Electronic aspects of iPhone 3.5mm audio output

Question

I am working for a while now on a small project of building an external mic for the iPhone to be hooked in parallel to headphones (via the 3.5 mm jack). Basically, I want to replace the iPhone's original headset microphone with another microphone, but still use the headphones to listen.

Below is an image of the prototype structure (based on 2 existing products, but it is actually not working; probably due to impedance issue):

Some clarifications on the image:

• item #5 - split point between microphone jack and headphones jack.
• item #4 - iPhone plug 3.5 mm
• item #3 - a microphone which for now is NOT working since the iPhone seems NOT to identify it (probably impedance issues - it's impedance about is about 650 Ohm)
• item #2 - simple headphones jack (any headphones with 3.5 mm plug can hook there)
• item #1 - microphone jack hooked to the external microphone I have for now.

I would like to focus my question on the electrical aspects of my project for now. Known data I gathered so far (please feel free to correct any mistake you identify):

1. iPhone supply's 1.5 V on the TRRS 3.5 mm jack.
2. the TRRS plug of iPhone is built of from 4 pins: Left/Right/Ground/Mic

Questions:

1. What's the power consumption per EACH part of the iPhone headset? Each part means that there are 2 components - headphones & microphone and I need the separated power consumption (especially the microphone!)

2. What's the current the iPhone drive on the headset microphone and what's the current driven on the headphones?

3. Read some other answers on the impedance topic, that the iPhone identifies external microphone (on the headset for example) only if the impedance is ±1650 Ohm, but than, I read another answer which claims the required impedance is ±5000 Ohm. Any ideas what is correct?

4. Should I plan that the microphone impedance (includes the wire) to be 1650 Ohm (or 5000 Ohm based on the answer I will get for question #3) OR the whole prototype (microphone + wires + regular headphones I will hook to 3.5 mm connector) should be together the 1650 Ohm / 5000 Ohm?

5. Is this correct to say that the 1.5 V supplied by the iPhone 3.5 mm audio output, means that the right/left/mic pins are positive contacts with 1.5 V vs. the ground pin (means that we have 3 parallel circuits sourced by one 1.5 V power source).

Answer 1

This is the data I can share with you after having run my own set of experiments and having search (extensively) through the Web for other people's real hands-on tests. I have discarded / omitted the data which I have been unable to reproduce:

1. The impedance of the standard Apple miniature hands-free microphone, the one integrated with the headphones they include in the iPhone 4th gen device is about 1600 Ohms. This means, if you simply connect a 1.6K resistor between the MIC and GND connections of the TRRS connector, the iOS will switch to the external microphone (in fact, a resistor, not a microphone).

   Having said that, the truth is that different versions of the iOS and devices will employ different decision algorithms when trying to "guess" if there is an external microphone connected to the iPhone, iPad or iPod. You can find some references in the Web (I won't cite them here as I consider the information misleading) stating different threshold impedances and behaviours for the algorithms of each iOS version and device.

   My advice is simple: forget about the gory details of the different versions of iOS out there. Simply use a 1.6K resistor, mimicking the genuine iPhone miniature microphone. I'd bet Apple is not going to change the iOS behaviour in the near future ruling out millions of hands-free headphones!

2. The iPhone as well as other mobiles will apply a DC voltage about 1.5-2.5V to the microphone. The objective of this is twofold: it serves the iPhone as a way to measure the external DC impedance of the microphone and also powers the preamplifier built into many miniature electret microphones.

   The above means you should be careful when interfacing to the mic connection of an Apple device, either by AC coupling your signal on top of the 1.6K previously discussed resistor or by using a voltage divider with a bigger resistor and again the said 1.6K resistor. Which leads to the next topic...

3. The microphone input of the iPhone will saturate at about 40 mV peak (millivolts). Thus, you must adapt the amplitude of your audio signal to similar level.

4. Beware of the audio 3.5mm TRRS plug (male) connectors out there! Before using one, look carefully at the connection at the base of the plug (the S or "sleeve"). Some connectors will end in a circular metallic disc connected to the sleeve.

   The problem is, many Apple devices (for instante, the iPhone 4) have a metallic enclosure internally connected to ground and this type of connectors will make contact between the enclosure (ground) and the sleeve (microphone), rendering useless all your tries to inject an audio signal. I have suffered myself this problem in the past, wasting some hours trying to figure out why the iPhone won't recognize a properly connected 1.6K resistor.

   You can find some nice pictures of this problem and a temporal homemade solution for it: http://martinjohnsoncommunications.blogspot.com.es/2012/04/iphone-external-mic-connection-solved.html

Answer 2

I've had a bit of a go at answering the wiring part with a resistor.

Answer 3

I don't have an iPhone, so these are guesses:

What type of mic are you using? The headset mic is probably an electret mic, and the 1.5V is used to power it. An electret will look high impedance to the mic input, as opposed to e.g. a dynamic mic.
I would guess the bias resistor (internal to the iPhone) is probably about 1kΩ (this interesting paper says 640Ω)
You may be able to test what impedance it detects at by connecting a pot/resistor in place of the mic (from mic wire to ground) Vary the value from 500Ω upwards until it detects a mic, then note the value.

The 1.5V is probably only present on the mic wire (relative to ground wire), unless the headphones do something "special". You can test this easily with a multimeter.

The current for the mic is likely to be pretty small, less than a mA. The headphones will probably be around 20 - 40mW.

Make sure you are connecting to the correct wires - IIRC the sheath is not ground on the iPhone jack.

Answer 4

I had similar issues when trying to connect a line signal to the headphone jack of a Iphone 6s. I also used a TRRS splitter to headphone and microphone jack. While the headphones seem to work fine, the Iphone was always using the internal microphone instead of the external input. Sometimes when plugging in the headphone jack while recording, the Iphone switchted to external input for about 1 s and then back to internal.

Thx to this thread i found the solution in the missing impedance on the line signal path. When plugging in the headphone jack, I could see the Iphone was providing 2.8 V for about 1 s to detect the impedance of the microphone. I added a 2.1 kOhm resistor between mic+ and ground and now the Iphone switches to external input and does not switch back.

The cable with the resistance must be plugged in first though. Only after detection it can be plugged to the line out, not sure yet why this is the case.

Also my headphones stopped working after switching to external mic and playing around with the headphone jack like plug/unplug is activating voice control on the phone. This seems very weird like there is still a input on the headphone path of the splitter.