For powering up a preamp from the USB port of a cheap spectrum analyzer, I needed a converter that would provide me with 15V/50mA at extremely low levels of ripple and noise. USB is not very clean, converters are (normally) not very clean, so this became quite a task. At the end of the day, I found an (inefficient but clean) solution.
A USB Powered 15V Supply
Electronic Projects for Fun 