iPod adapter

[jcc4507fly]

Hello,

I'm trying to make what should be a simple circuit. It will be used to attach an aviation headset, and an iPod at the same time. So that I can listen to tunes and hear radio transmissions.

Parts include:

Project box 2"X3"
1/4" male audio stereo jack(plug to airplane)
1/4" female audio stereo jack(headset jack)
1/8" female audio stereo jack(iPod input)
couple of feet of shielded audio wire
I also have a few diodes I would like to throw into the mix, to protect the iPod, and airplane radios!

The Plan:
1/4" male to airplane, wired to box. Box will have 1/8" and 1/4" females side by side.
I was thinking I should wire in diodes on the iPod line and also the line from the airplane. So current only travels from iPod, and from airplane, then both travel to headset. This to prevent damage to either one.


Audio INPUTS: Audio OUTPUT:
1/4" male--------->diode>______________1/4"female output(to headset)
1/8" female------->diode>



Does this seem like the best solution? Should I integrate a Volume control in the box as well? Would I need a transistor and battery to boost the sound? Any thoughts would be great. This is my first attempt at something like this!!

 

[MikeMl]

Post a schematic of how you propose to mix the IPod audio to the aircraft headset audio. Dump the diodes. Buy a modern headset that includes the entertainment input like a Lightspeed.

 

[jcc4507fly]

The Clarity Aloft, and Light Speed would not work very well in the aircraft I fly do to excessive wind noise. I fly with several guys that have them and they don't feel they are worth the money. That's why I'm opting this route. I can use my current headset and these components only cost around $20 vs. Clarity =$695, Lightspeed =$500. I also found this iPod/Cell phone Adapter, **broken link removed**. =$110. This is what I'm after, however, I don't want the cell phone input. Besides the pricetag!

I'm not sure exactly how I was going to "mix" the audio sources. Thats why I was thinking of using diodes. Would it work if I just spliced the two inputs together and then sent it out to one single 1/4" output to the headset? I'm not sure how the headset would handle this?? Also wouldn't everyone plugged into the intercom then be able to hear the music? I'm trying to avoid this.

 

[MikeMl]

Mixing two audio sources can be done (sort of) with a passive network, or with an active mixer.

The problem is that the aircraft headphones require a much higher signal voltage than the headphones supplied with your IPod. Have you tried to drive your aviation headset directly from the IPod?

Does the aircraft in question enable you to set the listening level individually for crew member?

 

[MikeMl]

Can you find a small audio coupling transformer with 10 to 30 Ohms on one side; and 100 to 500 Ohms on the other?

 

[Dragon Tamer]

try taking an aviation headset, cut wire about half way between the jack, and the headset. here's the tricky part, you need to find the wires that go to the mic, and the wires that go to the earpeices. reconnect the wires for the mic, soldier them, put some heat shrink tubing on them, and never touch them again. after you have the wires for your earpeices, go out to wallmart, or the dollar store, and buy a cheap pair of headphones. cut the earbuds off of the headphones and strip the wire back. look carefuly, if there is a copper wire wraped around an inner insolated wire, your in luck. just strip the wire back and attach a 0.1μF capacitor (can pick some up at radio shack for... cheap (i don't know how much they cost)) to each signal wire(including the signal from your aircraft radio), and again solder them in place. If you cut the earbuds off and find 2 non-insolated wires (as this is the most likely cenario, be prepaired), these non-insolated wires can be found in a lot of headphones now. you can't solder directly onto the wire. first you need to take some fine grain sand paper (finest grain you can find), use it to rub off the nonconductive coating on the wire. then you can soldier everything in place. the wires for the ground never need a capacitor. the capacitrs value should be a minimum of 0.1μF. you can go higher, but try to make sure that you have the polarity correct on the capacitor. sence i don't know what the aircrafts output level is try to use ceramic disk capacitors (they're usualy rated for a couple hundred volts).

 

[MikeMl]

Dragon Tamer, your suggestion of bridging the IPOd across the aircraft headphone audio is naive at best; dangerous at worst. One of the OP's requirements is that he not backfeed the IPod audio to the other occupants in the aircraft. Your method provides NO ISOLATION whatsoever!

 

[Dragon Tamer]

i think you may be under estimating the robustness of an ipod. mine took a good deal of abuse before it finaly broke. it broke because i had the audio from the ipod shorted across 12V of power that was conected with the polarity reversed in respect to the signal. but like i said, i'm not an expert in aviation equipment, please enlighten me so that a better solution may be found.

 

[MikeMl]

i think you may be under estimating the robustness of an ipod. mine took a good deal of abuse before it finaly broke. it broke because i had the audio from the ipod shorted across 12V of power that was conected with the polarity reversed in respect to the signal. but like i said, i'm not an expert in aviation equipment, please enlighten me so that a better solution may be found.

I dont give a crap about your IPod! I'm worried about what the IPod is doing to the headphone audio buss in the aircraft.

The nominal impedance level of an aircraft headphone earpiece is ~500Ω. The IPod headphones are low impedance, nominally ~30Ω. If you bridge the IPod audio output to the aircraft headphone output, the IPod will effectively short circuit the aircraft audio system, pulling down the audio level, causing severe distortion, making it impossible for the aircraft crew to communicate with each other, or over the aircraft communications radios.

 

[Dragon Tamer]

i think that we may need to consult audioguru in this. i can't think of any way to protect the ipod or aircraft equipment without some sort of complex mixer. you could always take an old radio that you don't need any more and experiment using that. and just so we're on the same page, you do know that i said (i think i said) to have a decoupling capacitor on the output of the radio and ipod, right?

 

[jcc4507fly]

Hey guys, Thanks for your replies!
Just a quick update. I built an adapter 1/4male that plugs into airplane. It is wired into a box that splits into female 1/8" plug(iPod) and a female 1/4" plug(headset). When only one audio source is played, ie iPod, or airplane radios, the audio sounds great. But when I turn them both on the volume is greatly reduced. The iPod audio still sounds great, however the airplane gets distorted if I turn its volume over half way. Any ideas??

BTW, The audio doesn't bleed to the other passengers in the aircraft with this setup.
There is NO voltage over the audio jack on the aircraft. Only about 10ohms.

Is it possible to amplify the signal from the aircraft to solve this? Or is there a way to mix the audio?

 

[jcc4507fly]

Aviation Headset Specifications

Here's a link for the headset I'm using.

 

[Dragon Tamer]

Thanks for that link, and mike was right, I went and made my simple circuit with a 500 ohm resistor to simulate the load. There is a lot of feed back into the aviation radio from the iPod. I had the output of the simulated iPod set to 2V pp. I didn't know what I should set the output of the simulated aviation radio would be. Thanks for specifying that the impedance of the head set is 150 ohms, that changes everything. More testing is needed!!!

 

[jcc4507fly]

Mixing two audio sources can be done (sort of) with a passive network, or with an active mixer.

The problem is that the aircraft headphones require a much higher signal voltage than the headphones supplied with your IPod. Have you tried to drive your aviation headset directly from the IPod?

Does the aircraft in question enable you to set the listening level individually for crew member?

Mike, How would I go about creating a passive network, or an active mixer?

I have listened to the iPod with my aviation headset, and it sounds great! The configuration I have set up now does not affect the other pilots audio, so we are good there. Just as you suspected the aircraft audio is distorted ONLY when the iPod is plugged in and audio is playing. Would your suggestion, "Can you find a small audio coupling transformer with 10 to 30 Ohms on one side; and 100 to 500 Ohms on the other?", possibly solve the distortion problem by bringing the ipod resistance up to a comparable level with the airplane?

Do you know anything about impedance matching??

 

[audioguru]

I would make an audio mixer circuit with pots to adjust the levels.

 

[jcc4507fly]

I would make an audio mixer circuit with pots to adjust the levels.

How would I go about doing this?

I dont think I need pots, because I can adjust each sources volume independently.

 

[audioguru]

A simple inverting opamp makes an audio mixer. It has an input resistor from each audio source, a feedback resistor and two or three resistors to bias it at half the supply voltage. It also needs a few capacitors.

But a mixer mixes. Then when you are listening to an MP3 you won't understand that the air traffic controller is telling you to quickly dive to avoid crashing into the airplane directly in front of you. I think the law says the MP3 audio must be muted when the aircraft comnmunications radio has speech. So you need a VOX circuit not an audio mixer.

 

[jcc4507fly]

I think I will stick to a mixer, as I do not want or need it to mute with incoming audio signal. I can listen to multiple inputs at the same time, if the volume is adjusted properly. The mp3 audio is to be used as background at a low volume, therefore will not hamper my communication with ATC, or the other pilot.

What kind of input resistors, feedback resistors, and capacitors do I need to use? Thanks again.

 

[audioguru]

The parts you need depend on many unknown factors. Maybe you can experiment with different values to get it sounding properly.

 

[MikeMl]

Maybe this will give an idea. Two transformers, one for each channel. Put them in the box between the aircraft jack, and where you plug your aviation headset.