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David Clark Headset / Tape recorder

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by kill_switch, Jun 27, 2009.

  1. kill_switch

    kill_switch New Member

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    Hello all.

    Lets say I have a tape/digital recorder with a preloaded massage that is to be played in a few languages, and it has to be transmitted from an aircraft. So basically I'm trying to make a box that will do the following.


    I can plug my digital tape recorder into that will transmit into the Mic.
    I can plug my headset into and listen to the tape recorder, to make sure I have the right message before playing, while still maintaining aircraft comms as usual.

    Tape recorder uses standard 3.5mm jack.
    Aircraft uses the jack in the diagram.
    [​IMG]

    However I would like to have a Switch Installed for TRANSMIT/LISTEN. So as I'm listening it's not feeding back into the AC comms. Will I need a diode to throw in there to only allow the one way direction? But will an, for example, ipod have enough power (.7v) if my memory serves me correctly to bypass the diode?



    I know this can be done. I know it can. Am I on the right track with this wire diagram? Any help is greatly appreciated.


    diagram.jpg
     
    Last edited: Jun 27, 2009
  2. kill_switch

    kill_switch New Member

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    Actually I won't need a diode to stop any feedback into the aircraft, as Hot Mic would have to be disabled and you need to push a button to talk, so when reviewing the message you just don't push the talk button.


    So it all seems like it should work. Anyone see anything wrong?
     
  3. xiptron

    xiptron New Member

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    LOTS!!! There are several problems with your circuit and what you want to do (which may be ILLEGAL!).

    Aircraft audio systems are very different from civil/domestic systems. The first thing is that ALL aircaft headset mics and handmics are POWERED types. The mic input circuit of the transmitter supplies a 'bias' voltage of 6V to 8V dc to the mic connection on the plug. This goes back to the days when aircraft mics were carbon, like in old telephones, hence the powering voltage. If you look at the schematic of the microphone part of the headset you will see a right-pointing triangle - this represents an included pre-amplifier, powered from that voltage. You cannot connect your ipod (or whatever) output directly across the mic line as the low output resistance of the ipod will seriously interfere with the amplifier circuit, preventing the aircraft microphone from working (which you wouldn't want to do!) while you also risk damaging the output circuit of the ipod (or other device) by connecting this voltage to it. Note that when not transmitting, this voltage may rise to full rail, depending on how the PTT is arranged. A diode won't help.

    Next problem: Aircraft radios have a headphone output typically sourced from a couple of hundred ohms. If you measure the dc resistance of your headphones, you will find it varies from a few hundred ohms to over a thousand, depending on the position of the volume control. If you were to make a parallel connection across this circuit, as you have shown, the low-impedance ipod output circuit will (again) effectively "short-out"
    the higher impedance radio output, dropping essential comms volume to the headphones and may cause distortion as well.

    In short order, your circuit will NOT work in any successful manner, as you will now see, and could compromise safety of the aircraft.

    CAUTION: I have to make comment on your intention to "broadcast" messages in this manner. Firstly, International Aviation Regulations clearly PROHIBIT "external" or "adapted" connections of any type to aircraft radio systems, unless they are approved by the CAA in that country, and secondly, the intention to broadcast prerecorded messages IS, unless approved, contrary to CAA regulations, period.
     
    Last edited: Jun 29, 2009
  4. dave

    Dave New Member

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  5. kill_switch

    kill_switch New Member

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    The headset amp and mic amp are part of the actual comms control box. Each station has it's own independant headset amps. Not the microphone. I know the diagram says otherwise, however that diagram was not taken from the aircraft, only to show the connections, which remain the same. I probally made the diagram worst by leaving that mic amp in.
    Actual diagram for headset here, last page

    The mic has an impedance of 5 ohms and is dynamic, not carbon.
    The resistor is 100ohm (variable of course)

    Being in parallel with the headset does actually work. Bench tested with calibrated equiptment.

    I do understand there is a legality issue here, so this is all "if's and can's" at the moment. However, I'm sure if I can build this prototype correctly, it'll be sent through to get approved, as its for search and rescue. I'm not trying to break any rules. Let me get that out there. I know no one said I am, but maybe why no one wanted to shed any light.

    Also, there are MP3 adapters available for David Clark Headsets.
    For users audio use - CD / MP3 Player Headset Adapter - Fixed Wing - MyPilotStore.com
    For distribution into ICS - CD / MP3 Player to Intercom Adapter - MyPilotStore.com
     
    Last edited: Jun 29, 2009
  6. kill_switch

    kill_switch New Member

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    Got one more question.

    I googled the output impedance of an ipod, and I found it to be 5 Ohms. Now with the Dynamic Mic being 5 plus/minus 1 for the most part (I know the diagram says 9 max, its actually 5.something usually) Is that considered impedance matched?
     
  7. xiptron

    xiptron New Member

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    The picture is becoming clearer. The headset you use is a "passive" military type and entirely incompatible with the GA (general aviation) type system I first thought you were trying to inferface with. There are conversion kits available for this headset that make it usable with GA civilan radios. However, I now understand that that same conversion is taking place in the extender boxes that you describe. That's why the ipod drives the headphones adequately too.

    You could try the circuit as you drew it. Set the modulation level with the ipods volume control, Remember your transmitter is AM so overdriving could cause splatter. It's trial and error thing, but the impedances and levels are similar so it may well work as is.
     
    Last edited: Jun 30, 2009
  8. kill_switch

    kill_switch New Member

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    Glad its coming around. I plan on not putting the ipod's volume too high maybe the first 2 "clicks" or so. Let the Box amplify as stated.
     
  9. xiptron

    xiptron New Member

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    Carry out your checks on the ground first. Tune the transmitting radio to an unused frequency during a quiet time (middle of the night is good) and have someone else listen for you in another aircraft, preferably a good distance away, while you adjust the ipod volume. Bring the level up to the point where they can hear obviouse distortion on the audio signal, then back it off until it's clean. It's not a scientific method of setting the modulation level but you will be able to determine if you're in the right signal matching range. Ideally the ipod's volume control should be at about 1/2 to 2/3 setting for a clean transmission. If it's a lower setting than that you will need to make up a resistive divider between the ipod output and mic input to reduce the latter's sensitivity.
     
  10. kill_switch

    kill_switch New Member

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    Will do, plus we have our own frequency for contacting maintenace. Which will be used.

    Thanks for the audio advise, already had that in my plan. Someone will be on cellphone while everything is adjusted.

    Thanks xiptron for the help.
     

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