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 left channel (tip) arm of the splitter. Plug a PTT switch into the right 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.