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I am trying to connect wired earphones to a SIM800 GSM modem so the user can talk over phone calls. After a successful test with a terminal block and hand-soldered connectors, a PCB was designed. The prototype setup was tested with the same earphones and as far as I know, the pinouts of the test setup and the PCB match.

However, when I connect the PCB to my module, the earphone audio is extremely quiet, and the microphone doesn't work (can't be heard on the other end) at all - even with the same audio settings in the module that worked earlier. Additionally, the person calling the GSM modem hears a clear feedback of their own voice (exactly what they say, just delayed), which wasn't present before.

Other components on the PCB include an I²C IO expander, filtering capacitors, some USB wiring and status LEDs. I don't think any of these would cause the issues above (apart from perhaps adding a little electrical noise).

Since the PCB was designed for "Apple pinout" earphones, I have tried switching it to Android pinout with bodge wires, but that did not fix the issue (made speakers even quieter). Judging by the resistances measured with a multimeter (very high between Sleeve and all others, low between Ring2 and Ring/Tip, very high between Ring2 and Sleeve), the earphones match the Apple pinout.

I have also checked continuity between the differential audio pins of the module this PCB is connected to and the PCB. Inserting a 3.3V Vpp frequency onto the MIC lines with a signal generator (actually the calibration output of an oscilloscope) produces a tone that can be heard clearly on the other side, so the MIC signal path is clearly working.

Now, I realise that trace impedance mismatch (in the 0.254mm/10 mil audio traces) compared to what the earphones and modem are expecting might be causing my problems. Is this likely to be the case?

What (else) might be my issue here?

Relevant schematic section and PCB top/bottom layers are attached below. Relevant part of the PCB schematic Pinout of J1: Pinout table of J1

All components are mounted on the top ("red") side of the PCB. Top side of PCB

Bottom side of PCB

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  • \$\begingroup\$ Are you using the same earphone and connector as you used before, and are you sure that it follows the iPhone "standard" and not the Android version? \$\endgroup\$
    – brhans
    Commented Feb 28 at 21:00
  • \$\begingroup\$ There are at least three problems. First, contrary to your claims, the board is already using the "Android" or CTIA pinout, LRGM. Second, as the connector footprint is symmetric, we can't know if you mounted the connector on red side or blue side of the PCB. And third, we don't know the pinout of some random connector called J1 and how it relates to some SIM800 module. Did you perhaps wire MIC GND and SPK- together by the way? \$\endgroup\$
    – Justme
    Commented Feb 28 at 22:11
  • \$\begingroup\$ Are you using an electret microphone? Have you provided a bias voltage (power) for it? None is shown in your partial circuit diagram. You don't need to worry about the characteristic impedance of PCB traces for audio signals. \$\endgroup\$
    – Graham Nye
    Commented Feb 29 at 0:58
  • \$\begingroup\$ @brhans I am using the same earphone that was used before. Not sure how to name the standards now, but the earphone pinout matches the PCB connections. \$\endgroup\$
    – Liftyee
    Commented Feb 29 at 8:31
  • \$\begingroup\$ @GrahamNye The module has a bias voltage output and it is enabled in software. \$\endgroup\$
    – Liftyee
    Commented Feb 29 at 12:41

1 Answer 1

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Do not connect the SPK- output to MIC ground.

The speaker is driven by a H-bridge (maybe a D class amplifier) so both SPK+ and SPK- are active audio signal wires and neither of them is ground.

The MIC- seems to be a differential input too, not ground.

Which basically means the module pins cannot be directly wired to a 3.5mm TRRS connector for headset connectivity.

Headsets have one shared ground for mic and left/right drivers.

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