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I have a microphone with an impedance of 194 ohms.

I want to be able to connect this to my smart phone via the CTIA standard. In order to do that, I need the impedance of the microphone to be at a minimum of 1k ohms as specified here https://source.android.com/docs/core/interaction/accessories/headset/plug-headset-spec#electrical

If I directly connect it with the 194 Ohm mic, my smart phone will not detect it (As expected).

If I then add a 1k ohm resistor in series, it now detects the mic, but there is a lot of noise and you can only hardly hear me talk.

In order to fix this I have used this circuit (sorry for the crude drawing)enter image description herewhich works great, using mic output to 100nF cap -> 1k Ohm resistor -> 3.5mm jack 4th poll. I then use a 2.2k between the 4th pole and 3rd GND pole.

Now as I said above, this works great (and the new impedance is at 2.194k ohms), but I am looking to see if I can boost the volume even more as I am not sure my values I am using here are optimal. Is there a better way to achieve my goal using passive components?

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  • \$\begingroup\$ It depends on what type of mic it is. The phones are designed to be connected directly to an electret mic capsule with built-in FET. Whatever mic you have needs to be adapted to look like standard electret mic and how to build an adapter circuit depends on what mic you have. \$\endgroup\$
    – Justme
    Commented Apr 19, 2023 at 7:19
  • \$\begingroup\$ @Justme not sure on the exact mic, but they call it a "magnetic-dynamic noise cancelling microphone" \$\endgroup\$
    – jLynx
    Commented Apr 19, 2023 at 8:26
  • \$\begingroup\$ You need to know what it really is. If it is a dynamic mic, it's just a coil. If it has noise cancelling features that are made with electronics, it needs a power supply and output has nothing to do with the actual microphone capsule and again the circuit to adapt a buffered output to look like electret will look different. What mic you have and what electronics it has inside? \$\endgroup\$
    – Justme
    Commented Apr 19, 2023 at 9:55
  • \$\begingroup\$ @Justme this is for a Motorsport comms system. It's a Stilo helmet with built in comms (Stilo ST5 F). I can't seem to find any specifics on the exact mic they are using in it. But the helmet connector has 4 poles, S- S+ M- M+. So I'm assuming it's direct connection to the mic itself \$\endgroup\$
    – jLynx
    Commented Apr 19, 2023 at 9:59

2 Answers 2

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you have a dynamic microphone and you need to connect it to a port that expects an electret insert instead, to do that you'll need to use a pre-amplifier.

maybe something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

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Now as I said above, this works great

As drawn, your components form a 500 Hz high-pass filter. I'm not sure what you mean by "works great", and the audio fidelity might be just fine for voice comms.

If it were me, I would increase the coupling capacitor (reference designators - !) by 10 x. This will increase the low frequency response, but possible let in some power line hum.

If all still is well, next I would eliminate the 1 K resistor. Depending on the phone's input impedance, this should increase the audio signal level, maybe by as much as 30%.

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  • \$\begingroup\$ Thanks! yes this is actually just for voice coms, so I hadn't noticed the drop in fidelity. When I say works great, I mean I can hear my voice on a recording with no noise in the background. I'll give it a try by switching to a 47uf cap and then try eliminating the 1k \$\endgroup\$
    – jLynx
    Commented Apr 19, 2023 at 3:32

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