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My google+ friend Bruce Turner recommended this Comprehensive iPhone Mic Adapter cable for hooking up an XLR mic to the iPhone. IMO this is a great plug-and-play solution for getting great audio into the iPhone. Especially for interviews in the field. Put your iphone on a tri-pod using The Gliff (another must-have iPhone 4 accessory) and then plug in this cable. Plug in a Shure SM58 and you're good to go. You do need to speak right into the mic but this is a good thing as this will help keep background noise down.
I bought the cable on Amazon for $29.95 and got FREE shipping with Amazon Prime. I'm ordering another on today to have as a backup and to keep in my second kit.
Amazon link: http://www.amazon.com/Compatible-Microphones-Professional-Recording-Headphones/dp/B004Z97CQW
The above setup is for the iPhone. Special note: To hook a mono mic like an SM57 or 58 to a stereo camera like the Sony NEX-7 you can use this simple cable: http://www.amazon.com/HOSA-XLR-3-5mm-Sleeve-wired/dp/B000068NZC/ Special Note: To mount your smart phone on a tripod use this: http://www.amazon.com/gp/product/B005LY9FVU/
Sennheiser EW100 wireless kit I use:
Also see: https://www.youtube.com/watch?v=yJ06qn0czak
and for iPhone 5 & 5s users see: http://www.amazon.com/Glif-Plus-deluxe-package-iPhone/dp/B00AAKO646/
More product links on amazon:
Rode Smartlav Lavalier Microphone for iPhone and Smartphones: http://www.amazon.com/gp/product/B00BHN05H2/
extension cable for Rode Smartlav: http://www.amazon.com/gp/product/B008DWGLLO/
Glif Plus: A deluxe Glif package, iPhone 5 Tripod Mount & Stand:
F-S(Y): i.Trek SUPER MOUNT F, Metal Smartphone Tripod Adapter:
http://www.amazon.com/F-S-Smartphone-Motorola-Blackberry-Smartphones/dp/B005LY9FVU/ - Special Note: Be sure to lock down the iPhone on the tripod prior to turning on the camera app.
This video is also worth watching: http://www.youtube.com/watch?v=U2fYf970GAM
Finally here's a test of the Rode SmartLav using the iOS app: https://soundcloud.com/craigshipp/recording-5
Looking for the cheapest and easiest way to get good audio into your phone? Check this out: