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I'm a pilot and we have a comm setup that I need some help with because of noise. We have a USB cable that supplies power to one of our GoPro cameras mounted in the cockpit. When this cable is plugged into the Apple USB adapter (or any 110 USB adapter for USB A (male) to USB mini, we get a lot of "hum/noise" in the audio. When we disconnect the USB from power, it's a clean recording.

A link to the noise and video is here, if anyone would be willing to offer their opinion. We have tried USB shielded cables and ferrite filters that go around the cable. No luck. While we don't have to have the cable plugged in for power and the camera is designed to run on battery, we would like to see if we can filter out this noise. We are not sure if it's something in the camera or if it's the line picking up noise.

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  • \$\begingroup\$ Do you have any noise problems if you connect something else to the USB power adapter? First thing here would be to isolate if it's the noise from the camera or from the adapters. In general, USB power adapters aren't known for their output quality. \$\endgroup\$ – AndrejaKo Dec 20 '13 at 15:53
  • \$\begingroup\$ Intuition says ground loop. Go to Sporty's and order a cigarette lighter adapter that is appropriate for your aircraft's electrical system voltage, and leave AD-DC conversion out of the mix entirely. \$\endgroup\$ – Matt Young Dec 20 '13 at 16:09
  • \$\begingroup\$ Doesn't aircraft AC work on a different frequency? \$\endgroup\$ – pjc50 Dec 20 '13 at 16:26
  • \$\begingroup\$ @pjc50 Airliners 400 Hz AC buses. I'm sure there's an exception somewhere that will make a liar out of me, but light aircraft do not have AC buses. They're all 14 or 28V DC systems. \$\endgroup\$ – Matt Young Dec 20 '13 at 19:32
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    \$\begingroup\$ Gopro's all suffer from this noise when plugged into USB to run from mains - I use one plugged in to record theatrical performances and we get noise - we have to record the audio separately which is a pain. I'd also be interested in a solution. \$\endgroup\$ – user122840 Sep 5 '16 at 21:44
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I get basically the exact same issue with my car stereo if I charge my phone/MP3 player while having it connected to the line-in of the stereo. The ground loop is allowing common-mode noise in (in this case, the ignition system of the car is producing huge amounts of noise, and it's getting in through common-mode coupling).

Since this is common-mode, galvanic isolation will not necessarily fix the issue, since the critical measurement is common-mode coupling capacitance, which can be fairly high even on brand-name hardware (basically: efficiency, cost, and common mode isolation. Choose any 2).

What you need is a "hum eliminator", e.g. an audio isolation transformer.

They seem to run about $10 in general:
enter link description here
enter image description here


It may seem counterintuitive that the solution to your issue of noise when you connect the USB line is to put a filter in the audio line, but that is indeed the proper solution.

The noise is a function of the loop created by the two cables. Basically, both the USB and the audio cable are grounded together in the gopro. As such, if there is a voltage differential across between the audio-connection in the plane, and the power connection, it will cause current to flow through the audio cable, through the gopro, and out the usb cable (or vice versa).

As such, breaking the connection in either one will solve the problem. Audio isolation transformers are much cheaper and easily available, which is why I suggest it over a isolated power supply.

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  • \$\begingroup\$ Worth a try but I need to find something that's USB. I'll search and see if I can find that type of device in a USB format. We have an adapter that's for aircraft comm and GoPro.. It has all the adapters to plug into our panels (stereo plug) and then the USB to power and connector to USB Mini for GoPro. The maker of the cable put a 60Hz filter on the adapter but it made no difference. Since we plug the adapter into a 110 outlet below the throttle quadrant, would something like this maybe do the trick? amazon.com/Ebtech-Hum-Voltage-Filter/dp/B0002E4YI8/… \$\endgroup\$ – agorski0489 Dec 21 '13 at 16:23
  • \$\begingroup\$ This doesn't go in the USB line, it goes on the audio line. As long as the gopro power and the adapter power are the same source, it won't present an issue. The thing you link is probably mostly bullcrap. It can only be a common-mode choke, and they're not going to fit much of a choke in that little box, considering the fact that it needs to handle ~10A. \$\endgroup\$ – Connor Wolf Dec 21 '13 at 16:34
  • \$\begingroup\$ Basically, your issue is caused by the interaction of the USB line and the audio line. You need to properly filter one of them, but the USB interface is a lot harder to do because it's carrying a lot more power, and is DC. You could use a well-isolated DC-DC for that, but an audio isolation transformer is much easier, and can run off the audio-signal itself without issue. \$\endgroup\$ – Connor Wolf Dec 21 '13 at 16:36
  • \$\begingroup\$ @agorski0489 - Ok, reading the reviews on that linked device, it definitely wouldn't do anything. All it does is allow the protective-earth of the connected device to float a few volts. The apple USB adapter doesn't even use the protective earth. It would to nothing at all. \$\endgroup\$ – Connor Wolf Dec 21 '13 at 16:41
  • \$\begingroup\$ Connor, thanks a lot.. your recommendation above is worth a shot. i'll have to get a couple phono to 3.5mm adapters but shouldn't be an issue.. \$\endgroup\$ – agorski0489 Dec 21 '13 at 16:50
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You could try to find a galvanically isolated USB power adapter; i.e. a psu using using a good old fashioned transformer. That would be better for two reasons:

  • No noise generating switch mode psu
  • You would isolate the camera from the "ground" (for a lack of a better word), thus avoiding turning your camera into an antenna.

Edit: I assume that Phil's comment (below) is correct. However, my point on switch-mode vs. transformer still stands. A cigarette lighter-adapter might also be a good idea.

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    \$\begingroup\$ The Apple USB adapter is already galvanically isolated. Were it not, it would need a connection to a safety ground to pass safety regulations. \$\endgroup\$ – Phil Frost Dec 20 '13 at 16:15
  • \$\begingroup\$ @PhilFrost - Galvanic isolation is measured at DC. Most power adapters have crap common-mode (e.g. capacitance) isolation, which is where the problem is. \$\endgroup\$ – Connor Wolf Dec 21 '13 at 16:38

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