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I'm trying to build my own headphones to mic pad. I want to connect the input of the pad to a phone's headphones conector and the output of the pad to a second phone's mic connector. Both phones feature a TRRS 3.5mm audio jack (4 contacts, CTIA standard). Here's the circuit:

                     C1
    +Line level in --||----R1----+-- +Mic level output
                       +         |
                                 |
                      +----R2----+
                      |
    Ground (input)----+--------------- Ground (output)

I found the schematics in this page

I've measured the headphones output signal in a couple of phones. Each brand seems to have different amplitudes, almost nobody follows line level standard. I measured amplitudes ranging from 500mV to 1.8V peak to peak.

I've chosen R1=20K, R2 = 1K, so the output amplitude of my pad should be 1/21 the input (23mV to 85mV).

This pad works fine when plugged to any device that only has a mic input. But with phones it's a bit more complicated. Phones already have a built-in mic, and there is some mechanism in place to detect whether or not an external mic has been plugged to the audio jack (e.g. a handsfree). So with phones and tablets, this very same pad works only sometimes: when it works, the pad is detected as an external mic, and it displays the "headset+mic" icon in the status bar; when not working, the phone switches to the built-in mic, and the "headset with no mic" icon is displayed in the status bar (as if only a TRS connector were plugged in). It looks like a random issue, and I don't know why, because I used exactly the same phones and tablets for my tests. My pad is well soldered btw.

And now here comes the odd thing. If I add a 1V DC offset to the pad output then it works always. No random malfunction any more.

Could it be that the unbiased pad output signal (which is centered around 0V) is detected as "ground" by the phone and it switches to external mic? Or is it a bad choice of resistor values?


UPDATE: it could be the case that my voltage bias circuit is providing aditional impedance and that makes it work. I'll play with a potentiometer in R2 and see what happens.

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Looks like R2 needs to be higher. The Android specs call for minimum of 1K from mic to ground. Your 1K resistor is right at the limit. An example is given for a microphone with a 5K DC resistance.

So, use a larger R2, and proportionally larger R1. (R2=5K, R1=100K)

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  • \$\begingroup\$ Wow that's totally counterintuitive. I thought that the lower the impedance the less chance the phone thinks the mic is grounded. And doesn't explain why the DC offset does the trick. But yes, I'll try different resistors. \$\endgroup\$ – Mister Smith Dec 10 '15 at 11:56
  • \$\begingroup\$ On the other hand, my biasing circuit would add more resistors..., so you might have a point after all \$\endgroup\$ – Mister Smith Dec 10 '15 at 12:24
  • \$\begingroup\$ At the end of that specs document you linked it says the threshold to detect the 4th segment in the TRRS connector is 100 Ohm. So theoretically either 1K or 2K resistors should work. I'm more and more confused. \$\endgroup\$ – Mister Smith Dec 10 '15 at 12:37
  • \$\begingroup\$ The 100Ohms is the resistance between segment 3 and segment 4. If it is less than 100 Ohms then it is assumed that you have a 3 segment (TRS) connector. If it is over 100 Ohms then it is assumed that you have a 4 segment (TRRS) connector. This is an additional requirement beyond the mic button detection. \$\endgroup\$ – JRE Dec 10 '15 at 12:42
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    \$\begingroup\$ You were right, it was an impedance issue. Playing with a potentiometer I found it stops failing when R2 > 3.6K. 4.7K works always. \$\endgroup\$ – Mister Smith Dec 10 '15 at 21:42

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