Can the output be floating, or does it need a common ground with the supply?
Start with the circuit for a 12v 250W rms bridge audio/servo amplifier and then drive it with a frequency of your choice. Take the output, full wave rectify and smooth it and sample it to give a voltage to control the frequency generator output amplitude.
If common ground:
Stick a 1:1 transformer with appropriate ratings between output and rectifier.