Piezo Preamp + Mute True Bypass + Clean Boost + DI Box

[brazuca_nz]

Hi everyone, newbie here!

I play the Cavaquinho, a sibling of the Ukulele that uses steel strings and has a bright sound. We use piezo transducers to pick up the sound, normally two small discs underneath the instrument's top board.

It works well, but it has a high impedance, and we need to put a lot of gain in the mixer. So I would like to make an all-in-one pedal to make it better. The chain would be something like this:



The Digital Tuner is something I worry about later.

For the Preamp with tone controls, I thought about using one described by Rod Elliot at https://sound-au.com/project202.htm



Then I need to create the boost and DI box output. Can I use the OPA2134 to achieve this as well (I bought 5 of them)? I would like to power this either using a 9V battery or a rechargeable 18350 lithium battery.

Is my project feasible? Am I missing something?

Cheers!

[PRR]

For the Preamp with tone controls... ... Am I missing something?

Tone controls?

I suspect you meant to show us Figure 4:

[brazuca_nz]

For the Preamp with tone controls... ... Am I missing something?

Tone controls?

I suspect you meant to show us Figure 4:

Right you are! I have updated the post. Thanks!

[Jim Hagerman]

Is my project feasible? Am I missing something?

Interesting concept. Normally piezo transducers like to see very high loading impedance (10M), but here you load it with zero ohms, or basically run it in current mode.  Does it actually work?

[stallik]

I built a Barcus Berry 3000a clone for this purpose which works quite well..
Barcus Berry Piezo Pre-Amp & EQ

[brazuca_nz]

I have been working on some schematics, and this is the result so far. I am not sure about the resistors' and capacitors' values, though.

What do you think?

[antonis]

I'm confused a bit with your switch wiring (it seems like PREAMP & BOOST outs prang each other) but see below to get an idea about Booster configuration:



Booster's gain is X57 (1 + R18/R10) so you might wish to tweak it a bit..
Also, C10 value might be enlarged (up to 220pF, say)

[brazuca_nz]

I'm confused a bit with your switch wiring (it seems like PREAMP & BOOST outs prang each other) but see below to get an idea about Booster configuration:

Hi there, sorry if it is not clear, I don't really know how to represent it.

The idea is that there are 3 circuits (1, 2 and 3). The circuit 1. Piezo Charge Amplifier w/ Tone Controls is always ON.

The circuit 2. Booster w/ Tone Controls can be turned ON/OFF using a DPDT switch, where the default state is OFF (the OUT_PREAMP goes straight to OUT_BOOSTER, OUT_BOOSTER is only a representation of the circuit 2 OUTPUT).

The circuit 3. Balanced and Unbalanced Output is always ON and gets the OUT_BOOSTER (either the dry signal or the boosted signal) and sends it directly to a unbalanced TS port and to a balanced circuit that ends up in the XLR output.

Is that clearer now?

Thanks!

[brazuca_nz]

So, what I need is a DPDT switch (true bypass) where the C7 (in your schematic) is. The gain of the boost can be fixed, let's say +20db, and we can have a potentiometer to attenuate it.

[antonis]

In case of Preamp + Tone is always ON you don't need another Tone control inside Booster circuit..

If I got it right, the only ON-OFF (in ther mean of series implementation) effect should be the Booster so let's make it more functional: 




SW1 can be 3PDT switch with LED indicator when BOOSTER is ON..



P.S.

The gain of the boost can be fixed, let's say +20db, and we can have a potentiometer to attenuate it.

+20dB gain walks the line between boost and overdrive (most possible to slip over the later..)  and there is no benefit from attenuating an already distorted signal..
(you'll simply lower its amplitude..)
IMHO, up to +15dB should be marginally OK - lower should be better..
After all, you simply want to boost a signal, the level (amplitude) of which is already set by VR3 pot (preamp OUT)..

[brazuca_nz]

I built a Barcus Berry 3000a clone for this purpose which works quite well..
Barcus Berry Piezo Pre-Amp & EQ

Here is the schematic for it, using the op amp I have available:


Question: Let's say I add another OPA2134 IC and want to use the same 9V rail. I connect pin 8 to 9V+ and pin 4 to GND. But what about pins 3 and 5? Can I use the same VREF, or must I create another voltage divider?

Thanks!

[bluebunny]

You can use the same V_REF.

[amptramp]

The Barcus Berry circuit is a lot better.  It has a high input impedance, which a piezo sensor needs.  It has a Baxandall tone control and an output level control, all of which are useful.

[brazuca_nz]

Here is the current circuit:



Some considerations and questions:

• The first circuit is the preamp, I am using the Barcus Berry 3000A that was suggested here.
• How can I change the tone controls' frequencies for that circuit? Because the Cavaquinho has a pitch much higher than the Guitar.
• If I don't have the exact values for the resistors, can I combine two to achieve the desired value?
• Now, regarding the booster, circuit 2: There are some very high resistor values there (22M, 10M). Can you help me bring them down? How could that affect the boosting quality?
• Another thing about the booster: I would like to have the potentiometer (Boost1) controlling only the boosted signal and not attenuating the original signal, that means if the pot is completely turned anti-clockwise and the boost is ON, the audio output would be the preamp signal. Is that possible?
• Finally, about circuit 3, how can I modify it to use the same V_REF used by the other op amps?

Here is a conceptual 3d image of the pedal (using a 1590BB enclosure):


Many thanks!

[antonis]

Now, regarding the booster, circuit 2: There are some very high resistor values there (22M, 10M). Can you help me bring them down? How could that affect the boosting quality?

R16 & R13 only affect U2A input impedance..
(R13 also contributes to input noise, but let it be for the moment - or just add a low value cap in parallel..)
You can safely lower them down to 1M each, if you're OK with a 500k input impedance..
(although, 1M equivalent impedance should be more OK for preamp Volume pot value..)

P.S.
Make both of them 2M2..

I would like to have the potentiometer (Boost1) controlling only the boosted signal and not attenuating the original signal, that means if the pot is completely turned anti-clockwise and the boost is ON, the audio output would be the preamp signal. Is that possible?

Unless I don't get you, that is already taken care by Boost switch..
If you mean to have Preamp's output going through Booster unaltered, wire a SPST switch in series with R14..
(you could turn Booster into Buffer..)

How can I change the tone controls' frequencies for that circuit?

Further analysis
Pages 2.46 to 2.55

[PRR]

> change the tone controls ....Cavaquinho has a pitch much higher than the Guitar.

Aside from the mess of math that Antonius gave you.... If it is pitched an octave up, cut all cap values in half. You normally do not want to mess with resistor values because they are about loading and loss, not primarily frequency. None of the values are what we call "critical". Simple filters like these, even 10% "error" is unimportant, hardly heard.

You have an electronic error:

[brazuca_nz]

P.S.
Make both of them 2M2..

Thanks!!!

Unless I don't get you, that is already taken care by Boost switch..
If you mean to have Preamp's output going through Booster unaltered, wire a SPST switch in series with R14..
(you could turn Booster into Buffer..)

Sorry about my cumbersome English, non-native speaker here
Yes, the idea is that the Boost potentiometer doesn't interfere with the preamp signal, only with the Boosted signal. For example, let's say the preamp signal is +10dB (just an example, idk if this is an actual value) and the boost switch is ON. Then the boost level knob is completely closed (all way down to the left), the output signal should be the original +10db. Then, I turn the knob all way down to the right and the signal is boosted by another +10dB, the output should be +20dB and, if I toggle OFF the boost switch, the output signal is back to +10dB.

As far as the tone circuit is regarded, I have found some good explanation here https://www.guitarscience.net/tsc/circuit_analysis/baxandall_2.pdf and a nice simulator here https://www.guitarscience.net/tsc/baxandall_2.htm#RIN=600&R1=22k&R2=22k&R3=22k&R4=10k&R5=10k&RB=100k&RT=100k&RF=600&CB=47n&CT=560p&RB_pot=Linear&RT_pot=Linear (you can change the components values and "rotate" the linear pot for both bass and treble).

You have an electronic error:

Thanks for pointing that out! I changed the V_REF global label to the new position, but the capacitor value is indeed 100nF at https://tagboardeffects.blogspot.com/2017/05/barcus-berry-3000a-buffer-preamp-eq.html, should I really change it to 10uF?

[brazuca_nz]

Another thing that I noticed is that this Barcus Berry 3000A Baxandall circuit is slightly different from the one in the article I have mentioned:

Mine:


Article:


Not only there are 2 capacitors instead of 1 for the Bass, but they are connected to R3 (in my case R4). Why is that?

[PRR]

really change it to 10uF?

That's barely below the audio band, and you have a lot going back to V_Ref. If you will build a million then you do want to minimize cap-costs; in DIY a 0.1 (film) and a 10 (electrolytic) are about the same price and the difference doesn't justify a lot of figuring. I'd do at least 10. But use your own judgement!

[brazuca_nz]

really change it to 10uF?

But use your own judgement!

The reason I asked you that is because of my ignorance of the subject, I really don't know why that cap is there and what impact it's value has in the circuit. If you say it would be better, I am definitely using a 10uF then! Thanks