Iphone Audio Cable

Here's the quick and dirty iPhone audio interface that I whipped up today. It plugs into the headphones port on an iPhone or similar phone that uses a TRRS for mic and speaker, and sends the headphone out of the phone to a stereo channel on the mixer, and balances a line level output from the mixer and matches it to the mic level input of the iPhone. I will use this cable to take calls during live shows.

The trick is to making something like this work is getting audio back into the phone, its extremely simple just plug the left and right channels of the headphone output into the mixer, but convincing the powered microphone input to take a line level signal is a bit trickier. We'll make the conversion using two resistors, check out the diagram after the break.

 R1 bonds mic signal to ground so that the iPhone "sees" the presence of a microphone and listens for signal on its mic input. R2 attenuates the line level signal to match the impedance of an elect. microphone. These values don't have to be super precise, I actually just grabbed two resistors out of a bag without checking their values. I figured that since I was only sending telephone audio the quality didn't matter too much. To correctly execute this circuit you should use a small capacitor to block the dc voltage present on the microphone input, and a correctly calculated resistor to match the impedance of the input. By adding a small potentiometer you can adjust the impedance of the input on the fly, based on the signal you're feeding it.

A beauty shot of the resistors, pretty simple once its explained.

So that's it! when its all hooked up the iPhone gets its own stereo channel and aux group, you can adjust the mix going to the phone independent of the main mix and of course mix audio from the phone in just as any other input. The only other important step is to make sure that no signal from the phone is present in the mix that gets sent back, obviously, even the slightest signal will cause catastrophic feedback.