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I'm trying to solve a seemingly simple problem with (hopefully) an equally simple solution. I'm strictly an amateur when it comes to electronics, so I'm hoping someone here can guide me in the right direction so I don't do something stupid and fry my iPhone...

I have an iPhone dock in my car, and the dock routes the audio output from the iPhone's dock connector to the line-in jack on my car's receiver. Over time, the line-in jack on the car's receiver has become worn and loose, and as a result it seems to disconnect for just a tiny fraction of a second whenever I go over a bump in the road. The disconnect/reconnect cycle is quick enough to be inaudible, but the iPhone detects it and pauses audio playback because it thinks I've unplugged it from the dock. This is annoying, because I have to keep hitting the play button on my phone every time I go over a bump.

Based on my (admittedly limited) knowledge of electronics, I'm guessing that the iPhone detects the disconnect by measuring the impedance on the audio output and pausing playback when the output impedance goes above some threshold. Based on that assumption, my theory is that I should be able to build a cable that will fool the iPhone by providing a small load on the audio output circuit even when the other end of the cable is completely disconnected. That way, when I go over a bump and the line-in jack does its microsecond-disconnect-thing, the iPhone will still see a load and think everything is still connected.

In its simplest form, this could just be a resistor strung across the two wires of the audio cable, but I have no idea what value to use for the resistor. I also don't want to fry my iPhone, so I thought I'd ask around first to make sure I'm not doing something totally stupid.

Will this work? Is it as simple as the resistor idea, or do I need something more complex?

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  • \$\begingroup\$ The direct solution would be to replace the worn jack on the receiver ... \$\endgroup\$
    – Dave Tweed
    Feb 28, 2014 at 14:33
  • \$\begingroup\$ @DaveTweed: Yeah, but I'd need a chainsaw to get the receiver out of the dash, and then a large hammer and crowbar to get it open. And that assumes I could find the line-in jack once I got in there, without destroying everything inside. Since the problem is otherwise inaudible, I figured a rigged cable would be easier and just as effective. :) \$\endgroup\$
    – Jeff Loughlin
    Feb 28, 2014 at 14:40
  • \$\begingroup\$ You can usually get the reciever out without a chainsaw, but it can involve taking out the entire dash, instruments and all. Is there a cars.stackexchange.com? :) \$\endgroup\$
    – pjc50
    Feb 28, 2014 at 17:32
  • \$\begingroup\$ THIS IS NOT A REPAIR QUESTION! FFS! \$\endgroup\$
    – Passerby
    Mar 1, 2014 at 0:27
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    \$\begingroup\$ @Passerby: Thanks. I tried the resistor idea and it helped a lot, but I still have the problem occasionally so I might try your idea when I have some time. I agree this isn't a repair question and shouldn't have been closed, but that's stack exchange...c'est la vie, I guess. Thanks for the help. \$\endgroup\$
    – Jeff Loughlin
    Mar 1, 2014 at 14:18

2 Answers 2

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A parallel load, like resistors, would work, but like any parallel load, will draw more current or share the existing one, resulting in a lower signal at the input. Anyone that has used a headphone splitter will notice this. The solution to that would be an amplifier, like used in powered headphone splitters. enter image description here

But your issue doesn't require a parallel load. The powered splitter is just a couple of opamps as buffers and amplifiers, and is the same thing you need.

An opamp in buffer mode, will provide a constant load for the iPhone that should prevent the impedance check from pausing the music, and will prevent any lowering of volume that a parallel load would. Since it is being used to buffer and not amplify, there should be no clipping and very little to no audiable change in sound quality.

enter image description hereenter image description here

Any low voltage opamp will work, with a minimal number of parts. A dollar store car usb charger can be used to power it.

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    \$\begingroup\$ I built this over the weekend, and the result is much better than the simple resitor load I tried previously. I drove all the way to work this morning over bumpy terrain without any problems. I ran power for the op amp right from charging circuit on the dock connector, and it all fits neatly inside a tiny plastic box about the size of my thumbnail. Thanks! \$\endgroup\$
    – Jeff Loughlin
    Mar 3, 2014 at 15:05
  • \$\begingroup\$ @JeffLoughlin nice. Pics? \$\endgroup\$
    – Passerby
    Mar 4, 2014 at 0:28
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Potentially it may work and should be safe, although resistors of lower value than headphone impedance (32 ohms) could theoretically be damaging. I would try 100 ohms and 50 ohms first. Standard 1/4 watt resistors will do. Note there will be a loss in signal amplitude through the resistor. Make sure you avoid the possibility of shorting out the wires by careful construction.

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  • \$\begingroup\$ The dock connector is line level, not headphone out. The input impedance is not the same as 16 or 32 ohm headphones or aux in. \$\endgroup\$
    – Passerby
    Mar 1, 2014 at 0:50

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