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# DC power to audio jack? Answered

I have a pair of airline headphones that have a standard 3.5mm jack for the audio but require power to an additional 2.5mm jack in order to work. I'm looking to use them with a desktop PC so thought about getting the power supply from the USB but am a little confused about how I can get it there, I've had a look around on ebay to see if there was someone that was selling a 2.5mm female to USB jack but havent managed to find what I need.

Do these things even exist? Or would I need to get myself a 2.5mm female connection and wire it up to a USB lead myself?

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## Discussions

To sort of summarize, what I wrote here yesterday.

The hard part of this question was figuring out what kind of headphone plug you've got, out of the half a dozen different "standard" kinds that exist for "airplane headphones". But I think we've narrowed that down, and the secret special name for your headphone plug is, "ARINC C1", and the for the pins that supply power, the voltage is nominally 12 volts DC.

That pdf I linked to previously, with pictures of those various ARINC plugs and jacks, I link again here,

and a picture of just the ARINC-C1 is attached below, with the picture-circuit-diagram I drew.

In response to your question, "Do these things [ARINC C1 to desktop computer adapter exist?", I am guessing the answer is "No"

So, that means you have to build one. As part of the design for this, I think there should be some kind of current limiting thing, placed in series with the 12 volt supply. The reason why I want a current-limiter there is because if the computer power supply gets shorted, even if only for an instant, it decides to turn itself off completely. I am guessing a 50 ohm resistor will work, and that wild-ass guess is
based on the assumption that the headphones only want a small amount of
current, only about 20 mA, or so. Or, in other words, the voltage drop across that resistor is 1 volt, for every 20 mA of current, so the change in voltage seen by the load in the headphones is small; i.e. it sees 11 volts instead of 12, at 20 mA. 10 V instead of 12, at 40 mA, and so forth.

It turns out I have a bunch of the parts, for to build this adapter, so for the sake of illustrating what they look like, I took a picture of my suggested circuit diagram, with some of these parts thrown on top.

I don't have the 2.5 mm mono phone jack. I am guessing the inline style would be appropriate, and I found a picture of one those, here:

http://www.digikey.com/product-detail/en/cui-inc/M...

Have you tried probing your exotic headphones using an ohmmeter, or equivalently, using a digital multimeter set up in resistance measuring mode?

I mean, you can even wear the headphones on your ears while touching the pins on the plug with the multimeter probe, so that you can hear the test current pulsing through each speaker coil.

When I try this with an ordinary pair of headphones, that I found on my desk here, I hear, "Scritch, scritch, click, click," in the right speaker when I have the ohmmeter leads connected across the right speaker coil, and the meter sees about 20 ohms of DC resistance.

Similarly for the left speaker, I hear, "Scritch, scritch, click, click," in the left speaker when I have the ohmmeter leads connected across the left speaker coil, and again the meter sees about 20 ohms of DC resistance.

I mean this is a simple trick, that tells me which wire goes where, for ordinary headphones.

I am somewhat skeptical about the part of the story where you say your exotic headphones require power in order to work. Moreover, I do quite not grasp how you made the leap from headphones requiring power, to knowing the kind of power they want is 5 volts DC, like from USB.

I dunno. Seems like all these questions could be answered if there was a way to see inside these headphones, and discover which wire goes where.

Regarding finding adapters: If you know which wires go where, you can make your own adapter, or instead cut off the exotic plug and replace it with a more common one.

When i first tried the headphones through a standard jack there was no sound, I discovered that they needed power to operate by connecting batteries to the 2.5mm jack. You are correct that I've just plucked the 5v out of thin air but USB seems the easiest source of this power.

I assume they need power due to noise canceling, what type of connection do nois canceling headphones generally use?

I don't know. I've never seen a pair of noise canceling headphones, in real life.

I am trying to find pictures of the thing, er plug shape, you described, using Google(r) Images.

I think I might have found it, with some help from dumb luck, in a picture from a US patent application, specifically, US20030153205, uh, here

http://www.freepatentsonline.com/y2003/0153205.htm...

Maybe there are some clues in this document. Haven't read it yet.

I think your plug might be "type C1", shown in Figure 8.

The smaller, 2.5 mm, plug has two conductors: TS. Tip is labeled "PWR (V+)". Sleve is labeled "PWR (V-)"

The larger, 3.5 mm, plug has three conductors, TRS, and is labeled like an typical, 3.5 mm, Tip-Ring-Sleve, headphone jack.

OK. I think I found your plug, in this pdf,