[sdiy] paralleling piezos

[Harry Bissell]

Roland has at least one piezo on the frame they would subtract from the others
to try and isolate the pads better, and detect clumsy (rim) shots :^)

H^) harry


----- Original Message -----
From: karl dalen <dalenkarl at yahoo.se>
To: sdiy DIY <synth-diy at dropmix.xs4all.nl>, Ingo Debus <igg.debus at t-online.de>
Sent: Mon, 07 Feb 2011 16:51:21 -0500 (EST)
Subject: Re: [sdiy] paralleling piezos

How do they solve polyphony on touch sensitive pads that Drum machines
like the AKAI MPC series uses?

In the past i recall drum machines used to be one piezo per pad and
quite high tresholds to reduce interference then i recall Roland used
something like 4 for all 16 pads spread out and a smart algorithm to
detect which one where hit and how hard, now i heard that on later
MPC's they use some sort of resistive math with a in built grid or
something to detect pad and velocity?

Hal Chamberlin did a polyphonic velocy sensitive stick board for
Kurtweil using a resistive, (cant remember the name of the material). 
It's in this interview somewhere.
http://interviews.sonikmatter.com/2002/hal_chamberlin.php

Reg
KD

> Ingo Debus <igg.debus at t-online.de>:

> Am 06.02.2011 um 22:47 schrieb Ian Fritz:
> 
> > If you have two devices in parallel and strike only
> one, you will obtain half the expected voltage.  (The
> same charge but twice the capacitance.)
> 
> Yes, but only if the two piezos are equal.
> Today I went to the shop and bought another 20 piezos. I'll
> check their capacitance before using them.
> Meanwhile I found a circuit called charge amplifier. It's
> basically just the piezo connected between ground and the
> inverting input of an opamp and a capacitor (with a bleed
> resistor parallel to it) from output to inverting input.
> Since the piezo sits at virtual ground, the additional
> capacitance of the other piezo won't matter. But this would
> require an extra opamp per key. I could just as well have
> separate buffer amps for the two piezos (which I plan to do
> anyway, to detect the position where the key has been hit).
> Here's a nice writeup, I found this through Wikipedia:
> <http://focus.ti.com/lit/an/sloa033a/sloa033a.pdf>
> 
> >   If you want to add the responses, I'm
> pretty sure you want to connect them in series.
> 
> If they're viewed as voltage sources, yes. I thought they
> are more like current sources, given their very high DC
> impedance. But obviously that was over-simplifying.
> 
> I haven't tried to put them in series yet (to answer
> Harry's question). After replacing some piezos, 13 out of 16
> keys are working ok-ish; so I don't want to make radical
> changes now.
> 
> > Careful, they can put out large voltages.  I have
> a couple of experimental tap sensors where the tap bends the
> sensor. One of them easily puts out 30-40 V
> 
> Yes, I have a 5V zener diode across each imput, just in
> case.
> Hey, piezos are used in cigarette lighters to generate the
> spark that ignites the gas :-)
> 
> 
> >   And there some cool folks online who
> have used these cheap beepers to make sensitive
> seismometers!
> 
> Cool. I always wondered if they could be used as a heart
> beat monitor?
> 
> 
> > Are you trying to detect the magnitude of the strike,
> or just its occurrence?
> 
> The magnitude of course. And maybe roughly the position too
> (later).
> 
> 
> As to Harry's suggestion:
> > The piezos might load each other, the resistor would
> decouple them. Maybe a resistor in series with
> > each (a passive mixer) might work better.
> 
> Well, yes, with two unequal resistors, compensating for the
> unequal piezos...
> 
> Ingo
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-- 
Harry Bissell & Nora Abdullah 4eva