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I've added an AUX input to my Mercedes Audio 10 casette radio using an online tutorial. Essentially hooking up a 3.5mm jack direct to the output of the audio processor.

I'm familar with this procedure having done it with some USB-only Harmon Kardon soundsticks and other devices.

It works well with my ipod, music plays in the car at the correct volume with no ground buzz or artefacts I can hear.

When I attach my phone however, the phone immediately hard-resets.

I've poked around with a DMM and there seems to be about +4v DC between each signal wire and ground.

As a comparison the AUX-input wire I soldered onto the Soundsticks puts out about 2.2v DC between each signal wire and ground. The phone works fine with the Soundsticks.

Could it be that the phone doesn't like this extra voltage and is resetting to protect itself?

How should I go about lowering this voltage? Pulldown resistor to GND? perhaps a shunt?

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    \$\begingroup\$ Series capacitor. \$\endgroup\$ – JRE Jan 29 '17 at 12:06
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You should consider using capacitor coupling for the audio signals to block any DC bias that exists between the two units.

Placing a DC bias on the phone output(s) is highly likely causing latchup to occur inside some chip in the phone. You should avoid further testing in this mode as you run the risk of permanent damage to the phone.

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