Me and my high school friend made a DC to DC converter that would give us 30 or so volts at a couple amps from a 12 volt battery. We used germanium power transistors which could not go much above 10 kHz. The Delco’s and the 2N458A’s were our favorites – we managed to push them to their limits and burn out a number of them.
We built a HiFi amp, and added the converter to it and it could really put out the power – probably about three times what the normal 12 volts could put out. But the problem was that the toroid core on the converter screamed real loud! It was magnetostrictive! Either Hypersil or Deltamax, I forget which. They were sealed in a metal donut, along with silicone oil to reduce the friction.