Preamp for 3D printed piezo equipped Cello.

[yeeshkul]

Hi guys i was asked to search for a preamp for a 3D printed Cello. It uses a piezo pickup with 3k5 output impedance an 65nF capacitance.

I found this page that deals with the topic
https://sound-au.com/project202.htm

this one looks like the thing


It is a charge amp, so i understand i will have to play with Cf and Rf as well as the James tone control.
Is there anything else i should consider? Cellos can produce quite deep tones.

[PRR]

> play with Cf and Rf as well as the James tone control.

Rf is subsonic, leave it as-is. Cf can be selected = to pickup C for unity-gain, then down or up for good signal level.

I don't see why you'd tinker the James network? The bass corner is 75Hz and pushing it lower is mainly wasted, cello (65Hz) or full orchestra or rock&roll.

FWIW, Dave Baker used to rock the cello through a open-back Fender twin. It speaks with authority but is no earthquake-shake. EDIT- ah, Dave and his Fender with Ted Dunbar and his??  Is the photo really 1998? Doubt it.

[cspar]

I've been looking into peizio preamps for non guitar use also, although the instrument that I'm working with is a uke.

I'm not sure what your amplification end goal is but mine is to not be bound to a peticular peizio or instrument and to run the instrument into a PA not into an Amp.

Chooseing a design/designing for the desired end result is the most important part of building.

Most of the designs I see posted around seem to be focused on getting the sound out more than sculpting the tone despite there being a tonestack and or resonance or presence controls.

My goal is to build a preamp that puts the end tone in the hands of the artist more than the soundman.

I've come to the conclusion that the best way to do that is combine the circuit blocks of a small pedal board into a single condensed "lunchbox" unit.

I'm still in looking into the virtues of different input and gain topology's but I've been focusing on a chain of blocks that goes something like this aside from effects loops or any compression/limiting/gating;

Variable impedance matching input with slight variable gain control,
Bax or James tonestack,
Dual parametric mids,
Gain stage,
Cab sim,
DI

By dual parametric mids I mean something along the lines of a Pearl PE-10 so that there can both be a notch and a peak or dual notches as needed for the peticular instrument.

I'm going for a much different texture and use but I'm drawing inspiration from the IVP preamp more than from most of the basic peizio preamps. I'm just seeing those as frontends.

Depending on your amplification method the schematic you posted
might work well by itself, I don't know.

An easy route to go that might better match the 3d printed cello in concept would be a sending signal into an IR loader if you can find or make a preset IR for cello.

I don't have much experience with peizios and had a similar request from a friend these are just some thoughts from my recent brainstorming and really a stop along the way to designing a non peizo ivp inspired preamp/di for my own use.

[Rob Strand]

[remove stuff until I get time to come back to it]

It uses a piezo pickup with 3k5 output impedance an 65nF capacitance.

I'm the 3k5 output impedance is the 65nF impedance at 1kHz, although I get 1/(2*pi *1k *65n) = 2.4k ohm.
To get 3k5 you need 700Hz, almost like a nominal frequency for the impedance value.

[Lino22]

Rob i suppose you can measure the capacity of the piezo, right?

[Rob Strand]

Rob i suppose you can measure the capacity of the piezo, right?

A capacitance meter should be OK.   Most sensors should handle the drive from the cap meter.

For accurate results you need to remove the lead capacitance.  Guesstimate at 100pF/m.  Not a big deal for 10nF + sensors.

[Rob Strand]

[remove stuff until I get time to come back to it]

There's a few tips in,
https://www.ti.com/lit/an/sloa033a/sloa033a.pdf

Here's some example of what happens when you try to add some RF filtering.   In some ways you are better off with circuit 2 with a very small input resistor.  After that it gets messy.   (That's why I pulled what I wrote before.)

The problem with the large capacitance sensors is it requires a very small input resistor to avoid HF-roll-off.  Even 100 ohm is too high.

The green trace is essentially flat.

[anotherjim]

HF roll-off from 10kHz might only benefit this application. The string family doesn't output much over 5kHz.
There are questions about the performance of the instrument/pickup arrangement that I would want to answer before it's worth providing a preamp that does anything more than buffer/amplify.

[yeeshkul]

Thank you guys for your precious input.

[yeeshkul]

If i want to add a headphone output to it, can i wire it straight to the output or shall i use a buffer?

[Jarno]

You'll need a headphone amplifier, which can be quite a rudimentary affair, or something more elaborate.
Rudimentary would be two parallel sections of 5532 per channel.

Like here:
https://www.haraldswerk.de/In_Out/Headphone_Euro/Headphone_Euro.html

Edit:posting this I do realise that you are probably on a single ended supply, don't have any schematics of that off hand (I also build modular synth modules and had a look at Haralds' site this morning, hence I knew he had a 5532 based headphone amp on there).

[anotherjim]

Headphones are very good idea for this application. Electric string instruments can sound dreadful directly while the pickup sound can be spot on. If you cannot have an amplifier loud enough to drown out the squeaking and scraping, headphones are a good option.
You will need the high-impedance buffer since headphone amp designs don't care about catering for pickups. Look for Cmoy DIY headphone amps but beware there's a lot of audiophile voodoo stuff about them.

For a readymade preamp + eq, there are plenty of guitar piezo modules out there for cheap that run on x1 9v battery. You can even get a tuner built-in. You can beat them for noise, and maybe looks but it will be useful to test the water.

[yeeshkul]

So here is what i am gonna test. I used a simple headphone preamp stage with a LM386. Thank you, Rob, for that roll-off ideas, i will employ it if needed.
I will use TL062 for the charge amp. OPA2134 are hard to get here, although they seem to have a fancy noise ratio.