i’ve found an old battery charger from nokia, it provides 5.7 Volts DC, 800 mA…
do you think can be enough for a midibox?
Smaller or bigger one?
In bigger ones there is just trafo and four diodes - it could be used to power midibox using low-drop pull-down voltage regulators (not 7805).
In smaller (“fast” chargers) ones you can find voltage converter which is probably to noisy to audio equipment and couldn’t be filtered enought.
probably to noisy to audio equipment and couldn’t be filtered enought.
True, but midi isn’t audio, and 5V is only enough for the digital circuits 
So, first I would pull apart that charger, and post a pic if you’re unsure… If it’s 5.7V unregulated, then it won’t be enough to drive the regulators and output 5V. If it’s already regulated then maybe you could mod it to reduce the output (not sure about that…)…
But I imagine it will be unregulated 5.7V, designed to drive regulators in the phone which output 3.3V, and not suitable for a midibox. Sorry 
But what about this ones?
LP3875 - 1.5A Fast Ultra Low Dropout Linear Regulator
LP3856 - 3A Fast Ultra Low Dropout Linear Regulator
It has 0.38 Volt dropout, so it’s possible to run +5V stabilised voltage starting from 5.4V!
Feel free to try it out
I just received some TexIns TPS7150QP for my battery powered MidiMon (4x1.5V/6V).
http://www.ortodoxism.ro/datasheets2/d/0jpp0yois67wsye8qa4a6u59dfcy.pdf
It has about 100mV DO.
As soon as I find time, I will check it out, if it works.
Greets, Roger