[Jan] is one of those people who always plays with synthesizers, and nowadays that means a lot of USB cables supply power. If you want to do a portable synthesizer setup, the best option is to look at USB powerbanks with their sophisticated lithium cells. They will work well, but remember: you can buy D cells almost anywhere and they actually hold a ridiculous amount of energy. They are cheap, albeit disposable and disposable, so why not create one USB power bank from a massive pair of batteries?
The compilation began, of course, with a pair of Energizer D cells that hold 20,000 mAh. The battery holder for these cells is inexpensive and easy to source, leaving the only other necessary component a cheap 5V amplifier converter. It was just glued hot on the back of the battery holder in parallel, a simple switch was added and the whole thing was mounted in a neat little 3D printed case that looks like a car (motorcycle?) Battery.
Testing with a phone revealed that this thing will charge at 570mA from 3V, which is more than enough for [Jan]the needs. Of course, the use of disposable batteries in 2018 is more than a little wasteful, but a project like this is designed to be a simple solution to the problem of providing power to USB devices everywhere. You can take D-cell batteries anywhere, and what produces this design to the detriment of the environment is more than offset in its convenience.