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Strangly when i insert an audio to audio cabel into the the smarthphone audio jack the multimeter shows me max. 0.1 V.

But if the cabel is just half inserted in the audio jack the multimeter shows me a Voltage of 2.4V

Can someone explain how this is possible and how much Voltage i can exepct from and smarthphone audio jack?

I am mearsuring between ground and mic on a headset connector.

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    \$\begingroup\$ When I plug in a cable into my smartphone, my voltage meter doesn't change at all. Ofcourse, I forgot to mention that I didn't actually connect the voltage meter to my cable, but you don't mention how you connect the voltmeter so I assume you are doing the same... Voltage is measured between two points and if you don't tell us which two points, we can't help you. \$\endgroup\$
    – Joren Vaes
    Aug 13, 2017 at 13:19

1 Answer 1

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Depending on the standard used, the microphone pin will have a voltage (generally with a large resistance in series). This is needed to allow the microphone circuit to work. The large resistance in series is there so that when the microphone pin is shorted out by the headphone cable, it doesn't damage anything (almost no current can flow and the headphone is not damaged). Since the current draw of the microphone electronics is very low, when connected it does not cause a large voltage drop allowing the microphone to work.

Here is a similar question that might shed some light on this matter: Microphone voltage on a 3,5mm PC microphone/headphone jack

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  • \$\begingroup\$ Can I assume that the microphone is an electret condenser microphone, and that the voltage being provided by the smartphone is a bias voltage that powers the JFET? \$\endgroup\$ Aug 13, 2017 at 17:33
  • \$\begingroup\$ @Robert Harvey I think there are usually full ICs in those microphones, I don't think the signal is consistent enough to use as a reliable direct bias. However I'm far from an expert so I could be wrong \$\endgroup\$
    – Joren Vaes
    Aug 13, 2017 at 17:40
  • \$\begingroup\$ Electret microphones usually only have a simple jfet amplifier built in. Nothing complicated, and usually not an IC of any kind. \$\endgroup\$
    – JRE
    Aug 13, 2017 at 17:48
  • \$\begingroup\$ The "microphone" ic is not just there for the microphone, also for you teack/volume whatnot. Putting all that in a simple CMOS die seems like a sensible approach. But again, not an expert on this, just reporting my observations \$\endgroup\$
    – Joren Vaes
    Aug 13, 2017 at 17:49
  • \$\begingroup\$ Here is a teardown of the apple ones, that shows that there is more than a simple JFET amplifier in the microphone module ifixit.com/Teardown/Apple+EarPods+Teardown/10501 \$\endgroup\$
    – Joren Vaes
    Aug 13, 2017 at 17:53

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