Electronic – Confusion about TRRS jack and mic input

Line levels are 0.5-2.0V range. Mic inputs are expecting millivolts. You just need what is called an L-pad attenuator. Your drop should be 40-50 dB.

If you are using a consumer line level, you will need about 40 dB attenuation. (Professional audio line input would need about 50 dB.)

Attenuation in dB = 20 * log10((R1 + R2) / R2)

Try R1 = 10k and R2 = 100.

If you get accurate measurements of your line source and a know good mic signal for the iPhone, you will be able to calculate more accurate values. Or you can put a 500 ohm trimmer resistor in place of R2 to allow varying of the signal if the line out doesn't have adjustment.

You cannot put a capacitor in series because it blocks the DC supply. Your resistor in addition pulls the DC onto its knees, because the typical resistor between mic+ input and system's supply voltage (3 or 5 volts) is 2...3 kOhm.

The only electrical circuit which works is a full filtering circuit which provides the needed supply voltage to your mic capsule. I bet you want something maximally as complex as your own attempt due the limited space and before all, due the wanted simplicity. I guess you want to avoid external battery.

If you can afford the inconvenience of a dedicated battery, the obvious solution is simple (assuming RC high-pass filtering is enough):

ADD: seemingly you have already figured this principle by yourself when this was under construction.

The voltage 3V is a guess. Your mic can be happy even with 1,5V and it can stand much more than 3V.

R1 is not a part of the filtering circuit. It's DC path to mic and it surely has some recommendations in mic's datasheet as the voltage. Too high resistance causes distortion. Too low resistance degrades the sensitivity but I'm sure it's not critical, maybe just opposite, you want to make it less sensitive, I guess. Even as low as 270 ohm can be good for this purpose. I would start with 2,7kOhm.

C1 is essential for the filtering. It breaks the DC path, so the included battery is a must. The resistor of the filter is in your camera, it's the DC supply circuit. If that has too high resistance you can insert a resistor in parallel with the camera mic input. I would try at first C1 = 0,47uF . If it still is too bassy, reduce the capacitance or add a resistor in parallel with the camera mic input.

Some metal shielding can be needed against electrical disturbances like in mic cables. Connect it to the minus pole of the camera mic input.

If simple RC filtering isn't enough (=idiotically gritty speech sound, but still too much low bass noises) you need 2nd or higher order active filter which needs an operational amplifier. Electronics application notes, web and books contain thousands of schematics for active filters as well as calculation receipes.

Alternatively you can do it acoustically. Put the capsule inside a rigid small bottle with thick walls. Let the bottle have small hole near the sensitive end of the capsule. If the mic is near enough user's mouth, it can still get enough sound.

Another idea is to punch a hole which lets the sound enter to the back side of the membrane of the mic. When user's mouth is remarkably closer the front side than the hole, the mic can well still produce enough signal. Here's the common mic capsule opened:

https://www.personal-view.com/talks/discussion/comment/225394

Finally the ultimate solution: Try a dynamic mic. It needs no DC supply. A half of in-ear headphones goes well. You need only a series capacitor to get the highpass filtering, you already have the electret mic supply circuit as the resistor. Try series capacitor = 0,47uF at first.

A general note for all mic types: You must have some soft material which is sparse enough to let the sound go through, but prevents all wind turbulence near the mic. You need some space between the mic and this windshield. 1cm, even less can be enough.