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IPhone audio output

post #1 of 11
4/25/09 at 6:20pm
Thread Starter 
Guys/Gals,

I am not too techy, but can do some electronics work. I fly for the Air Force and I make cords that my folks can use in the plane to listen to audio devices. These cords are U174 plugs on one end and a 1/8" or 3.5mm jack on the other. I believe the headphones we use are 1200 ohms, instead of the typical 300 ohms. The cords I make always work even though the ohms are different. The problem is, some folks now have iphones, and they're telling me the audio is not working with the interphone system. Does the iphone's output have something other than 300 ohms? Do you think I need to put a resistor inline with my cords for iphones? What can I do to make these cords work? Thanks for any help I can get. BD
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post #2 of 11
4/25/09 at 8:30pm
first gen iphones cant use standard plugs as the entry hole before the actual jack is too small, that may be the issue they are having

btw: thanks for your service
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post #3 of 11
4/25/09 at 11:33pm
1200ohms seems like a lot to push especially when you consider the power on the output of the iPhone.

How are these being used in the system?

Is the ipod directly powering the headphones or is it being hooked up to the internal intercom system?

What you may want to consider is using an iPod Dock connector with teh specfications liste dhere to work with all iPod. If the size of the plug is the issue, get a set of cheapy $3 headphones from walmart or target, etc. that has a small plug similar to the iPod headphones, cut it off about 4 inches from the plug and solder a 3.5mm reciever or whatever size you want. It will make a nice little converter.

If power output is the issue, then you may want to consider adding something like a CMoy or maybe even a T-amp in the signal path. I don't know if you have power lines accessible by pilots or what you would need to do as far as that is considered. A T-amp usually needs ~12V and a Cmoy needs usually ~9V. Cmoy's aren't entirely suitable for 1200ohm, but a T-amp might be overkill. Something putting out 10w/channel would be quite a lot. Either can be made to run off battery power, but if you have DC lines to use, might as well do it.

Please do let us know how the whole thing goes together.

Anything we can do to help you guys!

BTW: feel free to PM me.
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post #4 of 11
4/26/09 at 12:49am
If the headphones have 1200 Ohm impedance, then the iPhone probably wouldn't be able to drive them directly because that requires more voltage swing than the iPhone can put out. Most portable headphones are actually rated for 32 Ohms which is likely what the Wolfson CODEC is designed for, and there is more of a current requirement than voltage when dealing with low impedances. Keep in mind that the iPhone battery is 3.7V, and the Wolfson likely puts out 1.5Vrms. I'd say use an amp with high-ish gain to boost the voltage if you want the headphones to work. You could use something like a Mini^3 or CMoy that plugs directly into the plane; I'm assuming they have DC power outlets like in cars. Just remember to tape it down.
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post #5 of 11
4/26/09 at 5:19am
I think even a portable amp would have trouble pushing 1200ohms; that seems ridiculously high. I dont even think my lisa III would get anything more than a squeek out pof them and its the most powerful portable amp available.
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post #6 of 11
4/26/09 at 9:31am
Quote:
Originally Posted by pistolsnipe View Post
btw: thanks for your service
I second that...
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post #7 of 11
4/26/09 at 11:35am
Quote:
Originally Posted by qusp View Post
I think even a portable amp would have trouble pushing 1200ohms; that seems ridiculously high. I dont even think my lisa III would get anything more than a squeek out pof them and its the most powerful portable amp available.
The Mini^3 was built to incorporate compatibility with high impedance headphones like the HD600, so maybe if one paralleled the outputs with 1.2k resistors, one could achieve the same signal swing with those aviation phones. That, of course, assumes that you want to build a headphone amp for the pilots at all. I wonder what would happen if you paralleled the outputs with ~32 ohm resistors...
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post #8 of 11
4/26/09 at 6:50pm
Most of the current would go through the 32ohms (path of less resistance), and since the same voltage drops on the 32 and the 1200's, most of the power would be drained through the 32 ohms.
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post #9 of 11
4/26/09 at 6:50pm
Wow, that's ridiculous impedance...you'd need to build a resistance adapter...I know they have those adapters for sale on Moon Audio but not for jobs quite that high
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post #10 of 11
4/27/09 at 1:01pm
Thread Starter 
Wow, you guys are seriously smart with this stuff. I will call the headphone manufacturer and find out the exact ohmage of the earphones on the headset. I'll get back to all of you. Thanks for you interest in helping me. BD.
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post #11 of 11
1/9/11 at 7:30pm

iphones only do decent with my 50 ohm sennheiser 558

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