[sdiy] paralleling piezos

[Ingo Debus]

Hi list,

at the moment I'm building a xylophone-type MIDI controller. The  
thing doesn't produce any sound, just MIDI note messages. The keys  
are wooden blocks, sized approx. 15cm * 4cm * 2cm (6 inches * 1.5  
inches * 0.75 inches). I use cheap non-encapsulated piezo buzzer  
disks as sensors (from Conrad, 690635).
In order to have a key "speak" uniformly, no matter where it is hit,  
I mounted *two* piezos under each key. Thinking piezos are just  
current sources, I paralleled them. The two piezos of a each key are  
fed into an amplifier with 1 Megohm input impedance.
Now I found that these piezos vary a lot in sensitivity. To my  
surprise this can be corrected somewhat by putting a resistor of  
100k...1Meg in series with the more sensitive piezo. The less  
sensitive of the two piezos even seems to become more sensitive with  
the resistor in series with the other piezo. I have to experiment  
more to verify this though. It seems that one piezo is a significant  
load for the other one; after all they have a few nF capacitance. Hm,  
would it make sense to hand-select pairs of piezos by capacitance?

FWIW, the 73 piezo pickups of the Yamaha CP-70B electric piano are  
all in parallel, only with one 4.7k resistor between keys 63 and 64  
and another one between keys 65 and 66. The signal is taken from the  
high keys side and fed into a preamp with 470k input impedance. This  
works very well, there are no noticable volume differences between keys.

Ingo