Author Topic: Preamp schematics for Piezo pickups ( for acoustic guitar)  (Read 216 times)

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alonper

Preamp schematics for Piezo pickups ( for acoustic guitar)
« on: January 02, 2018, 02:40:09 PM »
Hello experts,

I would like to design and build a Preamp for acoustic guitar with Piezo sensors (I am using this piezos www.kksound.com/products/puremini.php)
currently I've got this OP-AMP , MCP6021\4 ww1.microchip.com/downloads/en/DeviceDoc/20001685E.pdf
The most important thing is that I want the signal from the guitar to be able to connect to LINE LEVEL equipment such sound boards and etc...so It has to be also with low impedance..
and, I've got only 1 power supply, 5V DC to feed the op-amps :\

Any suggestions or links to a pre-amp for this kind of needs?

Thank you all!!

R O Tiree

Re: Preamp schematics for Piezo pickups ( for acoustic guitar)
« Reply #1 on: January 03, 2018, 08:05:43 AM »
Disclaimer - if some of this is already known to you, then I apologise, but your post gives me no clue as to your experience/knowledge levels.

Piezo pickups can push out up to 10V p-p, it sez 'ere, if you really spank them, so a 5V supply will not cope at times.  In addition, that op-amp you have will not cope with repeated over-loading of the input (actually, no op-amp is happy with this, but this device is not the one you need, I feel).

There are lots of pre-amp circuits out there that have high input impedance and low output impedance (google is your friend) with very low current consumption (up to 200 hrs out of a 9V battery) and the headroom required for piezo pickups.  I've seen criticism on other forums in my search today about the flimsy enclosure construction of the Pure Pre-amp designed to go with this pickup, as well as complaints that the skinny, little knobs on the tone control are hard to dial in accurately.  Those, together with the current $142 price tag mean that making your own circuit looks attractive.  You just have to be sure that you know what you're letting yourself in for, here.

Electronic components and a piece of vero-board will not cost more than a few $ (single figures), but that's not the whole story.  If you're going to mount this circuit internally to the guitar, then an enclosure is not going to be needed, although you will have to figure out some method of mounting the battery so it's accessible and secure - you need to get at it to change it, and a loose battery will inevitably thump against the guitar body, with obvious results... And you're going to have to perform at least some surgery on your expensive guitar - is $142 really too much when you consider that there is a risk of damage to your guitar?  If you're going to mount it externally (belt-box, perhaps, so as to minimise the length of cable between pickup and pre-amp?) then you have to find some sort of enclosure, jack sockets in/out, a patch lead long enough to get from your guitar to the belt-box, but short enough that it won't flap about and snag on things; pots and knobs to control volume (and tone?); paint and legends to make it look pretty... it all adds up.  Oh, and tools, as well... drill and drill bits, screwdrivers, soldering iron, solder, flux (for when you make a mistake and have to de-solder/re-solder something without damaging the board traces), de-soldering braid or pump, magnifying glass or, better, jeweller's loupe (for spotting short-circuits), hacksaw, files, pliers... if you haven't got the right tools for the job, they will have to be bought as well out of your $141.99 budget.  There's a great article here at R.G.'s website, which I have shamelessly paraphrased, which goes into much more detail.  You need to be part engineer, part electronics-geek and part artist and it's a steep learning-curve.

Suddenly, $142 doesn't seem so bad, if this is the only thing you're going to build. If, however, this hobby is something that you feel you'd like to get into, then all those tools and the skills you will learn will be an investment.  It is immensely satisfying to build a new circuit, modify it to taste and then know that no-one else in the world has one quite like yours.

There's a simple circuit at Cafe Walter that he will supply for $49 already built.  He also, very kindly, supplies the schematic so you can build your own (note that sourcing the 2 JFETs might be pricey when looking for through-hole ones).  He designed it for a bass guitar, but the frequency range is more than enough for a "normal" guitar as well.  I'm sure that others will post their suggestions for alternative circuits.  For example, do you want tone controls, like the Pure Pre-amp?  Or will your sound engineer do that bit for you?  Do you want to set the volume with a trim pot, or do you want to have some control over that yourself, bearing in mind that your sound engineer can turn you up or down but, if you drive this buffer into distortion, there's nothing he can do except wave frantically at you to turn yourself down? The answers to these questions will inform us more about what you need and what would work for you.

I hope all this helps.
...you fritter and waste the hours in an off-hand way...

PRR

Re: Preamp schematics for Piezo pickups ( for acoustic guitar)
« Reply #2 on: January 04, 2018, 08:11:11 PM »
> I would like to design....

I sense that you are not ready to "design" a preamp, and want a ready-bake solution. (The high-Z to low-Z concept is opamp fundamentals.)

"Line level" means many different things. 0.1V to over 10V.

Defining "5V supply" means you can't get near the top of this range, only about to about 1.6V. That should be enough for many things.

Piezo output can be low or frightfully high. On another forum there is a report that simply loading a piezo with a capacitor gives a lower level and impedance. Buy a 10-pack of 220pFd or 330pFd and use as many as necessary.

« Last Edit: January 04, 2018, 08:12:55 PM by PRR »

samhay

Re: Preamp schematics for Piezo pickups ( for acoustic guitar)
« Reply #3 on: January 05, 2018, 07:41:56 AM »
If you want to dabble with design, I like to use a charge amplifier topology for piezo preamps.
https://en.wikipedia.org/wiki/Charge_amplifier
https://www.google.com/patents/EP1338879B1
This would let you pad down the input (if necessary) so that it plays nicely with your 5V supply, and I find you get much better behaviour if you are using the preamp off-board. You don't lose bass due to cable capacitance.
I'm a refugee of the great dropbox purge of '17.
Project details (schematics, layouts, etc) are slowly being added here: http://samdump.wordpress.com