Transmit Audio and Compression with the Elecraft KX3

This is long, but it combines multiple recommendations from the KX3 and Elecraft mailing lists into a single procedure.

First, update to the latest KX3 firmware. There was a new compression algorithm in 1.50 and fixes in 1.61 and 2.30.

Then, get your KX3 manual. If you can’t find a paper copy, download the latest KX3 Owner’s Manual. You will be looking up a few menu settings.

The audio adjustments are done in separate steps:

  1. Transmit audio equalization (TX EQ).
  2. Microphone bias config.
  3. Microphone gain.
  4. Compression level.


Adjust TX EQ. The KX3 manual explains equalization settings under RX EQ, which works the same as TX EQ. See the section titled “Receive Audio Equalization” (page 20).

About half of the energy in speech is in the low frequencies, but that only adds 5% to intelligibility. So cut those and put all the power into the high-effectiveness frequency bands.

Several experienced ops gave similar recommendations for TX EQ. These settings are from Jim Brown (K9YC) and I used them without any change.

Freq. Band 50Hz 100Hz 200Hz 400Hz 800Hz 1600Hz 2400Hz 3200Hz
EQ -16dB -16dB -16dB -3dB 0 0 0 +3dB

The 3.2kHz boost is optional, use it if it works for you. I did not.

This chart shows this equalization and compares it to the well-known Heil recommendations for Elecraft. I chose the settings for Heil’s wide-range electret element (iC), since that seemed most similar to the CM500 electret mic. For easy comparison, I subtracted 4dB from the Heil settings. This normalizes them to 0dB in mid-range and uses the KX3 maximum cut (-16dB).


Mic Bias

If you are using a Yamaha CM500 headset or other mic that gets mic bias from both the tip and the ring, you can do one of two things for better transmit audio.

If you don’t know whether your mic takes bias from ring, turn off mic bias. If you still have audio from the mic, it does take bias from the ring contact.

The best option is to use a $6 stereo splitter to disconnect the logic bias from the mic. For details, read my earlier post on better audio from your Yamaha CM500.

If you have one of these mics and don’t have a splitter, turn OFF mic bias. This will increase the mic output. Ring on the mic connector is always biased by the KX3 PTT Up/Down logic. That logic supply is noisier than the mic bias supply, so you may want to adjust TX GATE to 1 or 2. That should kill any low-level buzz between words. This is not needed for the Elecraft MH3 mic.

Mic Placement and Gain

Set the TX power to 0.0. Turn off compression (KX3 manual, page 14).

Choose your mic position. Many people like to position a microphone just off the corner of the mouth. This can reduce pops from plosives (P, B, and T). It also reduces breathing noise so you won’t sound like Darth Vader on the air.

Adjust mic gain as described in the KX3 manual under “Basic Voice-Mode Setup” (page 15). While speaking, adjust mic gain for about 5 ALC bars (see below). Try to get four bars solid and one bar flickering. It is harder than it sounds.


After all that, turn on compression and find the right amount for your voice.

To evaluate compression, use the two digital voice recorder (DVR) buffers and headphones. Record into a buffer then play it back for evaluation. This is better than listening simultaneously with TX monitor (that includes bone conduction) or on-air evaluation (with even more variables).

Read about how to use the DVR in the KX3 manual under “Digital Voice Recorder” (page 21).

Record a message into one DVR buffer with no compression. I used “CQ SOTA Kilo Six Whiskey Romeo Uniform” with repetitions. That has plenty of sibilants and hard consonants (the “K” and “X”) to make problems with too much compression. I recommend including some K’s, T’s, P’s, and S’s in your test message.

Then record the same message with a medium level of compression in the other buffer. The KX3 manual suggests starting with a level from 1 to 10. Make notes. Listen to the two buffers and choose the winner. Try a different level for the other buffer (make notes). Repeat with different compression levels until you are satisfied.

You may want to come back and recheck in an hour or so. My ears got acclimated after several rounds of A/B comparison.

A final on-air check is a good idea, too.

Then remember to turn the TX power back up!

Final Notes

I ended up with mic gain set at 51 and compression set at 20. Any more compression than that and I heard too much distortion. This was with MCU firmware 2.33.

When you change microphones, turn off compression and reset the mic gain. Then turn compression back on.

If you want to experiment with TX EQ settings, use the DVR A/B comparison method, then recheck compression.

Special thanks to a pair of experienced hams who shared their knowledge. I always read their posts.

  • Lyle Johnson, KK7P: He suggested using the DVR to evaluation compression settings. He also worked out the interaction of the logic bias and mic bias and posted the original detailed analysis and workaround to the Elecraft KX3 mailing list.
  • Jim Brown, K9YC: An experienced contester and audio professional, he is a prompt and patient elmer with help on mailing lists, conference presentations, and papers. It is all published on-line and I recommend you read and study his work.

Better Yamaha CM500 Audio with PTT on Elecraft KX3

With a $6 cable, you can get cleaner microphone audio from your CM500 headset and add a jack for a PTT switch.

The Yamaha CM500 is a very popular headset for amateur radio use. It is rugged, effective, and affordable — $60 at B&H Photo and $55 at Amazon. B&H has a much more accurate description of the product.

The CM500 electret boom mic can be powered by the Elecraft KX3, but it takes bias from both the tip and the ring on the plug, like many headsets and mics designed for use with computers.

If you don’t know whether your mic takes bias from ring, turn off mic bias. If you still have audio from the mic, it does take bias from the ring contact.

The KX3 provides mic bias on the tip, but also provides a (noisier) logic bias on ring. The logic bias is for the PTT/UP/DOWN buttons on the MH3 hand mic and cannot be turned off. The workaround is to turn off mic bias to get more output from the CM500 mic, but this leaves a bit of noise on the audio.

I was about to build an adaptor to clean this up when I realized that an off-the-shelf stereo to mono splitter would do the job! This disconnects the logic bias from the mic connector, providing a low-noise bias supply. The other arm of the splitter is now available for a PTT switch. Bonus!

Plug the CM500 into the right channel (tip) arm of the splitter. Plug a PTT switch into the left channel (ring) arm of the splitter. I like this splitter because the arms are labeled “tip” and “ring”. Also, the “red is right channel” works for “red is transmit” in my head.

KX3 CM500 adaptor

What about PTT? Around this time, my beloved Grado SR-60 headphones died after fifteen years of fine music. I decided to repurpose the excellent cord for a PTT switch (the right ear driver failed, so the cord was fine). I found a hand-held pushbutton for $15 and added that to the project.

Here is the full setup with headset, PTT, and the KX3, of course.

KX3 CM500 overview

For my music headphones, I upgraded to the Grado SR-225e. Very nice.

Many headsets designed for computers have an electret mic that takes bias from tip and ring, so this may work for those, too.

Special thanks to two relentlessly inquisitive hams who shared their knowledge. This would not have happened without them.

  • Lyle, KK7P: He worked out the interaction of the logic bias and mic bias and posted the original detailed analysis and workaround to the Elecraft KX3 mailing list. I always read Lyle’s posts.

  • Fred Cady, KE7X: Author of The Elecraft KX3 Book, where I verified that ring2 (logic ground) and sleeve (mic audio ground) on the mic connector were wired together inside the rig. That was the last bit of information that made this hack work. Otherwise, I’d be wiring a custom TRRS adaptor.

Update: The original version of this article mentioned the Koss SB40, which looks very much like the Yamaha CM500, but costs less. The Koss has a dynamic mic instead of the electret mic on the Yamaha. I asked on mailing lists, and some operators are not pleased with the quality of the dynamic mic on the Koss, so I’ve removed those mentions from the article. If you look closely at the picture with the mic plug, it says “KOSS”, which would suggest that Koss makes these for Yamaha, but with an upgraded mic.

Working All of Palo Alto with 5 Watts

How high can you get your antenna? I used a roll-up J-pole antenna on an 18′ collapsible pole supported by our patio table and could be heard across the city using only my 5W HT.

Every Monday night, I check in to the Palo Alto ARES/RACES training net. I can almost always hear net control, but they can rarely hear me. I guess it is good practice relaying traffic, but it makes it hard for me to contribute substantially.

5W Yaesu VX-6R connected to antenna

Last night, I realized that I could use the umbrella hole in our patio table to support an antenna mast! It worked really well. I got great signal reports from across the city. I was clear enough that I got a report of low audio, so we tested a bit more mic gain.

antenna mast supported by patio table

The pole is a Greenlee 18 foot fish pole that was left behind after some electrical construction. It weighs 2.5 pounds and collapses to under 28 inches long. The sections are friction fit and designed for pulling, so they won’t support a lot of weight.

The antenna is an Ed Fong dual-band roll-up J-pole. You can build your own from Ed’s instructions (downloadable from his site), but he builds them better than I could for only $28. I bought mine at our local Ham Radio Outlet in Sunnyvale.

IMG 9845 crop

If you have a patio table, grab some PVC pipe and get your antenna off the ground. It really helps.

Bacon Jerky!

Bacon is magic in food but a problem on the trail—refrigeration, skillets, grease, etc. Shelf-stable bacon makes tons of trash with strips wrapped individually. Bacon jerky to the rescue!

IMG 1093 crop

On a recent visit to Walgreens, I spotted bacon jerky and immediately bought it. It does not seem to have a lot of preservatives. It isn’t overly salty (beyond its bacon-ness) or smoky. It should be eaten within three days after opening the package. With Scouts, it would last three minutes, so that is not an issue.

$5.99 for 3 ounces. That is a decent amount of cooked bacon, so a fair deal.

When my dad and I backpacked in the Pecos Wilderness, we took Wilson’s bacon bars to crumble into our morning oatmeal. The bacon bars disappeared decades ago, but we finally have a good replacement.

I have only seen this as a Walgreens brand. It will probably spread, but pop into your local Walgreens and give it a try.

Petzl Zipka Headlamp

When my first Petzl Zipka headlamp finally broke, I bought another one. That is a sure indicator of the right gear for my style. Enough light, light enough (69g), and it fits in my pocket.

IMG 8840

The Petzl Zipka is a different headlamp. Instead of a wide elastic headband, it has a thin, retractable Dyneema cord. The cord disappears into the the headlamp. It seems like the skinny cord would be uncomfortable against your head, but it never has been for me.

One hint–when taking it off, pull the cord out farther, then lift it off your head. If you just pull it off, the cord will wind up bits of hair into the spring-loaded reel. Ouch!

It is also easy to put on non-head objects. I’ve put it around my wrist, hanging on the underside and lighting my cooking. I’ve put it around the top of a water bottle to elevate it as a lantern. I’ve hooked it to a clip on my tent as a reading light.

The button behavior is pretty straightforward. I don’t find myself confused and swearing at the flashlight trying it to get stop flashing or whatever. That problem is surprisingly common with modern LED flashlights.

It takes AAA batteries, which isn’t optimal, but they last a really long time, so whatever.

I’m pretty sure I’ve bought three different versions of this headlamp. One of them broke, I bought one for my wife’s pack, then I got tired of moving mine back and forth between my backpacking gear and my amateur radio emergency communications gear. Will I find an excuse to buy another one? Probably.

They keep tweaking the name, adding a “Plus” or a “2” or both. For the latest version (newer than my red one), it is back to just Petzl Zipka. They are also back to the Bondi Blue color, which is nice because the shape reminds me of the original iMac.

Whatever, buy the latest Zipka. It has always been around $40.

As the sun goes down, I drop my Zipka in my pocket. When I need light, I’ll have it.

BSA Youth Protection and E-mail

Our troop has an e-mail alias that can be CC’ed for any communication between an adult and a Scout. To satisfy the “no one-on-one contact” rule, we respond to a Scout’s e-mail CC’ing yp@our-troop. The mail goes to the Scoutmaster and the Committee Chair.

I do this all the time, even when working at a District level, approving Eagle Scout service projects.

The Boy Scouts of America Youth Protection Guidelines and Social Media Guidelines are online.

Supermarket Backpacker

I came for the flannel, but I stayed for Harriett. I didn’t see this book in 1977, but I’m glad I found it now.

I bought a used copy of Supermarket Backpacker by Harriett Barker and I love it. This sentence starting at the bottom of page one may be the truest thing ever written in a cookbook: “Don’t forget that water is the only thing you can cook really well when backpacking in the high mountains.” I have proved that it is true in the flatlands, too. Ask the other members of the Raccoon Patrol.

How many cookbooks have an intro with more information than the four pages in this book? Not many. Perhaps more trail cookbooks should be written by “an avid outdoorswoman as well as a trained home economist.”

For the perfect icing on the cake, a friend wrote haiku for each chapter.

Backpacking for days.
Found! New evidence of man…
Plastic container.

Also, lovely pen and ink illustrations from two other friends. We should all be so lucky in our friends.

This book has a huge amount of information. Brand names, vegetarian meals, kosher meals, a Mexican sopa seca recipe. You could camp for years on just this cookbook.

One more quote from page 86, in the dehydrating section:

A good rule to follow when making any leather…if it tastes good in the blender, it will taste twice as good at camp. Before drying, sample and make additions until the combination pleases you.

There is only one thing that makes me sad from this book. We can no longer buy a Wilson’s bacon bar. Dang, I miss those.