It has taken a while, but I finally answered one of my own questions about busking equipment. Last year I got a bass amp and plugged my phone into it to play backing tracks I made in BandInABox. That's last year. This year I don't have access to an electrical outlet, so I have to up my game.

It took a little research, a little luck and experimentation, but I wound up with this set up
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The power supply is running tunes from my phone through the amp. Unplugged. The amp was relatively cheap. I'm sure you can get these at pawnshops cheap. The power supply was $200, but when you're not using it you can use it as an uninterruptible power supply for your computer, network, or critical electronics (games). This power supply will run the amp for ~7 hours. I'm not going to be busking for that long. You could probably get by with a smaller one, and how long it lasts will depend on if you're powering more than just a single amp.

Anyway, I had a question that didn't get answered very well a year ago. This at least gives you some options if you have to play without a plug.

Cool! With a large enough card in your phone, you should have plenty of room for the wave or mp3 files you generate from BiaB!