Doepfer

I have looked at the Doepfer DIY page and did some web searching but have found no posted schematics or mod diagrams. Basically I want to mod a A-138 mixer to give gain if that is easily done. I would assume it is a simple resistor change in the feedback loop of the last stage op-amp but without a schematic or assembly drawing I don't feel like tracing the circuit out to figure which resistor to change, I am looking for modest gain, enough to boost a typical line/headphone out of a MP3 player to levels that work nicely with a synth rig. I have a pair of A-119s already but they seem wasted on some applications where I don't need the other bells and whistles they offer, sometimes all I need is gain - no gate detection or envelope followers. I know it is a dead simple circuit to make too but since I have six A-138s in my rig, I thought it would be easy to just mod one. All my A-138s are the older versions (without the DC offset) if that matters. Thanks!

-James