If you have an iPhone and play guitar you might be interested in this. There is an app called Amps & Cabs that gives you the choice of three different amp heads and three different speaker cabinets. The sound is surprisingly good for a 99¢ app.
All you have to do is get your guitar signal into the iPhone and away you go. But how do you do that?
Well I picked up a bargain bin iPod to RCA cord (red, white and yellow) and connected the red and white onto an 1/8" stereo headphone jack and the yellow onto a 1/4" guitar jack. Then I mounted them in the box that came with the iPhone.

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Here's what the app looks like.
