Buchla 205 Dual Matrix Mixer

I have not built a 205, only repaired them. The mixer is organized as two independent 5x4 mixers where each of the five inputs can be assigned a level to any of the four outputs. The switch also sends that signal to a pair of left and right monitor outputs. Although they are designated left and right, the monitor channel is mono. A separate set of four outputs in the center combines the left and right outputs.


The issue with this mixer was bleed to other channels. I measured the bleed as -51 dB, a low number but still audible. In looking at the schematics, Don used a 2R2 resistor between the CCW terminal of the potentiometers and ground. I have no idea why but this injects some signal into the remaining channels. I replaced R26, the 2R2 resistor, with a jumper and I could no longer hear or measure any bleed.


The mixer is inverting and the All Out jacks are again inverted so are in phase with the input. The monitor outputs are also inverted.

The V1.0 BOM specifies a TL071 for IC4 but the part number is for a TL072.

205 Corrected BOM


The jumper can be seen in this rear image to the left of IC4 in the center.