I am just a few parts way from having a passive (transformer based) mic pre that I'm cobbling together from a Rolls PB223 phantom power supply. I started with that, as it already provides phantom power, so that part is already designed. I have some very cheap impedence matching transformers that provide approx. +12 db of gain with the MSH-1O's i plan to use it with. My idea at this point is to add a basic transitor circuit (after the transformers) that will help blend in the line inputs when needed. I have severel mixer schematics that will work on 9-12v dc power, which is what the Rolls box runs on. Right now, I just need a descent dual 10k audio pot and a couple of RCA panel mount jacks and I'll have it done. The cheap transformers could easily be replaced in a later design with quality units. After the prototype, I figured I'd make a real nice one in a Hammond or similar enclosure. Maybe even make an integrated or removable 8x 6v DC small SLA battery pack for the phantom power, like Doug uses.
All it takes is time and money... I have time, with some motivation the money would come as well...