Several times we received questions about how to hook up a flight helmet and mask/boom mike to the Personal Computer. Especially the die-hard flightsimmers like to hook up the real gear to their virtual world. Therefore I decided to write an article about how to make an adapter cable for this purpose. 

The most commonly used earphones in military flightgear is the H-143 model earphone. The earphones are wired in parallel which gives a total impedance of 8 ohms. The biggest disadvantage of this system is that the sound produced by the earphones are Mono because both earphones receive the same signals. Also the quality of the earphone elements are good for communications but don't give a Hi-Fi quality.

Another problem is the signal coming from the microphone. This signal is very weak and should be amplified. You will also find a very simple but working microphone amplifier schematic which will improve the signal of the helmet microphone so it can be used in combination with the PC.

OK, lets get started! First we need to know what materials we need to built this adapter. 

• 2x mini-jack, mono (this are the mini jack plugs what goes into your PC soundcard but in mono instead of the stereo version which is commonly used. Difference is that the mono version only have two rings instead of the three rings)

• Speaker cable (don't use to thick wires, 20AWG is good enough)

• U-92A/U plug (or similar, see drawing below. This plug fits the U-174/U or U-93A/U plug which is commonly used at flight helmet and oxygen mask commcords)

I have done some measurements on the U174/U plug on one of my helmets. I have made following conclusion:

Contact number:

1. + microphone

2. + earphones

3. GND microphone

4. GND earphones

The contact numbers are corresponding to the numbers of the solder contacts in the U-92A/U plug.

With this information we're able to make the adapter cord:

• Cut two pieces of speaker cable of the same length (length you prefer) and solder both of the mini-jacks on one end of each cable.

• Now you have two dual wires, both with one plug. Mark one of these cables on both ends with a marker or a piece of tape. Just for recognition which cable is for what. 

• Solder the microphone wires on the contacts of the U-92A/U plug. Ground on 3 and + on 1

• solder the earphone wires on the contacts of the U-92A/U plug. Ground on 4 and + on 2

Now you have an adapter cord with an U-92A/U plug which fits your helmet with two dual wire cables coming out. At the other end the two jack plugs which fits the soundcard of your PC.

To Amplify the microphone signal you need to place a small amplifier in the mic.line (contacts 2 and 4 in the U-92A/U plug). The schematic below shows a simple but working amplifier. The amplifier works between 6 and 12 Volt so a simple 9Volts battery will work. I don't have done tests to this design but it seems to work. 

The parts list for this low-cost amplifier:

Semiconductors
1x BC548 NPN transistor
1x BC558 PNP transistor
Resistors (0.25W 5%)
1x 100 Ohm
3x 2.2 kOhm
1x 22 kOhm
1x 10 kOhm
2x 100 kOhm
1x 150 kOhm
Capacitors
1x 0.1 microF MKT Polyesther
1x 1500 pF MKT Polyesther
2x 10 microF Electrolytic capacitor
1x 22 microF Electrolytic capacitor
1x 100 microF Electrolytic capacitor

If you have further questions or suggestions, please contact me by sending an e-mail to sj.scheffers@home.nl