Parallel Piezo Pickup Problem
Check that there are no active electronics in the drum pads. If there are you would have the scenario described in Figure 1.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Parallel connected piezos.

On first glance you might think that either output will drive the next stage. With amplifier outputs the device with no output will try to maintain 0 V on the output. If the devices are the same type the best you could hope for would be mid-way between 0 V and the output of the drum that's been hit.

If this is the case then a simple mixer circuit would probably suffice.