Yamaha DTXpress/DTXplorer/DTXtreme

The first thing to do is check you are using the correct circuit (see
the files section here) because some of the circuits floating around
have been modified by other people and have a diode (D2 on my circuit)
missing from the rim pad to the tip of the jack.  Then check you have
built it correctly.  If you have had to subsitite a different
transistor make sure it has a decent current gain, although as the
Yamaha module has a fairly high input impedance even the most rubbishy
transistor should do - I haven't tried it.

The use of the Schottky diodes was largely theoretical to reduce the
voltage drop - in actual use I have never noticed any difference with
normal small signal silicon diodes (eg 1N4148). 

The piezos easily generate 10V or more and can happily drive 100k.  I
have used it with the KP65, TP60s, PCY65 and Pintech & Roland pads
with no loading problems so I think you have a construction/wiring
problem.

Keith.