Yamaha DTXpress/DTXplorer/DTXtreme

I have tried to make the circuit to work with two pads but run into 
problems. It seems to require the pad to be hit really hard in order to 
make the rim shot sound. Reducing the R1/VR1 combination did not help, 
and it caused the input impedance to drop, to the extent that it simply 
killed the sound completely.  

My guess is, becase the transistor has no bias circuit, to make it turn 
on, the piezo has to provide all the current and voltage (at least 0.5V 
B-E), and it is asking too much from the TP60 pad.

Has anyone tried to modify the circuit to make it more sensitive, maybe 
by using a FET instead of BJT? Any ideas?

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.

Thanks for the comment regarding my initial problem. I replaced the 
transistor with a new one with a higher current gain (580 versus the 
the old one with 290 hfe) and it did improve. But the sensitivity is 
still a tad low. I still need to hit the pad harder than normal to get 
the module to register a sound, and it will not pick up any rolls. The 
rest of the circuit is exactly like the diagram except R3 is omitted 
(open) and the diodes are 1n4148.
The DTXpress module I am using is the first version. I wonder if its 
trigger is slightly different than the later versions?

I am not aware of there being any major differences between the first
three versions of the DTXpress module.  Have you tried changing the
attenuator switches?  I haven't tried removing R3 - have you tried it
with R3?  Also, you could try it with D3 shorted.

Keith.