Impedance Matching For iPhone Microphone Input

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pittsburgh

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Joined
Nov 8, 2009
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Nashville, TN
  Anyone have a schematic or formula that works well for matching impedances? I'm trying to send a low impedance mic signal into the high impedance  mic input of my iphone. I've had luck using an a low to high z xlr to TS audio adaptor, but the sound is somewhat noisy, and I feel like that might be too much impedance at the input. I'd like to do it without a transformer if at all possible.
 
Hello,i have done this with a cheap ART di box conectet in reverse.Works perfectly.But I couldnt use my rig in the field recording of some short film shooting because after 15 mins working my Ipad (and iphone) entered protection mode from the heat.It was full summer i agree and to hot outside but just for letting you know.Otherwise the FIRE and external mic with i phone is a very cool Solution.Just wire the DI box (an Passive one not the active 48Volt phantom Type) in reverse and it should e perfect
Cheers
 
There's DC power coming out of the iPhone; you may need to block that.

The impedance is around 1K. For dynamic, I'd just add 10uFd blocking cap. A 150:600 tranny and blocking cap might be a db quieter.
 
pittsburgh said:
Junction that would be great. Thanks.
Not sure if this solves your problem, but this is the mod we do to be able to plug a mic into an iphone 4, the cap is for DC blocking and the resistor is a load for the iphone to switch to the extrenal mic, without the resistor the iphone switches to the internal mic
http://img593.imageshack.us/i/iphonemicinput.jpg/
 
pittsburgh said:
Just Curious, why is there DC power coming out of the TRRS connection for the iphone?
Don't know, but it is. The iphone seems to have some intelligent auto switching depending on what is plugged in, I guess thats what the DC is for and when it measure a certain voltage drop it switches, as mentioned without theresistor it switches to internal mic.

One other issue we noticed, is that if you use the standard TRRS connector with an inetgrated earpiece/headphone connected, the earpiece volume is automatically pushed down in software and is really low, you then have to manually adjust it back up on the volume control on the side of the iphone. If you plug a headphone with standard TRS conector, the volume stays high, so as you can see, there is some automatic stuff going on there.
Michael
 
pittsburgh said:
Just Curious, why is there DC power coming out of the TRRS connection for the iphone?
It is a power supply for microphone – just like phantom power for pro condenser mics, just lower.
Also, it's a pull-up voltage used for detecting headset button presses.
 
smotesko said:
It is a power supply for microphone – just like phantom power for pro condenser mics, just lower.
Also, it's a pull-up voltage used for detecting headset button presses.
I do not know what is inside an iphone but cheap oem mics, have internal FET stages that feed a pull up drain resistor, connected to a modest voltage.  Does apple make an external plug in mic? Then the pull up voltage would make sense.

JR
 
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