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enter image description here

Above is a mic jack going into a pre-amp. As you see the tip of the jack and the ring of the jack are both connected to the plus side of the electret microphone.

And when plugged in, it seems like the tip of the jack is coupled to the blocking cap of the pre-amp. And the ring of the jack establishes the connection for supplying biasing voltage for the mic through 2k resistor.

So as we can see the electret microphone's positive terminal needs two junctions to be coupled to two different points at the preamp side. There is two segments here: the tip and the ring.

But how about the connections in the case for a TRRS jack and a preamplifier? There is only one segment spared for the mic in a TRRS jack. How does biasing and AC coupling works in that case? I couldn't find any illustration or circuitry similar to the one I provided above.

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  • \$\begingroup\$ There isn't an industry-wide standard for 4-pole. And this impacts the microphone, in particular. Look at Apple (CTIA) vs Nokia (OMTP), for example. Just FYI. \$\endgroup\$ – jonk May 25 '17 at 21:31
  • \$\begingroup\$ but there is still one segment spared for mic in both types. \$\endgroup\$ – user16307 May 25 '17 at 21:32
  • \$\begingroup\$ Yes. Just wanted to call attention to polarity. \$\endgroup\$ – jonk May 25 '17 at 21:33
  • \$\begingroup\$ @jonk when I measure the bias voltage for iphone a laptop and a samsung 4s smartphone: for iphone the voltmeter reads 2.8V for the laptop 4V and for the samsung phone "6mV" what could be the reason for the mV bias? \$\endgroup\$ – user16307 May 25 '17 at 22:11
  • \$\begingroup\$ I just know that I have to use different TRRS headphones, depending. One won't work on device X, but works on device Y. The other one works on device X, but won't work on device Y. It is VERY annoying. It was only then that I went and looked this stuff up (a year back) and discovered there was a reason for my experiences -- companies INTENTIONALLY made their systems incompatible with each other ON PURPOSE! It's just a cautionary story suggesting you need to be comprehensive in view if trying to make something universally useful. \$\endgroup\$ – jonk May 25 '17 at 22:44
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Notice that the two pins on the trs plug are connected together in the microphone.

For trrs plugs where only one pin is allocated to the microphone, the two connections you had on the trs are connected together on the devicr side instead of the microphone side.

That's it. Nothing complicated.

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  • \$\begingroup\$ Thanks I got it. But can you please tell me what is the input impedance here? Is it Zi//Rb? And is that only for AC inputs? \$\endgroup\$ – user16307 May 25 '17 at 21:25
  • \$\begingroup\$ Same as it would be using the trs connection. Zi||Rb, ignoring the effect of the coupling capacitor. \$\endgroup\$ – JRE May 25 '17 at 21:27
  • \$\begingroup\$ when I measure the bias voltage for iphone a laptop and a samsung 4s smartphone: for iphone the voltmeter reads 2.8V for the laptop 4V and for the samsung phone "6mV" what could be the reason for the mV bias? \$\endgroup\$ – user16307 May 25 '17 at 22:12
  • \$\begingroup\$ Might be that the Samsung only switches on the bias voltage when it detects a connected microphone. \$\endgroup\$ – JRE May 26 '17 at 4:44

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