Piezo preamp yet again

[dunkelfalke]

Sorry to bother you with it again, folks, but since the tube preamp didn't work, I still need something.

I've got this scheme here and it seems pretty straightforward, except for one part:



the 1M resistor in parallel with the 1500pF capacitor.
As far as I understand, the resistor sets the input impedance and the capacitor lowers the input voltage so the OpAmp won't distort at voltage spikes.

The problem is that these values set the cutoff frequency to 106Hz. It is fine for a violin but not good for a guitar that can divebomb. If I set the capacitor value to 10nF I get a much nicer 16Hz, but then the input voltage would be reduced - if I calculated correctly - by 15 times.

I could take a 1nF capacitor, that would be fine, but then I've got to take a 10M resistor and I am not sure whether it would work (and it would have to be a carbon resistor, and those are not very good I think).

Any suggestions?

[JohnH]

Hi Dunkl. Its late here now, but tomorrow i might type that circuit into 5spice and we'll see how it goes....

John

[JohnH]

I had a go at this, with this idealised version on 5Spice:



and here is the response:



Things to note about it:

1. The piezo is effectively like a capacitor itself. I'm assuming it is larger compared to C1, or, consider C1 to be equal to the combination of the actual input capacitor and the piezo capacitance.
2. The voltage at the '-' input to the opamp is held very close to ground, so the input impedance of the circuit is virtually exactly that of C1.
3. The gain at higher frequencies is the ratio of the impedance of C2 divided by that of C1. Since higher capacitor value provides lower impedance, then gain = C1/C2. So as drawn, it has a reduction in signal since C2>C1 - not want you want with a piezo.
4. The gain falls at lower frequency based on the relationship of R1 and C2 impedance, and as you note, it drops at a higher frequency than you want.

So here's another version, with C1=2.2nF, C2 still at 1500pF=1.5nF and R1=3.3M



This response seems better, with a better low end and a small amount of gain.

You could try this, and be ready to tweak values. However it doesn't seem to do anything that can't be done easier and cheaper with less current draw, by a single JFET circuit.

cheers

John

[dunkelfalke]

Thanks for trying this out for me, but one thing, though, C1 is the piezo capacitance, it is the value of that electric violin pickup which was measured with a LCR meter. I don't have one so I cannot measure mine.

[JohnH]

OK, is your piezo actually intended for a violin?, so it may have a similar value to that C1.

Unless you can use a prebuilt preamp that is intended for your pickup, then I suggest that you mount your piezo and wire it to a stompbox (to get a high impedance input) and try it, record it to a pc. Then we can figure out what you need in terms of gain and frequency response.

cheers

John

[dunkelfalke]

Well, no, my piezo will go inside a guitar, but this circuit was originally for a violin.
Unfortunately there is no prebuilt preamp for my pickups, there were originally meant to work as passive ones but they are too weak for that and I want to "mix" them with my active magnetic ones anyway with a dpdt switch.

And to be honest, currently I don't have any stompboxes, but I should have enough parts to throw together a fetzer valve from runoffgroove.

[JohnH]

OK, what I meant was, is the piezo of a type intended for a violin?

The Fetzer valve is a good circuit. You may be better with a higher or lower value for the input resistor, instead of 1M. If your piezo is like the violin one, then 3.3M might be good. A capacitor from source to ground will increase gain - they show a 22uF one in the first diagram here:
www.runoffgroove.com/fetzervalve.html

J

[dunkelfalke]

It is a guitar double transducer, but I intend to split it into two single ones.

A question, though, can I use a Digitech GNX3000 multieffect instead of stompbox for this test? I've also just ordered a better multimeter which can also measure capacitance, but it will arrive next week anyway.

By the way, you have mentioned that the whole thing could be done in an easier way. I know about your preamp/mixer, and, while it is very interesting, it is also quite large and complicated. I also know about that little preamp from till.com, I even had one years ago in my guitar, it was pretty much my first project, but I was told that it is the wrong choice for a piezo, I am also not quite sure that it would work as intended, and I would need two of them.

But if you have a suggestion of a simple circuit that can preamplify two piezos and that they could be connected together after the amp stage and also together with a magnetic pickup just by a switch, then I'll happily abandon this one

[JohnH]

Thats fine, connect them to your modeller, with the modeller set to do as little as possible. Then, two questions: is it loud enough or does it need extra volume boost? and how is the tone, does it need more bass etc?

If both are fine, then a simple buffer wil be good (one for each piezo channel). If the tone is OK but you need more volume, then a circuit based on the Fetzer valve or Tillman preamp (similar) could work. If you need gain plus active tone shaping, then you are where I got to with my piezo system.

But check it out!

And getting a meter that reads capacitance will be useful, so Id be interested to see how your piezos measure, which could affect the optimum values for your input impedance of the new circuit.
cheers
John

[dunkelfalke]

Okay, I'll first have to glue the piezos to the sustain block.

By the way, what is wrong with the original circuit, anyway? If it is only the current draw then it should not be a problem because I will get an OPA2604 in two weeks.

[JohnH]

The opamp circuit you posted at the beginning of this thread had a reduction in signal, ie less than a x1 simple buffer. And it has no tone control. So my thought is, if you dont need tone control, you can use a simpler circuit like a Fetzer valve instead, and have gain too.

But opamps are fine in principle - Once you have determined what the preamp has to achieve, then you could use a suitable opamp circuit to do it. I have a personal preference for JFETS, but thats just me!

John

[dunkelfalke]

Ah, I see. Well, TL072 is a JFET opamp and it has the advantage of being more easily available to me than normal JFET transistors (I've got three TL072, but only two BF245B).
Would it be possible to use the opamp for a two channel fetzer valve?

I don't need any tone control, but I need the piezo output to roughly match the output of a EMG 85.

Oh, by the way, I've also got a Behringer GI100 active di-box somewhere, wouldn't it be even better than the Digitech?

[JohnH]

Anything with a high impedance input is fine to try it. But try it - there are so many differnt piezo circuits around and different transducers that you need to test. Just get it going and decide if you like the tone that it produces and how much gain you need. When you have worked that out, then those TL072 opamps can be used if you prefer them.

[sumgai]

dunk,

The problem with your initial circuit is that there is no "set" gain factor. As you've stated, it attempts to control the gain over a frequency band, and as corrobated by John, that cutoff point is right where you don't want it.

This comes about because the feedback resistor does not set the input impedance - it controls the amount of signal feeding back from the output to the negative input. Without an input resistance, there is only a net loss of signal, as John also points out.

While "cheating" at op-amp circuits like this is possible, it's certainly not common, and for good reason. Op-amps operate best when unimpeded by "tricks" - they are designed to be very wide-band amplifiers, and any frequency moderations should take place outside of the op-amp's internal workings.

Since this component is easier for you to work with (and it's a fairly low-noise device, so it's a good choice), then I'd suggest two mods:

1) Remove the 1500nf cap (just toss it out); and
3) Insert a 470K resistor between the piezo and the op-amp's negative lead.

This will give you an overall gain of about 2. The rest of the circuit is fine, and I'd think that you should be able to adjust the output volume for matching to the standard pickups. If not, then either raise the feedback resistor's value (or lower the input resistor's value) to get more gain.

It is also recommended that you use a resistor between the positive input and ground, that being the same value as the input resistor. This should reduce the DC offset voltage to near zero, which in turn should eliminate a possible distortion of your signal. But this is only a recommendation, and it's optional - if you don't hear any (unwanted) distortion, then don't bother.

If you experience sub-sonic noises, like from your hand disturbing the bridge, then insert a capacitor between the piezo and the input resistor. Start with 1.0 or 2.2µf, and a non-polarized tantalum would work well here, just like it does at the output. Change values as you see fit - greater values might allow the disturbance to get through, lesser values will cut it out, but may also affect your tone in an undesirable way. Again, this is optional, to be considered only if the bridge is too sensitive to your hand's movements.

HTH



sumgai

[JohnH]

Ive got some further thoughts and questions:

So, is it the idea to be able to adjust the mix of your two piezos with their own volume controls, and then blend them with the mag pups into a single mono output? Or would it be ok just to combine the two piezos as an equal preset mix, and then blend with the mag? Do you need a master volume control? ie basicly, what knobs do you want to have?

Sumgais idea, as appllied to the circuit that you posted gives two piezo outputs to a stereo jack, but we also need to address the blending. Also, depending on your piezos, I think the resulting input impedance of 470k may start to reduce bass on the piezo.

Its a great bonus that you have a cap meter coming, so lets get that data and also judge the gain and tone.

Im thinking of a circuit using maybe two of your chips (4 opamps, ie 2 per chip). One amp each for piezo 1, 2 and mag, and the 4th to blend them. We can use a non-inverting opamp configuration for the input stages that maintains the full high input impedance of your chips, and shape the tone if needed in the piezo-input feedback loop(s). Then input resistors to ground can set the exact input impedance needed to control subsonics (with piezos, lower input impedance cuts bass, so this can be set to give a roll off at say 50hZ and below, dependent on your piezo capacitance)

J

[dunkelfalke]

Sumgai: thanks for the suggestions, they are very insightful.
JohnH: mh, I'll try to describe it better

I've got a guitar with two active magnetic pickups (EMG 85 on the bridge and a Schaller SA on the neck). They output to a Gibson style three way select switch, then to the volume pot.

I've got those two piezo pickups (Monacor IMG stage line) which I want to preamplify and connect them in parallel together. If it won't work, I've got a couple of noname piezo elements that probably could be converted to pickups.

After the volume pot I want to put the signal of the magnetic pickups onto a DPDT switch, also the signal of the preamplified piezos so if the switch is in one position I've got only magnetic pickup output and in the other position I've got magnetic and piezo in parallel, but since the magnetic pickup output comes from the volume pot I could "blend" the magnetic output to piezo output that way just using the volume pot. That mixed output goes then into the Artex EXP active tone controller.

I hope the text above is understandable, if not, I could draw a diagram.

[ashcatlt]

Jun 5, 2010 16:55:22 GMT -5 dunkelfalke said:

...magnetic pickups onto a DPDT switch, also the signal of the preamplified piezos so if the switch is in one position I've got only magnetic pickup output and in the other position I've got magnetic and piezo in parallel...

You can do that with an SPST

...but since the magnetic pickup output comes from the volume pot I could "blend" the magnetic output to piezo output that way just using the volume pot...

...if the volume pot is wired "backwards". I'd think that the series resistance would be less dangerous with this low-Z active output, but we'd have to ask the experts.

[sumgai]

John,

I made that selection based on not wanting to have dunklefalke toss out any more than the minimum necessary. However, adjustments can be made, so long as the ratio between the feedback and input resistors remains in the vicinity of 2:1 (for the given gain of 2).

I'll defer to your experience in this matter.



sumgai

[dunkelfalke]

Jun 5, 2010 17:55:53 GMT -5 ashcatlt said:

You can do that with an SPST

I know but I have yet to see a push/pull pot with a SPST switch ;D

[JohnH]

Hi Dunk - with all the active circuitry that you have already got, adding the piezos might be easier than I thought.

Would you be happy to wire both piezos in directly in parallel, for an even blend of both? Then they could go to something like this:



The input resistance is R5, and it can be any value. The lower it is the more bass is reduced. So a good value will be to just roll off the sub-bass.

The max gain is (R3+R4)/R4, here set to x2 or 6db. But if needed, some extra tone shaping parts can be added, C4 and R6 - here set to cut higher frequencies, leaving a hump in the bass. This could be useful if he raw piezo tone to too tinny or lacking in bass - otherwise, they could be left off.

Heres the response:


Its not a full design - the op-amp needs a mid-level ground voltage and values need to be adjusted to suit your pickups - so just for discussion right now.

After that, if your active magnetic pups have a low value of volume pot, such as 25k, you could put about a 47k resistor from each of the mag vol pot and the piezo vol pot, then to your 3-way toggle, then to your active tone board. Slightly better however, would be to use the other half of your Op-amp chip to mix the signals. The benefit of this added stage can be that the blending can be 100%mag, 100%mag+100%piezo, 100%piezo. If you don't, then the middle setting will be 50%mag+50%piezo.

Anyway, all just chat, but I thought you might be interested in this as a way forward.

John

[dunkelfalke]

Hello :-)

Jun 6, 2010 7:40:05 GMT -5 JohnH said:

Would you be happy to wire both piezos in directly in parallel, for an even blend of both?

You mean before the preamp? Is it a good thing?

Jun 6, 2010 7:40:05 GMT -5 JohnH said:

After that, if your active magnetic pups have a low value of volume pot, such as 25k, you could put about a 47k resistor from each of the mag vol pot and the piezo vol pot, then to your 3-way toggle, then to your active tone board.

To be honest I wouldn't want to add another pot to the guitar. I've tried to do it with the same guitar, and while it works, it kills the looks and is very cramped.

Here is what I originally had in mind (simplified a great deal)

[dunkelfalke]

Okay, I've got the new multimeter and have measured the capacitance of those piezo pickups - 11.6 and 11.3 nF respectively.

Could not test the cheap piezo elements, though, have to find them first.

[JohnH]

Good info - and those values indicate quite powerful piezos in terms of their ability to drive input imepdances. Lets say youd like full bass from them, with sub-bass roll off with -3db reduction at say 40 hz, you could have an input impedance of around 350k. Or half that if you put two piezos in parallel (i think that may be a good idea). What that means initially is that you can mount them and wire them passively to your modeller or Di box, or to a guitar amp, and hear their full tone and volume. So that seems like a good next step - then adjust the opamp circuit to suit to match them to the magnetics.

John

[dunkelfalke]

Thanks for the info :-)

If it is not too much work for you, could you explain why connecting the piezos in parallel before the preamp would be better than after it?

[JohnH]

No problem, and you could do it either way, using separate opamps for each piezo or both in parallel into one opamp.

The second option is simpler of course, and given that you don't want to actually control the blend between the piezos, it will work fine. the charges from the two piezos will just combine and give you the average of each. It will leave half your TL072 unused, but that might come in useful for the future.

Had your piezos turned out to be very low capacitance, which would require a very high input impedance (such as on that violin circuit in post 1), then putting the piezos in parallel would have helped to allow lower input impedances to be used, which usually result in lower noise in the system.

You have the options however

The one thing i would point out however, is never connect the output of one opamp directly to that of another, as in your diagram above. Whenever the two opamps are trying to produce different signals, then they fight each other and cause distortion. A series resistor from each output before they meet fixes this.

Anyway, I think you have a few options for your circuit, and I don't see any big problems with achieving what you want. So time to do the tests. It would be very interesting if you could post a clip of he raw piezo sound (via whichever box/modeler you chose, into the PC), and also the magnetic output at the same recording level. any chance of that?
cheers
John

[dunkelfalke]

Ah, great, thanks for the explanation. You have pointed out the thing with the resistor before, but now I understand the background :-)

I know that I should record some output, but it could take a couple of days because it looks like my USB audio interface seems to be broken and the internal soundcard hasn't got any input jacks.

[4real]

"I know but I have yet to see a push/pull pot with a SPST switch"

hmmm...you know that a dpdt will work just as well right? just use half of it!

...

I keep following these things, but just received today a $50 Behringer AD100 Acoustic modeler pedal, except for a bit of low hiss it has quite a convincing sound, better than a lot of piezos (and less his than many anyway) ...a lot more 'flexible' being able to use it on any guitar, 4 different models and sounds different on different guitars. It also has a direct out, so you could split the 'acoustic' sound to a separate amp or stereo recording if desired. Also, no handling noise that you so often get with onboard real piezo systems...

It's just about convinced me that this is a better way to go unless you are going for a ghost system to run a synth or something. It also means you can keep the guitar passive if you wanted and save all your switches and mods for the mag pickup options...these things make the pedal sound different too, so you get a range of acoustic sounds with your pickup selections.

Just a thought, but it is always interesting to see how these things turn out...

[dunkelfalke]

That particular guitar has got two active pickups, one active EQ system and a Roland GK-2a hexaphonic pickup for the guitar synth so it is not like I absolutely need the acoustic sound (I only play for myself anyway and I do have two acoustic guitars). It is just that I want a guitar that can do almost anything am a guitar nut ;D

Otherwise I wouldn't have bothered with the electronics, the blueing of the vibrato system and so on.

hmmm...you know that a dpdt will work just as well right? just use half of it!

That is exactly what I've done already. The wiring is already in place for two months, it just awaits the connection with the piezo preamp. I wanted to do a small internal tube preamp originally, but it didn't work (maybe I've damaged the tube somehow, I don't know).