The 10-4 QRM Slicer KX-3 Microphone

Note:  This article was written on Oct 3, 2012 and was posted on the old kr7w.org website- which went QRT.   Recently a fellow QRP enthusiast asked me where I got the 10-4 QRM Slicer Mic shown in some of my SOTA Blog entries.  "I built it", I said.  The mic seen in the SOTA blog photos is red... which is Ver 2 of the QRM Cutter mic for the KX3.  The red mic is exactly the same design this article from the old website:

Just recently retiring... I was feeling pretty frugal when purchasing my KX3 from Elecraft.  "$54 for a small microphone?!"  I quipped.  "By God! I could build for much less"... and it'd be just as reliable. and useful.

Inside of old hamfest CB mic.

Photo shows a $1 old CB microphone found at a ham radio swap-meet.  Its dynamic mic element did not have enough audio output to drive the KX3.  After experimenting with different mic elements, a Radio Shack Electret Microphone Element was selected.

Deciding: Each Electret mic element was plugged into the PC's mic input where my voice calling CQ or Test was recorded, then compared to determine which mic best resembled a Dynamic element that produces a clear crisp sound- like a Heil HC4 contest mic element.  

After a couple of mic element candidates were selected, the computer audio editing app, Audacity's EQualizer effects were adjusted make the recording as crisp sounding as possible.  Then those settings were emulated in the KX3 XMT audio equalizer settings to be as close as possible. 

The old CB microphone mic metal weight was removed.  The plasting housing that the old mic element sat in was closed off with a piece of blank printed circuit material.  A 1/16" hole was drilled in the PCB material. This is where your voice/audio will pass through to the mic element.  The mic cartridge was hot melt glued to the PCB, centered to the hole. A pigtail wire was soldered to the mic element which passes through a hole drilled in the housing.  All needed parts were reassembled. How heavy is it?  ~3 oz. now.

The PTT switch in this mic is DPDT switch. Only one set of contacts were used to operate PTT.

The Mic schematic in the KX3 manual shows Ring 2 as the UP/DN control wire.  This feature is not used in this project.

The cable in the photo below came from a 10 ft audio/video jumper from Ebay.  It was advertised as a 'Phillips' cable.   With the cable cut in half, ohmmeter continuity testing the TIP / Ring 1 / Ring 2 / SLEEVE colored wires to know where to connect to in the microphone.

Note: It is not necessary to use a 4 conductor cable. Another mic was built that used a cut up Radio Shack T-R-S 1/8" cable. In this case, use Tip for Mic, Ring for PTT, and Sleeve for GND return.  It works just the same.  In this case- a 4 conductor cable was used because it has a right-angle plug that doesn't protrude too far from the KX3 housing... also the option to add Push Buttons KX-3 like pushbuttons can be added if desired. 

After wiring was completed- The PTT function was continuity tested with an Ohmmeter then again plugged into the PC and audio tested one more time. Then was disassembled and spray-painted Home Depot Orange.  It took a couple of coats, then a coat of clear lacquer to give it BLING.  The 10-4 label is the 'model number'.  Note:  I have never been a CBer.

Back to the ORANGE color... After using a Yaesu FT-817 whilst out SOTAing or backpacking... I was always stepping on the black microphone.  This orange mic is hard to miss when it is laying in the duff. 

KX-3 Testing:  The model 10-4 was plugged in with RF power output adjusted to Zero. Listening to myself with headphones and using the KX-3 MON function- it sounded pretty good- especially with the compressor turned up to 16.  Note:  This Radio Shack electret element has high output. The KX3 mic gain control was to  approx 7 for proper ALC action.

During a SOTA activation I someone finally answered my SSB CQ.  Even though I sound like Kermit T Frog, I was told "Great audio Old Man"

Lessons Learned... 

> The hole in the mic housing must be very small.  Take a look at your mobile radio mics... there's a small hole to diminish the amount of wind from your voice hitting the mic element.

> If your re-purposed mic has foam rubber and a metal screen in front of the mic element... be sure to reuse it. This helps keep the wind from hitting the element.

> If the mic element enhances Ss or the ESSSSS sound when you speak...  a low pass filter or a filter that shunts highs to ground may need to be included. Whilst experimenting with this, it became a hassle to get it just right. After finally settling on the Radio Shack mic element and adjusting the KX3 audio EQualizer settings... it turned out OK for me. Google 'Sibilance Filter' to find out more.

>  This mic can be plugged in your PC to make audio files for SSB contests to add to your logging software.  Audacity software can be used to enhance the audio.  "CQ Sweepstakes"

> Hot Melt Glue Gun is my friend on many DIY occasions- be careful.

10-4? Over and Out... Best Regards,

KR7W