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.
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