okay, if i buy a linetransformer and the ad20, i have to find a way to connect my okm2 mic´s.
the okm2 has a stereo miniplug ....
Typically, you shouldn't need both the line transformers AND the AD-20. The line transformers apply gain. The AD-20 applies gain. You don't need both. And you'll need some sort of power for the mics (sorry, I'm not familiar with the OKM2's, so I don't know if they require internal power, battery box, or phantom). At any rate, with the gear you've mentioned, you'd run one of these setups:
[1] OKM2 > power for mics > line transformers > JB3
[2] OKM2 > power for mics > AD-20 > JB3
In [1], you're using the line transformers for gain and the JB3's ADC. In [2] you're using the AD-20 for gain and ADC. I think [2] will sound better, but it's also the more expensive option. Line transformers are cheap (approx USD$40/pr), so if you're on a budget I'd recommend [1] and see if you like it. If you decide it doesn't sound good enough for your tastes, then you can always sell the line transformers and pick up an AD-20.