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https://github.com/jonnor/piezopro

(INACTIVE) Piezo contact microphone preamp
https://github.com/jonnor/piezopro

Last synced: 10 months ago
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(INACTIVE) Piezo contact microphone preamp

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README 

          

Piezo module with integrated preamp in fully shielded package.



# Status

Developing first prototype



# TODO



Preamp Revision 2 testing



* Measure frequency response of amplifier with Bitscope

* Measure signal/noise ratio or noise floor.

* Test with 5V power supply

* Test overvoltage on connector. VGND,HOT,COLD



Preamp Revision 3



* Depending on test results

* Test a charge amplifier?

* Test a JFET directly on piezo?



Final revision



* Document test procedure for PCB and/or assembly.



## Questions



* PCB stackup retains piezo sensitivity.

Put piezo onto PCB. With glue? Use adhesive tape to attach to instrument.

Measure output level. Compare to without PCB. Compare to coppertape+covertape.

Maybe the bottom should just be piezo+coppertape, sides+top is PCB? Can still look "perfect" from outside.

* PCB stackup is a good way to shield.

Test having just a few (4) connected holes in mid layer (from top/bottom). Can noise get in? 50Hz

Emulate side-plated PCB using copper tape. How much does it improve?

* Frequency response should include resonant peak.

For 27mm element is as low as 4kHz, within fundamentals of piano. 12mm=9kHz. 9mm=18kHz, edge of human hearing.

How pronouced is this peak actually when mounted on instrument?



# Design test



Revision 1. gain. 220k/22k. 10M input.

20mm Murata. Piezo in tin, glued. Lying on top of guitar. 

Measured GND, HOT. on output.

low E string. 80mVpp sustained, 500mVpp attack

G string. 70mVpp sustained, 700mVpp attack

high E string. 70mVpp sustain, 800mVpp attack



Current consumption 0.68 mA.



## Learnings



* Basic circuit works. Both with balanced and unbalanced inputs.

* Using a tin box works well as a shield.

* With microphone level input, signal is too hot.



# Worklog



Rev1: Circuit design verification. 22 hours



* Sept 08. 4 hours. Research exiting designs, initial schematic.

* Sept 09. 4 hours. Part selection and initial layout.

* Sept 09. 1 / 4 hours. Setup simulation in KiCAD.

* Sept 14. 1 hour. Sketching piezo mounts with rim/center attach

* Sept 19. 7 hours. Assembly and testing revision 1 PCB.

* Sept 22. 2 hours. Test overvoltage, calculate new component values



Rev2: Assembly concept verification. 7 hours 



* Oct 4. 4 hours. Update schematic and layout for revision 2. 

* Oct 8. 3 hours. Go through revision 2.

* Oct 3. 1 / 4 hours. Find out how to import artwork in KiCAD



Rev3: Production tweaked



* March 16. 2 hours. Make Python tool for vias shielding, use instead of edge platings



# Requirements



Electronics



* PCB is an enclosure for 20mm piezo. 25mm outer diameter

* BOM max 3 USD

* 100 hours++ on 9V battery. Max 2mA avg draw

* Low noise

* Balanced output. But also works with unbalanced inputs

* Line-level output, +4dBU

* 20Hz-20kHz bandwidth

* Reverse polarity protection

* Input overvoltage protection



Mechanics



* Cylindrical

* Fully shielded

* Output: 1/4" TRS female

* Power: 9V battery clip

* 1m cable, soft/flexible



# Sourcing



* Capacitors.

[0.1uF](https://no.farnell.com/w/c/passive-components/capacitors/ceramic-capacitors/smd-ceramic-multilayer-mlcc-capacitors?capacitance=0.1uf&voltage-rating=25v|35v&range=inc-in-stock-grp1&sort=P_PRICE)

[10uF](https://no.farnell.com/w/c/passive-components/capacitors/ceramic-capacitors/smd-ceramic-multilayer-mlcc-capacitors?capacitance=10uf&voltage-rating=6.3v|10v|25v|35v&range=inc-in-stock-grp1&sort=P_PRICE)

* NMOS. [BSS138L](https://no.farnell.com/on-semiconductor/bss138lt1g/mosfet-n-50v-0-2a-sot-23/dp/1431319)

* Diodes [SOD-532](https://no.farnell.com/w/c/semiconductors-discretes/diodes/small-signal-diodes?diode-case-style=sod-523|sod-523f&no-of-pins=2pins|3pins&packaging=cuttape&range=inc-in-stock-grp1|exc-obs&sort=P_PRICE)

*  Zeners [SOD-523](https://no.farnell.com/w/c/semiconductors-discretes/diodes/zener-single-diodes?zener-voltage-vz-typ=3.3v|3.6v|3.9v|4.3v|4.7v|5.1v&diode-case-style=sod-523|sod-523f&no-of-pins=2pins|3pins&packaging=cuttape&range=inc-in-stock&sort=P_PRICE)

* Opamp. [TL062](https://no.farnell.com/search/prl/results?packaging=cuttape&range=inc-in-stock&st=tl062&sort=P_PRICE)

* Piezo [Farnell](https://no.farnell.com/w/c/sensors-transducers/transducers/sensing-transducers/piezo-transducer-elements?external-diameter=27mm|35mm&range=inc-in-stock)



# BOM



[Octopart BOM](https://octopart.com/bom-tool/TbIQ2Cbo/OKDwL97yteywY4BA)



## Testing



Testing aspects



* PCB design verification

* Enclosure/mount design verification

* Production checks.



Tough-but-normal conditions



* 3-8 microphones

* Plugged into standard mixer with 50m cable

* 1 9V battery per microphone



Reverse polarity:

Hold battery on wrong way for 10 seconds, plug right way, ensure works OK.



Overvoltage:

Hit piezo with hammer. Ensure performance good afterwards.



Frequency response:

Connect signal generator at PCB input, with capacitor simulating piezo capacitance.

Sweep sinewave, record output, calculate frequency response.



Noise.

Internal noise. No signal. Measure output

Injected noise/interference (on output line, at microphone).

Attempt to inject 50Hz by wire. Lightbubl+dimmer.

Measure without shielding, and with.



## References



Electronics



* [Elliot Sound Products: Balanced Line Driver](http://sound.whsites.net/articles/dwopa3.htm#s10)

* [Elliot Sound Products: High-Z ](http://sound.whsites.net/articles/high-z.html)

* [WikiPedia: Line level](https://en.wikipedia.org/wiki/Line_level). Consumer: -10dBV 0.894 Vpp, Pro: +4dBu 3.472

* [StackExchange.com: Piezo FET preamp balanced output](https://sound.stackexchange.com/questions/687/diy-contact-microphone/37714#37714).

Developed by Alex Rice, www.zachpoff.com/diy-resources/alex-rice-piezo-preamplifier/.

Extremely simple design. Used as base for DIY kits and complete microphone Cortado MkII from Zeppelin Labs. 

* [Elliot Sound Products: Piezo Pickup Preamplifiers](https://sound-au.com/project202.htm).

Tested high-Z preamp versus Charge Amplifier.

First opamp stage is charge amp with 1x gain, then second stage sets desired gain.

Found that high-Z would pick up (50hz) hum, where as charge amplifier did not. When piezo was connected with bare leads.

Tested with TL072 and found it very quiet.



Shielding



* [Shielding fabric](https://www.aliexpress.com/item/Emf-shielding-Fabric-Signal-Block-Fabric-Military-Nickel-Fabric/32795159060.html)

* [Shielding paint](https://www.stewmac.com/Pickups_and_Electronics/Shielding/Conductive_Shielding_Paint.html)



Simulation



* [ngspice: Using KiCAD as GUI](http://ngspice.sourceforge.net/ngspice-eeschema.html)

* [eCircuitCenter: Basic SPICE opamp model](http://www.ecircuitcenter.com/Circuits/opmodel1/opmodel2.htm)

* [WikiPedia: Piezoelectric sensor](https://en.wikipedia.org/wiki/Piezoelectric_sensor).

Basic model (disregards resonance): Voltage source with a capacitor in series.



## Existing products



Active contact microphones



- Schertler. [Thommann](https://www.thomann.de/gb/schertler_dyn_uni_p48_contact_microphone.htm).

48V phantom power.

310 EUR.

- Cortado MkII [from Zeppelin Labs](https://zeppelindesignlabs.com/product/cortado-mkiii-contact-microphone/).

48V phantom power. Two gain settings, low-frequency cutoff switch.

150 USD assembled.

- K&K Pure pickup plus K&K Pure preamp. [Thommann](https://www.thomannmusic.no/kk_pure_preamp.htm?sid=fd268c89787f256dcc79b210d18a3667).

Total around 300 EUR



# Usage ideas



* Guitar. Acoustic, electric, traveller.

* DIY Cigarbox

* CNC mill an electric travel guitar from single piece of wood. Rosewood/maple.

https://www.aliexpress.com/item/Hot-Sale-Custom-Electric-Travel-Guitar-Portable-Style-Rosewood-Fretboard-22-Frets-with-Soft-Bag-can/32803999609.html

* Violin,viola,chello. 

* Double bass. 

* Stomp box