C-130 Hercules AC Feedback in Communication Cord

Discussion in 'The Projects Forum' started by C-130 Navigator, Jul 27, 2008.

  1. C-130 Navigator

    Thread Starter New Member

    Jul 27, 2008
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    When hooking up a miniphono jack from my laptop to the airplane's comm system we are able to play music on our headsets while we are flying.

    If I plug the power cord into my laptop, we get a loud feedback in our headsets.

    The AC powercord is fed from a DC/AC inverter powered from the aircraft (which in turn is turned back into DC before feeding the laptop.)

    How can I filter out the noise from that we are receiving in our headsets?

    Am I getting this noise because I need to ground the inverter somehow?

    Any help would be appreciated.

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    First, Thank You for your Service :)

    Secondly, I don't think it's a good idea for you to connect an inverter to the A/C's system, as if it's a civvie inverter, it's designed to run from 12VDC, whereas the aircraft's electrical system is 115V, 400Hz, 3-phase, and 28VDC. I have no clue as to what kind of interference the inverter may be causing to other critical systems, but I suspect that it is not good, and I strongly recommend that you discontinue connecting such equipment to the electrical system of the A/C.

    However, if your laptop's power adapter is rated for 28VDC or less, it would not be difficult to plumb it in to the 28VDC system, using a linear regulator circuit with noise isolation capacitors.

    The first hurdle would be to get an electrician to install a cigarette lighter socket on a panel somewhere convenient, with a suitably-sized breaker.

    (This isn't an original idea. During the VietNam War, many Marine F-4 Phantom aircrew had their electricians install cigarette lighter sockets in the cockpits of their birds so that they could plug in civvie radar detectors for audible warning of SAM site launches.)

    Getting access to the DC of the aircraft is the first step. Regulating the DC down to what's compatible for your laptop is the next step.

    So what is the power requirements for your laptop? Your AC adapter will give the output voltage and current rating.

    BTW, I've spent a number of hours in C-130's - many years ago. Certainly not as many as you have; and I was part of the cargo. ;)
     
    Last edited: Jul 27, 2008
  3. C-130 Navigator

    Thread Starter New Member

    Jul 27, 2008
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    Sgt Wookie,

    Thanks for your quick reply.

    The inverter used is standard and IAW normal ops. The laptops typically used run moving map software. So the inverter being used is okay.

    The backfeed that we are getting (didn't hook it up today) only occurs when the comm sys is plugged into the headphone jack of the laptop and the laptop is powered up through the inverter. If the power is disconnected, then the noise goes away.

    My first thought was to find a way to "ground" the inverter but the DC system is already a ground, right? (one hot, one return).

    The inverter supplies the AC circuit to the laptop that has hot, neutral and ground--though no ground is occuring...I think.

    I tried to ground out the power supply by running THHN #12 from the ground wire of the power strip (on the downstream, AC side of the inverter) to bare metal on the airplane. This had no effect.

    We also experience this problem if the laptop is not powered up but connected to any other plug and play system with it's own power source (e.g. sat phone, repeater scopes, etc.) Usually we have at least one other external, self-powered device on the mission laptop, so we try to bring a personal laptop for playing tunes, watching movies, etc. There is amply current to run multiple laptops off of the power strip coming off of the inverter. There is no way to hook the laptops directly off of the main DC bus without tearing apart the aiplanes and getting thrown in jail we land for illegally mod'ing the planes.

    My new thought that I came up with today would be to add a butt load of resistance on the line from the headphone jack on the laptop to the comm system. I just don't want to add so many OHMs that I lose any audio fidelity.

    So, since we can by flying for up to 10 hours and our laptops can't run on batteries for that long of a time, still looking for a way to filter out the stray electrons ruining our tunes.

    Thanks for the help! Let me know if you can think of anything else.
     
  4. roddefig

    Active Member

    Apr 29, 2008
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    Last edited: Jul 28, 2008
  5. C-130 Navigator

    Thread Starter New Member

    Jul 27, 2008
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    Roddefig,
    Thanks for your post.
    Is the iso xfmer something that I can buy at a Radio Shack?
    I saw the schema on the webpage you listed; how big of device are we talking here?
    What is the difference between jacking up the impedance and using the iso xfmer?
    Thanks again for your input.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Ever hear a 400Hz tone? The reason I ask is this is the frequency used by aircraft, I'm wondering if somehow the 400Hz isn't making into the mic input, or some other mechanism.
     
  7. roddefig

    Active Member

    Apr 29, 2008
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    Yes, but the webpage author seems to indicate that the ones available from Radio Shack are not very good.

    I doubt very large. I have used the Radio Shack transformers before and they're about 3/4" square. I haven't used them in a hifi application (only for a simple AM transmitter) so I can't vouch for their quality audio-wise.

    The problem is that there is two paths to ground, one through the laptop and the other through the comm system, which introduces the ground loop. I've had the same problem happen when I plugged my computer into A/V equipment to play movies. In my case you would only notice the hum during the quiet parts of the movie, so I just suffered with it (and it was at 60 Hz, which is much less irritating than 400).

    Anyways, we need to break the DC connection to ground. Using a resistor will only attenuate the noise and your music, so you're not improving the signal to noise ratio, only reducing the volume.
     
  8. C-130 Navigator

    Thread Starter New Member

    Jul 27, 2008
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    Bill,

    Thanks for the lead. I'll pass it by the flight engineer (who know the freqs & other electrical minutia) and see if we can't test out something in that range.

    Cheers,
     
  9. C-130 Navigator

    Thread Starter New Member

    Jul 27, 2008
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    Roddefig,

    Once again, thank you very much for sending me down the right track!

    Cheers,
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Seems like you could use a couple of capacitors to break the ground loop, rather than an isolation transformer. And yes, I concur - sounds like a ground loop problem.

    Can you:
    1) Disconnect the audio out from the computer.
    2) Plug the computer into the charging circuit.
    3) Measure the voltage from the ground side of the audio jack to the ground side of the audio plug.

    I'm thinking a couple of 100uF low-ESR capacitors would take care of it, if you knew the polarity and voltage of the ground loop offset.

    If you wanted really good audio response, get a couple of 100uF poly caps rated for 100V. Not cheap, but they're non-polar, and should be GTG for many years. You'll need one for the "ground" circuit, and one for each audio circuit. Your computer has L & R stereo output, the comm system is monoraul (unless they've installed stereo headsets in the last few years.)

    You'll need to mix the stereo output to a monoraul output, and then feed that to the comm system.

    I don't happen to have a decent stereo to monoraul mixing circuit on hand. I'll bet AudioGuru would. This is normally done at a "line" level, which is much lower than the output of your headphone jack.

    Do you have a "line out" audio jack on your laptop?
     
    Last edited: Jul 28, 2008
  11. roddefig

    Active Member

    Apr 29, 2008
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    Ground loops are still a bit of a mystery to me, but are you sure this would work? I thought the whole idea of using a transformer was for isolation and that to break the ground loop we need to isolate the two circuits. I know you can use transformers and opto-isolators to provide isolation, but I didn't think you could use capacitors. Am I mistaken?
     
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