardly ever used a microphone anyway. But I finally decided to come up with a better solution - hence the switchbox. It was easy to make and I thought it would be within my capabilities. Here is a picture of the internals.

In one position of the front panel switch the microphone signal is switched straight through. In the other, the audio comes from the computer sound card headphone output via a potentiometer. Since taking that picture I added some resistance to the hot side of the pot so the control can be used over a greater range as the K2 mic input is very sensitive and needs some attenuation. As you can see, I don't bother with isolating transformers. I've made dozens - well at least a handful - of computer/radio interfaces in my time and I have never, ever found the need for one. YMMV.

To celebrate the restoration of my constructional skills I have ordered a W5OLF WSPR beacon kit. John Harper, AE5X, recently reviewed it and wrote that it took him 50 minutes from starting to build to receiving his first spot. So I figured it shouldn't be too difficult, though it will probably take me longer than 50 minutes. It would be nice to successfully complete a kit before I start undergoing the treatment that I expect will make me too tired to do all the stuff I am managing to do at the moment.


Casey Bahr said...

Cool! I hadn't even thought to look for such a mod as I don't use the mic anyway, but I think this is a good idea. What value of pot did you use?

VE3WDM said...

Great to hear that your feeling better and yes I have the same problem with my K2 with regards to a fixed audio. I have a small board that when finished will give me a fixed audio out.

Julian Moss said...

Casey, I used a 10K pot as that's what I had and added 39K on the hot end. A 5K or less would work better though as even with a low output from the computer there is still more than enough audio to drive the DK into ALC.

BX2ABT / PA2BX said...

Julian, I applaud you. Great that you still have the drive to do stuff and the positive attitude to make it happen.

Unfortunately wsprnet.org is down, but I am interested too in providing a wspr signal from over here in east Asia. Keep us posted.

73 de Hans

Anonymous said...


I'll look forward to seeing your results on both your blog and the WSPR page (once it's back online).

John AE5X

MrJoshua said...

Hi Julian

Great news that you're feeling a bit more human again.

I'm now looking into that WSPR kit :)


Josh - M0JMO

MrJoshua said...

Hi Julian

I'm receiving your WSPR transmissions on 30m :)


Josh - M0JMO

Julian Moss said...

Excellent! I was testing out the Wonder Loop on the workbench with a microbore copper element instead of coaxial cable. It seems to perform very well. I was actually running 0.5W - I erroneously thought the low power setting of the FT-817 was 1W. Not WSPRing now as the antenna was getting in the way.