I found in the magnetic parts dept a few more cores of the right mechanical size, with around 60-80 nH A sub L range. This gives some more options. I'm also looking at a two-stage type setup, going 20:1 then 5:1 for 100:1 ratio overall. If all goes well I expect it might go from about 5-10 kHz to 200 MHz. Not flat though. From what I've seen so far, it's probably better to just regulate to output level closed-loop.
Ed
