Here's a light-medium gain overdrive designed by RNFR from the Apocalypse Audio blog. Here's what he had to say about it:
I thought I would build something that would turn on the guys that were into the lighter side of things- something different than my usual wall of fuzz tones. With the MLOD, you can get a heavy boost, or a light overdrive reminiscent of vintage microphone preamps. The fancy thing about this circuit is the lack of coupling caps in the audio path- something usually reserved for mic pres and hifi circuits. This lends itself to the "transparent", uncolored sound happening here. If you encounter any oscillation at the highest gain settings, simply raise the value of the 100 ohm resistor near the gain control to lower the amount of maximum gain.
Use sockets for the transistors. I'd start with J201s or 2N5457s for Q1 and Q4, and 2N2222 or 2N3904 for Q2 and 2N2907 or 2N3906 for Q3.
This is a library of perfboard and single-sided PCB effect layouts for guitar and bass. I'm not an electrical engineer by any stretch of the imagination, just a DIY'er who likes drawing layouts. It is meant for the hobbyist (so commercial use of any of these layout is not allowed without permission) and as a way to give back to the online DIY community.