This circuit arose from the need to couple the outputs of two CDROM drives to the input of a single sound card. Simply ‘stacking’ the connectors is possible but not particularly elegant, and the result is somewhat unpredictable due to mutual interference between the two drives. Using a quad opamp and handful of passive components, it is possible to quickly put together a mixer that first buffers the signals before adding them together. This completely eliminates any possibility of feedback or mutual interference. The actual addition of the two stereo signals is performed by R3/R9 and R6/R12.
Since these resistors also form voltage dividers that reduce the amplitude of each signal by half, the gain of the four buffer amplifiers is set to 2 as compensation. The required 12-V supply voltage can be drawn from the PC without any problems. The current consumption of the entire circuit is practically negligible.