Earlier this summer I purchased an Elecraft KX3 to use as a portable radio as well as my main station radio when paired with my Hardrock 50 amplifier. To facilitate both of these scenarios I had to make a couple of accessories to adapt my existing gear to the KX3.
The KX3 has several 3.5mm and 2.5mm jacks on its left side for the microphone, headphones, accessory connections, RX I/Q data, etc. Since the mic jack is the only way to get audio into the radio and I operate both voice and digital interchangeably quite often, I didn’t want to have to unplug the mic connection every time I changed modes. To solve this problem I built a microphone connection switch box. The box allows for the connection of a common 8-pin round microphone plug (in this case wired to the Kenwood/Elecraft wiring convention) as well as a 3.5mm jack for the output of the sound card I use for digital modes. The two are switched via a DPDT toggle switch and connect to the KX3 with a TRRS 3.5mm right angle plug. Both connections also feature 0.1uF DC blocking capacitors and the sound card side includes a 10:1 voltage divider which results in a 20dB reduction of the sound card output. This allows for much easier control of the audio going into the radio and helps prevent over-driving of the mic input. The blocking capacitors were intended to prevent any mic bias voltage from affecting my dynamic desk microphone, however, I found that some still got through. To solve this issue I made some macros for the KX3 to switch between desktop voice, digital, and portable voice configurations. This allows me to turn the microphone bias on or off and adjust the mic gain and compression depending on whether I am using the desk mic and foot switch in my station or a handheld microphone while portable.
The other accessory I made is for portable digital operation. I wanted to keep my setup as compact and light as possible, so I wanted to avoid having to take my SignaLink USB with me in the field. I found a good compact USB sound card, but I wanted to add some isolation from the radio as well as some signal reduction from the sound card output to the radio similar to what I did in my microphone switch box. For isolation I used ground loop isolators to reduce the possibility of interference between the radio and the sound card. To reduce the sound card’s output I used another 10:1 voltage divider to reduce the output by 20dB. In this case I only did it to the left channel since this is the default used by most digital programs and it is the same position on the 3.5mm mic jack as the microphone input on the KX3. Fortunately I was able to install the voltage divider inside of the isolator case itself by carefully prying apart the two halves. The voltage divider resistors were then soldered directly to the isolator circuit board and fit inside the case with plenty of room. The isolator case snapped back together when I was done modifying it without any glue necessary.
So far my KX3 accessories have been performing well and they make an already great little radio that much easier to use.