I am trying to drive a motor H-bridge, but my control system can only support a 400 Hz PWM signal (black box, can't change it) which gives the motor an extremely loud 400 Hz whine as you would expect.

Is there any way to take the 400 Hz PWM signal and convert it directly to a 25 kHz PWM without passing it through a microcontroller (they are verboten for this project)? I can't put the input PWM into an RC filter because I need to preserve the system's time response.

Any ideas? I did a search on Digi-Key for any IC's that could do this, but came up empty.