This is an English translation of an article that appeared in 2018 below.
Translation was performed with the help of ChatGPT.
Abstract
This article describes the process of creating a conversion and power supply circuit for the so-called NATO plug, or the U-174/U connector, specifically for the David Clark H10 headset, which is employed in aircraft and helicopters, including by the U.S. military. The U-174/U connector was developed for aviation and military applications, providing output, input, and power supply through a single connector. The U-174/U connector uses a unique method for providing 8-15V power supply to operate the microphone. In this article, we explore powering the headset using an external DC power source and a 48V phantom power supply. The conversion circuit was successfully designed and built, enabling the headset to be used with computers and smartphones.
In this article, we will discuss the headset's testing process and the development of the conversion circuit, which aims to utilize phantom power. We will also introduce methods for converting the headset to a stereo mini plug, suitable for use with PCs, and a 4-pole mini plug, suitable for use with smartphones. The production process is generally described in chronological order; for information on the actual circuit construction, please refer to the "Implementation with DC Power Supply" and "Implementation with Phantom Power Supply" sections.
- Abstract
- David Clark H10-36
- Unboxing & Operational Test
- U-174/U(TP-120)
- DC Power Implementation
- Implementation using Phantom Power
- Conversion Box
- Summary and Future Works
David Clark H10-36
I purchased the David Clark H10 headset, which is officially adopted by the U.S. military. The model I bought is the H10-36, mainly used in helicopters. *1
Although I am not a military enthusiast, I wanted to use this headset for its original purpose. The goal is to use this military-grade headset, designed for extreme environments, in everyday situations. To achieve this, the headset's plug must be converted to a plug commonly used by consumers.
Unboxing & Operational Test
This product can be purchased in Japan, but it is expensive. Occasionally, it appears on online auctions. I bought what seemed to be an unused item for 6,250 yen. Included with the product were the original box, headphone hook, about 10 sheets of paper containing instructions, specifications, and circuit diagrams, separately sold ear pads, and strangely, an unopened pack of playing cards. The headset itself showed slight mold on the ear pads, but the product tags were still attached, suggesting it could be deadstock or a collector's item.
Moving on to the main unit, there is a volume control on the left ear pad and a semi-fixed volume control near the microphone. The headphone design is an around-ear type, boasting an overwhelming noise isolation different from typical around-ear headphones.
First, I want to perform a simple operational test. The device uses a peculiar connector, so I will use a test cable connected with alligator clips to a phone plug.
It can be made from materials lying around, and having one on hand can be very convenient. A simple circuit diagram is included in the instruction manual.
According to this, the pin assignments for this plug are, from the tip: Mic-(T), Speaker+(R1), Mic+(R2), and Speaker-(S). I will connect according to this and test.
First is the speaker test. Clamp S and R1, gradually increase the volume on the headphone amp. Sound comes out, but it is not very loud, perhaps due to the high impedance. The volume knob on the side of the headphones works without any issues.
Next is the microphone. Clamp T and R2, plug it into the mic preamp, and gradually increase the volume. The volume is turned all the way up, but no sound is produced, only a faint continuous noise. Is it broken? A bit of noise enters when turning the semi-fixed gain adjustment volume at the base of the microphone. It seems to be connected somehow.
I carefully observe the microphone head. The diaphragm is larger than usual for a headset, so it might be a condenser mic, but a dynamic mic cannot be ruled out. However, if it were a condenser mic, it would require external power unless it is an electret type. I start to suspect external power is needed and consult the included documents. As expected, there is a description about microphone power supply.
It says 8-15V power supply is needed, but it is unclear where to apply it. I try applying phantom power between Speaker- and Mic+ as commonly done, but the situation does not change. After looking through the documents, I could not find useful information, so I turn to the internet. I find only one piece of valuable information.
AMPLIFIED DYNAMIC MICROPHONE, MODEL M-1/DC & M-1A
http://skylife.co.jp/flyingdog/19507P21.pdf
This is a specification sheet for David Clark microphones. It contains a test circuit, which is not in the documents I have and is not on David Clark's website. I modify the circuit slightly based on available parts and use a 12V DC power supply to create a breadboard circuit.
Upon turning on the power and connecting it to the mic preamp, the sound finally comes out. It seems that just converting the connector is not enough, but it can be used if a circuit is assembled. For now, I will try to deal with this peculiar connector.
U-174/U(TP-120)
Helicopters used in NATO countries adopt the U-174/U(TP-120) plug, also known as the NATO plug, and the H10 headset (depending on the series) also uses the U-174/U. The U-174/U and TP-120 appear to be similar products with different standards, but no differences can be seen in the data sheets. Both were established around the 1970s.
U-174/U
https://www.mouser.jp/datasheet/2/18/amphenol_05022017_tp-101-cp-1156596.pdf
TP-120
https://www.mouser.jp/datasheet/2/18/TP-120-972911.pdf
As expected, this standard is relatively popular in helicopters, so there are converter jacks available. However, due to low demand, they are incredibly expensive. They are not available in Japan and cost around 10,000 yen on US online shopping sites. There are also some reports that they do not work well. In that case, the only option is to make one yourself.
The jacks compatible with the U-174/U(TP-120) plug are the U-92A/U (or TJ-102) and TJ-101. The U-92A/U and TJ-102 are the same product, but the TJ-101 has a longer, thinner shape.
U-92A/U(TJ-102)
https://www.mouser.jp/datasheet/2/18/TJ-102-972980.pdf
TJ-101
https://www.mouser.jp/datasheet/2/18/tj-101-972892.pdf
Basically, either one will work, so just buy one of them.
Now, these are special connectors, and it is challenging to find a store that sells them at a reasonable price. Casual browsing reveals that they are being sold for over 5,000 yen, which is quite a rip-off. I bought mine from Mouser, which I have been borrowing data sheets from.
Mouser Electronics
https://www.mouser.jp/
While it may not be as well-known in Japan, Mouser is a major electronics retailer, especially in terms of the number of items they carry. They have a Japanese subsidiary and a Japanese website, so even though it is an overseas shipment, there are not many barriers to purchasing. If you spend more than 6,000 yen, shipping is free, which is a nice bonus.
This time, I purchased the TJ-101 made by Amphenol Nexus Technologies. (It seems that no other company makes such a connector other than Nexus.) The price was about 1,600 yen. It feels incredibly cheap compared to other sites, but it is actually a reasonable or even slightly expensive price.
TJ-101
https://www.mouser.jp/ProductDetail/Amphenol-Nexus-Technologies/TJ-101?qs=4z3o3tOTJCGgUfnzq2dYkg==
It arrived from the United States in just about four days. That was fast.
DC Power Implementation
Now that the connector has been purchased, it's time to build the circuit. I created the following circuit based on the schematic in the specification sheet mentioned earlier.
As this is just a small circuit, I did not bother launching LTSpice, and instead drew it by hand, though it may be difficult to read. As seen in the diagram, a DC power supply of 8-15V can be used, such as a 006P battery or an AC adapter. I recommend the latter. I have also included the pin assignments for three types of connectors: stereo mini plug, phone plug, and 4-pole mini plug. Note that the pin assignments for 4-pole plugs are not standardized and may vary between different models. Simply create the circuit as shown on the left side of the dashed line, and assign it to your preferred plug as shown in the left diagram. This circuit will allow for basic line-level signal input and output.
Later, I plan to implement it using phantom power, so I created a test circuit on a breadboard. Unfortunately, I forgot to take a picture of the circuit, but it did produce sound. This circuit should work for most people who want to use it with their computers or smartphones.
Implementation using Phantom Power
However, DC power supply implementation requires a battery or a noisy AC adapter, which can be problematic. Thus, I decided to implement it using phantom power.
Phantom power is a high-quality microphone power supply method commonly used in studio equipment. It uses a 3-pole plug to apply a no-load voltage of 48V between the ground and the audio transmission path. It is not technically a constant voltage source but rather a low-current source that will drop voltage when loaded. *2 While phantom power applies voltage between three poles, the U-174/U applies voltage between two poles. Therefore, a mechanism that extracts phantom power from the XLR connector while separating the audio signal and reducing its voltage, and then returns it to the U-174/U connector, is required.
To be honest, I have no familiarity with analog circuit design, so I simply created a circuit based on what I found online and compared it with the DC power supply circuit above.
The resulting circuit may not be perfect, but it should work. The circuit seems a bit sloppy, and I'm not entirely sure about some parts, but it should be something like this. I made the circuit on a breadboard and it worked, so it should be okay. The loaded voltage was about 8.5V, which is within the operating voltage range.
Conversion Box
The circuit is now complete. Next, I will create a conversion box to make it more user-friendly.
As all of the plastic cases I had were the wrong size, I bought Sakuma-style drops at the supermarket instead. I ate them all first, licking them from time to time, and they were surprisingly gone in about three days. Then, I drilled holes in the steel can that remained, and wired it up randomly.
I have started working on a circuit using a universal board, with XLR connectors for the microphone output and phone jack connectors for the headphone input. I used the following parts:
- NEUTRIK NC3MD-LX (XLR connector)
- REAN/NEUTRIK NYS2162 (phone connector)
- CANARE L4E6S (cable)
- Amphenol Nexus Technologies TJ-101 (U-174/U connector)
- Nichicon FineGold 50V 22uF *2 (electrolytic capacitors)
- 6.8kΩ and 470Ω resistors
- and a scrap of universal board.
The excess use of NEUTRIK and Nichicon FG is just a hobby of mine. Since the wiring cannot be done later, I passed the cable through before soldering.
Once the wiring was done, I assembled the panel. The hole for removing candy matches the inner diameter of the XLR connector, so I can easily insert and remove the phone connectors and circuit from there. After insulating the circuit, I put it in the can and arranged the panel. Finally, I inserted and secured the XLR connector.
It is now complete. I connected the microphone and headphones to my mixer and turned on the phantom power. It worked without any problems, and there was almost no noise.
Summary and Future Works
In summary, I was able to create a conversion circuit for David Clark's H10 and its U-174/U connector, as well as a power supply circuit. Unfortunately, I don't have anyone to chat with, so I haven't been able to use it practically, but I was able to talk using Skype's audio test service and this headset.
In the future, I plan to consult experts and work on improving the circuit, as well as measuring the microphone's frequency response. However, there are no experts around me, and I currently lack the motivation to do the latter, so it may be some time before I can tackle these tasks. Being alone is tough, no matter what you do.
*1:It should be noted that, although the H10-76 and H10-66 models are more widely used, I could not confirm the use of the H10-36 in the U.S. military based on my research. For the purposes of this article, we will assume that the H10 series is being used.
*2:For more information, please refer to http://www.op316.com/tubes/balanced/bal3.htm.
