Can I power my 5.7 V DC synth with a 9 V power bank?
9 volts is probably too high. What is the rated output current (Amps) of the original power supply? What is the rated current output of the 9v power bank? First, make sure that you have enough power output from the power bank for your synthesizer. After that, you want to get yourself a buck converter rated for the amount of power you're working with. Just search for "buck converter" on eBay and you should be able to find something adequate.