Piezo Disc Preamp

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:smash:

Hi All,
I have modified/redesigned this Piezo Disc preamp for acoustic guitars.The original circuit was published in http://www.redcircuits.com on page 16,but it never worked with a p.disc! or may be I (had) overlooked something?

The mods & additions to the original circuit are as follows;
1.A high impedance buffer
2.All IC's changed to TL072s ( works better than TL062)
3.Dual (+/-) power supply...sounds more "spacier"!
4.The o/p capacitor 0.022uF of the 2nd stage from the original was changed to 0.33uF

PIEZO DISC
Diameter - 20mm
Capacitance - 14nF +/-30%
Resonance Freq, 6KHz +/- 0.5%
Resonance Imped; 200? (unit not specified)

The placement of the disc is very critical for optimum sound. It seems that, there's only ONE "sweet spot" on the guitar & then it sounds almost (realistically) acoustic!

Attaching/fitting the disc directly on to the guitar changes the sound drastically,when the volume "pot is turned more than half way up.Placing a small piece of masking tape between the disc/guitar & taping the disc over seems to cure the problem to a great extent at cost of the sound being a bit "thinnish"

Doesn't seem to work well with USTs... at all uhh! (sounds typical piezo!)
However, there are still a few niggles with this circuit. I'm not sure if I have configured the 2nd stage properly or not!

So pointing out any of my oversights or additional improvements to this circuit would be most welcome.

I've no SPICE or any other simulation programs...

Thanks.
 
Here's the schematic!
 

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Can anyone please tell me if there are any components /values which should be changed or added for optimum performance.

1.Is the value 150k resist; correct to the 2nd stage invert.i/p?
2.Should the non-inverting inputs connected by resistors to the
ground? what values should these be?
3.Is the 470k volume p. ok or should i add another low value R in
series?

I AM STILL NOTICING A SLIGHT LOADING ON TURNING THE VOLUME POT ALL THE WAY UP!(THE SOUND CHANGES) This is somewhat reduced by by not placing the piezo directly on the guitar.

The circuit drives the 1Mohm Piezo input of my Roland AC60 quite well.

Added a summing amp..& this drives the Line input as well.

Any suggestions?
Thanks.
 

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hello.
optimum performance........you will hear it (with your instrument).
i think:
the non inv. inputs of the fet -opamp can be connected to ground,some people like to build in a small res (to lower offset or so..)
the input cap 1uf is not essential,perhaps you try it without that
the 470k pot is a high value res that is sensitive to humming and noise........perhaps you can use a lower one (100k) ,and for the 150k use a 33k (and you can increase the 0,33uf to 1uf -filmcap),the voltage amplification is around 3 (=100:33)
do not forget to build in psu decoupling caps nearby the pins of the opamp (e.g. 47.....100nf ,and perhaps an elco 47.....100uf paralell)
try it out step by step with listening tests.............
greetings...........
 
Thank you indeed MJF,
As I 've mentioned earlier,it sounds quite pleasent except for the sound,which is a bit thin.But when I turn up the 470k pot 3/4 way up, there is a drastic change..almost like a filtering effect.The sound gets even thinner!

Measured all the IC +/- in & o/p voltages without any signal,& they seem ok..0-3mv???

Wonder,if this could this be anything to do with the resonance impedance of the piezo itself?
I also noticed,that placement of the p.disc & the amount of pressure exerted on it makes a BIG difference.

I will try with 2 piezos in series..

I did try without the 1uF i/p cap..it sounded very harsh & nasty.

I will definitely change both the 470k pot & the 150k res, to the values you've suggested.
What happens when the pot is completely turned off?

It's driven by 2 x 9v battaries (PP3) as i am planning to build this as an onboard preamp..

Yes,I've 0.1uF polester caps on all IC supply pins.

Shall keep you posted.
Cheers
 
hello.
an offset around +-5mv at the output of the opamps is o.k.
voltage gain of the first stage is around 3 (1+(10k:4,7k)),the second stage gives 3.............so it is 9x max. together.and the third stage amplifies too..........when you overdrive it the sound will be thin and hard.
amazing thing that it sounds nasty without input cap (1uf).......
the second opamp stage is like an active volume control (a volume pot with amplification): pot fully turned up :three times.......turned up to 150k (as the input res is) gives 1x.............turned down to zero ohm should give zero volume.
the tone control stage is a standard application similar to national lf 353 opamps or so.............the 470k treble pot is a little bit high,often there is a 50k...........100k pot.will have a look at it.
greetings..............
 
Hi,
Thanks again for taking the time & for all the useful info.I've replaced the values to 100K pot & 33k & it works well.

Yes,without the input cap,it sounds just like an UST..

I did try many types/values.Electrolytics give a slight time delay.
especially when you tap on the piezo itself & exhibit rather unmusical sounds!I think it must be due to the p.disc as the source with it's very high capacitance or it's electrical characteristics!!)

Varying the buffer o/p "cap, 0.47uF is a little low on the treble to my ears & 1uF sounds very dull indeed. I do admit,I am little inclined towards more treble on my guitar, the result of using "Teles" all my life, hence,0.33uF sounds just right to me.

I've also connected 22k res. to ground on the + i/puts.

I did have a look at the LF353 application notes & they too have used a 500k pot in one of their tone control circuits.I still h'vnt
tried with a lower value..but do so as soon as I finish my "casing"
for the pre.

Cheers.
 
hello.
i am not a piezo expert,so i cannot give you good hints for the position of this on your guitar and so on...........
sorry,question.......what means the abbreviation ust?
some years ago i built this (hifi) tone control (and simulated). the 500k (or 470k)pot is the treble control and i used a 50k (and 100k) pot in this place and it worked without problems.it is a sensitive tone control,so turn up and down the pot's carefully (i would turn up fully the volume control at the output (10k) and would try to adjust the volume with the second stage pot).
greetings...............
 
MJF,
UST's are abb;for UNDER SADDLE TRANSDUCER..the types usually installed under the bridge/saddle of most of the (electric) acoustic guitars. I am sure you would have encountered them some time or other!

Yes,you're spot on about not turning the pre.a pot fully with regard to the main o/p volume pot (the one on the summing/ mixer.

However, p.discs seem very peculier in their behaviour to fre.changes.. & the tonal characterictics seem to corrospond directly...(stating the obvious here)to their diameter...but I am no expert on this subject..it's just my experience trying different discs,but then they sound much more natural than UST's to my ear.

I wish some knowledgeable people out there would enlighten us more on this.

I will give you feedback with different treb. pot values. Here's the PDF file of the LF353 tone circuit.

Cheers.
 

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hello.
..............ust........we use different names here for these things......like piezo pickup.....piezo bridge.....coax piezo (a piece of piezo coax cable).............kontakttonabnehmer (piezo discs? ....to stick on the acoustic guitar nearby the saddle) and so on.
i read in a manual if the tone is not clean with the piezo pickup it has mechanical reasons.......e.g. the pickup doesnot sit tight enough at the corps of the guitar.
greetings............
 
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?
 
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?
 
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.
 
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.
 
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.







.
 

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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.com which 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
 

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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.
 
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