Piezo Disc Preamp

[mjf]

.......here is an example of a manual for a piezo pickup (schaller,germany).........BA.........
perhaps it can help......

http://schaller-guitarparts.de/hp135250/Oyster-721.htm

greetings.............

[teleman]

Thanks,yes,I too came across this link quite some time ago.I did try 27 & 35mm p.discs,not this brand,but they don't sound that good with dreadnought or jumbo type guitars.These sound rather lacking in hi's & tend to feedback easily at lower frequencies.

I've also seen these advertised elswhere,& they recommend these sizes for use in "Contra/Double bass/chello etc.

In my personal experience 20mm discs sound the most natural on (jumbo acc).guitars so far albeit a little "thin" with just 1 element.

Btw, http;//www.americanpiezo.com gives a lot of info on piezo technology.I realize now that piezos are not as simple or easy as one would like to think/use as p/u's in musical instruments, if sound integrity is of paramount importance.

I am no expert on piezos,but think have re-discovered the following from my own experiments.
1.With larger diameters..lower freq.response?
2 Larger diameter discs resonate at lower freq. This resonance
freq;which creates these drastic change in tone.
3.Smaller the disc,higher the res.freq.hence can avoid resonance
at lower audio frequencies.

Am I right in my thinking that higher freq;have low energy/amplitude,so they get blended easily with rest of the sound,thus giving much linear frequncy response?

[phase_accurate]

Due to the usual impedance characteristics of a piezo pickup some people recommend the use of a charge-amplifier as the fist stage instead of an ordinary amplifier with high-impedance voltage input:

http://scopeboy.com/elec/chargeamp.gif

Regards

Charles

[teleman]

Good suggestion,Charles.I was wondering,isn't this circuit meant to give a constant pulse/charge?

I once came across this on www.electroportal.net where many seem to complain that it didn't work.I couldn't understand the language,Spanish?
Anyway,I will hook it up & let you know of the results.!

Btw,I've been doing a lot of reading lately on Piezo discs, & will give you my findings soon.

Here's just a parting thought:

The piezo disc or any piezo element,when,it is triggered by the guitar strings & not driven by any external voltages or current, can be assumed as a purely LCR device,correct?

So it should be producing only marginal changes in it's LCR values depending on how hard or soft the strings are played right?

Then,it should be exhibiting almost a static or a constant energy- to the load,the preamp. In other words presenting almost the same LCR values.?..or am I up the creek again?

So,will the resonance freq,of the device play any part in the sound produced by the preamp?

Any thoughts?

[teleman]

Phaseaccourate Thanks a million!

The charge amp circuit DOES work!It was an amazing transformation to the entire circuit! The sound was so revealing!

I connected it to a dual battary supply +/-9, by removing R2=470K & replacing R3 with a 47K instead.

I didn't have a R4 - 220ohm- on hand,so hooked up a 330ohm instead. The 10uF o/p cap still was giving a little "time-lag" so left it out & connected the output directly to the 1uF i/p cap on the buffer. Yes,I did leave the i/p buffer in as it was & this works very well indeed with the charge amp.

I could, for the first time hear the natural "timre" of the guitar!
I had about a metre of "braided" lead from the p.disc, yet it was so quiet!

The piezo disc I'm using has Ci=14000 pF( 14nF) capacitance.So the 10nF= Cf= feedback/gain cap seems is just about right.

Vg= Ci/Cf =14/10= 1.4 (It's the reverse of opamp gain law)

However,there's still a little "Delay Time Constant" problem, which I'll try to remedy. The placement of the p.disc is still very critical & changes the sound dramatically on different locations on the guitar.

Since I had overlooked the importance of the "caps" on the battary supplies,I suddenly noticed that the - 9v battary had drained itself to -7.9v!My stupidity!(This, I've gathered is very important,when using high impedance low gain circuits.)

I've been doing quite a bit of reading on Charge Amps & will post my findings soon.

[teleman]

As promised here are some of my findings on the the secret world of piezo.I'm sure most of you already know about piezo discs.Any way read onyou may catch something new.

1.A piezo ceramic disc can be considered of consisting a capacitor in
parallel with a current source.

2. This parallel capacitance represents the disc sensor acitance,hich is proportional to the actice electrode area.

[teleman]

oops,as usual ..the pc beat me to it....so awiting permissin to post again! sorry!

[teleman]

A Piezo ceramic disc can be considered of consisting a capacitor in parallel with a current source.

1.This parallel capacitance represents the senser capacitance
which is proportional to the active electrode area,and inversely
proportional to the thickness of the sensing element.

2.This value is usually measured/given at 1Kz and pF or nF.
for relatively small diameter from 5 to 50mm discs.

3.This capacitance can be considered as the source impedance.

4.The physical constants of the ceramic crystal determine the
equivalent values of R1, C1,,L1, & Co (please refer to
attachment)

5. A piezo disc acts as a voltage source as well as a capacitor.

6. This voltage output is inversely proportional to it's
capacitance.ie: Low capacitance = high output
High capacitance = low output.

7. When connected with an interface circuit, a buffer there's a
resistance added to the circuit.

8. This series connection of the capacitance & the load resistance
forms a voltage divider.

9. The impedance of the capacitor varies with frequency, this
means,the voltage across the interface will also vary with the
frequency of operation.

10.As the frequency drops,the impedance of the piezo disc
increases,which in turn lowers the voltage to the interface.

11.It is important to note that this source resistance becomes
higher for smaller capacitance & lower freq;

So, from this one can conclude,that,
a. the resonance freq.of the disc doesn't play a big part whe the
disc is operated as a trigger device within the audio range.

b. smaller diameter capacitors,with lower "C"'s are more
suitable as acoustic guitar sensor pickups than larger ones!
ie: 5-20mm (these usually have 8000-15000 pF values)

Now I think I know why we often get away with sticking 35mm
discs directly to amps, since these have very large "C" hence relatively lower voltage out puts,which don't overload the amps!
puts or affect the lower freq response although lacking in high frequencies.(which many don't seem to mind)

So, by employing smaller diameter discs & utilizing a high impedance buffer or a charge amp we can both get a very good low & hi frequencis are attainable.







.

[teleman]

I know,I shouldn't be posting this here,but this is related to trimming/testing of my Piezo Disc + preamp,so please bare with me.

I have nearly completed this power supply circuit by Albert Kreuzer,on http://www.albertkreuzer.comwhich looks quite neat & simple.

After reading the excellent posting in "Another look at LM317 & LM377 by "jbau" in the power supplies section of the DIY forum,I wanted to give it all up & live in a cave for the rest of my life! What a brilliant analytical mind!Tremendous work & my hat's off to you Sir!.

Now I need someone's help & advice on 2 things.

1.In the circuit,the 0.1uF capacitars C3-C4 are are just "Film or similar types I would imagine.

Q:Can I replace these with the PME series capacitors like
840E,426,840M.844 or 845 for better EMI /shielding /protection? & as a SURGE protector at the same time? If it is so,then which ones from these series should I use?

My 2nd Q' is;
Will,replacing these reg.IC's with LM317/377 vastly improve the PS circuit? I do know, the voltages can be fine trimmed with these
regulators.

Any good suggestions? Thanks

[teleman]

I thought I'd make the following changes to the PSU circuit & would grately appreciate if some one could tell me if this would give adequate protection from surge voltages & EMI,as I don't want to blow any of my projects while testing!

Change C3 & C4 with PMR 209 series anti-surge caps of the same value.

Connect a PME 271 E or 271M series EMI cap across the primary.
would 0.1uF be ok?

Thanks.