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Modifying input impedance of Boss OC2 to use as piezo buffer
Modifying input impedance of Boss OC2 to use as piezo buffer
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Old 1st January 2018, 08:13 PM   #21
PRR is offline PRR  United States
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> a pizo as roughly a current source in series with an internal capacitance

More useful to imagine a voltage source in series with a cap.

Or a current source with a cap across it.

A current source is an infinite impedance, so it does not matter what you put in series.

The underlying principle of a piezo seems to be deflection causes voltage, so a voltage source appears to be the more natural model for the mechano-electric action.

Of course VS+C and IS||C are duals, equivalent, so on the electric side can be modeled either way.

Charge-amps have been used in condenser microphones. They are also a favorite tool for photo-diode work.
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Old 1st January 2018, 08:51 PM   #22
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by GeorgK View Post
This is interesting, I wonder what the downside is as I have never seen this in a PU amp stage yet?
Since I first found out about this approach, I too have wondered why it wasn't widely used for audio. (Though it seems to be used in various other fields.)

I don't have an answer, though. I never really found a downside, at least in this era of cheap JFET input opamps.

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Originally Posted by GeorgK View Post
I have always regarded the behavior of a pizo as roughly a current source in series with an internal capacitance (would you agree?), so the arrangement would make sense.
I found a couple of ways to make sense of it.

One way: most of the old electronics textbooks show a piezo equivalent circuit consisting of a voltage source in series with a small capacitance. Usually source resistance is assumed to be zero or near zero.

If you short this out, the current through the capacitance will be the usual i = C dv/dt.

The derivative tells you the current will be proportional to frequency, i.e., rising at +6 dB/octave. Feeding this into an integrator gets you a flat frequency response.

I was not satisfied as to where that equivalent circuit came from, so I looked at physics textbooks instead of electronics textbooks. The physical reality is that when you bend / deform a piezo element, it generates an unbalanced electric charge across its two electrodes. The amount of charge depends on the amount of deformation. There is also a small capacitance between the piezo electrodes.

The physics textbooks stop there. But consider the capacitor equation Q = CV, from which V = Q/C.

So if you deform the piezo and place a charge Q on its own internal parallel capacitance, you get a voltage which is proportional to the amount of charge, i.e., to the amount of deformation of the piezo.

So the traditional recipe of feeding a piezo into a very large load resistance amounts to making sure that the charge generated by the piezo is not drained off by whatever the piezo is connected to. Basically making sure the RC product is greater than the duration of one cycle of audio at the lowest frequency of interest.

On the other hand, if you short the piezo, you get an amount of charge dQ flowing through it, and this happens during a time dt which is the duration of one audio cycle (period T). So the current dQ/dt is equal to dQ/T.

Since T = 1/f (where f is the frequency), that means the current through the piezo is f dQ. dQ itself is proportional to how much deformation the piezo experienced, so basically, we have a shorted current which is proportional to the frequency, and to the amount of deformation of the piezo.

Once again, if you run that current into the input of an integrator, with its 1/f frequency response, you end up with a flat frequency response...

You can scale the integration capacitance to set the amount of output voltage, and put a resistor in parallel with the integration capacitor to set the lower - 3dB frequency.

I discovered this approach about twenty years ago, because the piezo elements I was trying to use for motional feedback in a subwoofer did not have much capacitance, and they needed to respond accurately down to 10 Hz to keep the servo feedback stable.

This meant that the "feed it into a large enough resistance" approach resulted in very large input impedance, and chronic hum and noise problems. Feeding the piezo into a dead short (integrator input) worked much better for me.

-Gnobuddy
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Old 1st January 2018, 09:06 PM   #23
PRR is offline PRR  United States
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Note that a piezo is also a bunch of resonances and a "speaker".

So open/short termination should give different internal responses (damped, undamped).

I'm inclined to think undamped is more "exciting" and damped is more "accurate".

There may be another side to this. As you say it is a Charge effect and that is not yet intuitive to me.

And as you say, 19 cent opamps can be wired both ways for cheap trials.
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Old 2nd January 2018, 01:47 AM   #24
Gnobuddy is offline Gnobuddy  Canada
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For my purposes (wide band acceleration sensor), I needed to ensure no piezo mechanical resonances within the bandwidth of interest (10 Hz to 5 kHz or so).

I used a generic disc (bimorph) piezo, which was fairly floppy on its own, with a first (bending) resonance around 2 kHz, if I'm remembering correctly after all these years. To stiffen it up, I machined a mount for it that supported it rigidly all the way around the edge. Mounted like this, the first mechanical resonance was up around 5 kHz.

Piezo material being dense and stiff, it would take sizeable electromagnetic or electrostatic forces to affect its motion. Very little electric power is generated by bending. So I suspect (but am not sure) that there is little or no mechanical damping from the induced electricity.

-Gnobuddy
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Old 2nd January 2018, 06:15 PM   #25
GeorgK is offline GeorgK  Austria
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Gnobuddy & PRR,
agreed with all you said.

Anyway... I still have difficulties to put this all into at least some kind of equivalent circuit.

My reason for calling it a current source is that a piezo is actually non-conductive, and I made this non-scientifical thought experiment:
All that a piezo can do is shift charges from the one side to the other. If you could bend it infinitely, you would have a DC source. I you could bend it infinitely fast, you had infinite current. Vice versa, Voltage applied to the piezo results in deformation.
So, in my mind, the element should tend to hold against mechanical force as long as the charge can not be balanced, meaning no current flows. Applying more force will make the unbalance (or voltage) higher as long as current is allowed to flow. Isn't that the reason why you get a spark in a piezo lighter?

This ability to build up nearly deliberate voltage combined with a high resistance reminds me of the definition of a current source.

Or am I completely mistaken?
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Old 3rd January 2018, 05:37 PM   #26
Gnobuddy is offline Gnobuddy  Canada
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My way of looking at it: a current source can supply a DC current indefinitely into a load. Since charge is current multiplied by duration, it can also supply an indefinite amount of charge (given enough time). When the load drains off some charge, the current source does not run out - it keeps supplying more charge.

A deformed piezo is different in a couple of ways: one, the amount of charge caused by the deformation is finite - if you drain it off, no more charge appears. Two, you cannot get DC current to flow out of it at all (as you pointed out, it is non-conductive). All you can get is a brief pulse of current. Three, if you vibrate it (AC charge) and short it, the amount of current rises proportional to frequency.

So the piezo doesn't behave much like a current source - but its DC behaviour is exactly like a capacitor that's been charged. And it's AC behaviour is also exactly like a capacitor with a fluctuating AC charge on it.

I think that mental picture is closest to the physical reality. A piezo acts like a capacitor, which can be charged by physically deforming it.

I think today's piezo ceramic discs are kissing cousins to ceramic disc capacitors even when it comes to the basic construction - a ceramic dielectric, metallized on both sides. The only major difference is that the dielectric is deliberately chosen to be piezo-electric in one case. (And sometimes ceramic caps exhibit unwanted, accidental piezo-like behaviour, too.)

Does that make sense?

-Gnobuddy
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Old 3rd January 2018, 08:06 PM   #27
curlewcat is offline curlewcat
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so...
I've put some capacitance across the pup and sure enough when I plug into my oc2 not only does it not distort but it also gives me all of my low frequencies into the 1Mohm Z

Lowest frequency on the violin is 198hz so I've used a total capacitance of around 1500pf (including the pickup and cable capacitance). Basically makes my fiddle a similarish signal level and output impedance as an electric guitar, ideal!

Talk about thread drift, ended up having very little to do with modifying oc2s in the end. Cheers guys
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Old 3rd January 2018, 08:18 PM   #28
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by curlewcat View Post
Basically makes my fiddle a similarish signal level and output impedance as an electric guitar, ideal!
Excellent! The perfect happy outcome!

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Originally Posted by curlewcat View Post
Talk about thread drift, ended up having very little to do with modifying oc2s in the end.
True, but it did give you the answer to the real question you were asking, the meta-question one layer above "How do I modify my Boss OC2?"

That meta-question was "How do I get full bandwidth and no noticeable distortion out of my violin, while running it through my Boss OC2?"

-Gnobuddy
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Old 3rd January 2018, 11:19 PM   #29
curlewcat is offline curlewcat
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Quote:
Originally Posted by Gnobuddy View Post
Excellent!


True, but it did give you the answer to the real question you were asking, the meta-question one layer above "How do I modify my Boss OC2?"

-Gnobuddy
Does that sound dangerously like a meta-meta-question to you?
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Old 3rd January 2018, 11:20 PM   #30
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by curlewcat View Post
Does that sound dangerously like a meta-meta-question to you?


Though it was a statement, not a question...

-Gnobuddy
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