Schematics for compact cassette deck's

I want to update a cassette deck output from the standard garbage innards to something a bit more high quality. I would like to build a very good preamp circuit to transfer some irreplaceable cassette recordings to 24/96KHz digital via an analog balanced input computer sound card. The thing is, I can't find any schematics anywhere on the net. Does anyone know of a site that has high-end schematics to build one?
I would post this question on the diyaudio site. I'm sure someone will answer or may even have already asked the same question.

Good luck!
I would first start with upgrading the parts in the deck. If it uses IC's I would look to upgrade those and upgrade the power supply caps, rectifier, in line resistors in the power supply, all caps resistors in the signal from input to output, and add better RCA connectors on the output or make a Balanced (XLR) output but you will have to find a circuit/schematic of the ICs that converts single ended to balanced >transmits>receives>converts back to single ended. Pick up the Randy Slone "The Audiophile's Project Sourcebook", this book has the circuits for these XLR's. I hope this helps you out.