Got several requests for this one. Basically, it's small preamp to plug into a mobile device for use with apps like iRig. Pretty simple circuit. Shouldn't be hard to fit in a 1590a or even smaller since you don't need to include a footswitch.
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.