I looked at the schematics that had been redrawn by Marcelo Johann, but to my chagrin Behringer has changed the design in the meantime - it now uses a single ADC/DAC chip per 2 channels, the CS4270. In some ways it has made it easier to modify, as the output of this chip is single-ended rather than differential. In the end, I was able to modify the output schematic somewhat elegantly, but it involves desoldering the op amps and swapping in a few resistors. It also requires a small circuit to produce a voltage reference to bias the signal back to 0v.
Before:
After:
Notes on my very quick schematics:
The op-amps were TL072, I subbed them for TL082 for better output offset performance. You could re-use the 072's. MCP6002 was used because it runs off of a single supply. I powered it & the voltage divider in front of it with the 5v supply for the ADC chip -- IC3 according to Marcelo's notes. Trim until the output measures 0v. I desoldered the IC's and bent up the appropriate pins (5&10 I believe) then bridged them. I made 2 voltage sources, one connected to the first 4 channels, the other connected to the other. Probably not necessary but I had an extra op-amp available. If you really wanted to get fancy you could probably make voltage sources for each one to trim them each to 0v. I haven't measured the difference between channels yet.
