Adapting an Aviation Headset for Amateur Radio
In my few years operating in the UM Amateur Radio Club shack I've usually used a well-worn Heil Proset Plus for contesting and when other members were in the room. The Proset has been with the club since at least 2006, and by now the headband is loose, the boom refuses to stay in position, and the earcups don't seal well; it's not the most comfortable when you're operating for more than an hour or so. I started looking for a replacement for myself on Ebay, and ended up scoring a great deal on a David Clark 40600G-17.
It seems the 40600G-17 was at once David Clark's top of the line model, or close to it. It features stereo audio input with independent volume controls, a noise-canceling electret microphone, and Electronic Noise Cancellation (more commonly termed ANR, for Active Noise Reduction). The headset came with a box which I soon realized was only a PTT interface. This meant I was missing the ~$100 power supply for the ANR circuitry. Additionally, I needed to interface the electret microphone properly for various HF rigs for a couple of reasons:
- Modern radios expect dynamic microphones, and don't provide power electret microphones need
- The noise-canceling M7A electret in the headset is amplified; its output is much higher than dynamic mics, so the signal must be attenuated before entering the radio
A helpful ham referred me to the June and December 2003 issues of QST magazine, both having information on interfacing Aviation headsets with amateur radio gear. Using the schematics from each article and from David Clark's website, I developed the simple circuit below:
With the switch in the above position, power is supplied from the 9v battery to the electret mic, and audio passes from the mic through the 10uF capacitors to the radio input. The 20kΩ potentiometer and 200Ω resistor create an adjustable voltage divider reducing the audio output level. With the switch flipped to the right, mic power is maintained and 9 volts is passed to the ANR circuitry in the headset. In the center position, mic and ANR power are turned off to save battery.
The entire circuit needed to be portable, so I chose a small extruded aluminum box made by Hammond Manufacturing. I mounted the headset connector (scavenged from the PTT interface) and the power switch to one end panel. A cable gland for the radio cable was installed on the other panel.
I built the circuit 'Manhattan style' on a small piece of PCB. All the wires and electronics, as well as the battery, fit securely inside the aluminum case. The short piece of cotton braid is for easy extraction of a dead battery.
Finally, I terminated the output with a 6-pin XLR so that I could fabricate adapter cables for various radios. An adapter to 3.5mm TRRS works with a Yaesu FT-60R. The UM club has a few Heil headset adapters, so a cable with a 1/4" stereo headphone plug and a 3.5mm microphone plug allows me to use the headset with our Kenwood TS-590.
I've made probably a few hundred contacts using the headset, and I've received numerous and frequent reports of strong, clear audio. Received signals are also easily understood, as the headphone's frequency response is tailored to voice. It's important to note the noise reduction of the headset's microphone does intrinsically add some compression, so its especially necessary to pay attention to the transceiver's audio processor settings to avoid over-compression or over-modulation.