How to modify a piezo preamp circuit for 2 inputs?

[simba]

Hello! Could anyone help me, pls?
I need to build a pre-amplifier for two contact mics made out of piezo elements.
I should amplify the sounds of small metal pieces touching glass jars one by one. The contact mics will be fixed onto two jars.
The preamp's output will be connected to a megaphone with a miniplug.
(It'll be for an experimental performance art project.)

This preamp design from http://www.megalithia.com/sounds/tech/piezo/piezoopa.pdf seems promising to me, but in stead of one piezo element I would need two.
Somewhere I read that I shouldn't simply connect the cables of the two incoming signals. I assume I should also re-calculate the values of some of my components if I get another piezo..?

I'm a complete newbie on electronics and don't know anyone who could help. Any tips/recommendations/further reading are super-appreciated.

[pebe]

Hi Simba,
As you say, you cannot simply join the two piezos together. They have high impedance outputs and must be buffered from one another.

Here is a way of doing it. You need the extra components shown in blue. The extra opamp has unity gain, and its output is fed into, and mixed with the signal of, the original circuit at 'A'. The original circuit is not affected and there will be the same gain from both piezos.

Best of luck.

[simba]

Thanks a million, Pebe!

Your reply is already very helpful. But did you attach a file or link to you post? Or were you meant to? I can't find it, but I understood that you refer to something with "Here is the way of doing it. The part shown in blue (...)". Sorry if I missunderstood or just fail to see it...
Heyhey!

[simba]

Hey the computer I'm using just didn't show your attachment at first. I got it now. sorry, wrong alert!