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.

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