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Old 18th June 2013, 06:12 PM   #1
RipX is offline RipX  France
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Default Guitar piezo preamp

Hi,

I want to build a cheap preamp system for my guitar. I have seen this piezo transductor : TRANSDUCTEUR PIEZO 3 A 30 VAC - Elecdif.com
But now I want to build a preamp for this ch*t

I have seen this post : http://www.diyguitarist.com/PDF_File...iezoPreamp.pdf

Idealy, I want to build a model like this but with an adujstable gain. So, carac :

- 9V battery alim
- for piezo
- built in the guitar
- with an adjustable gain and a on/off button
- cheap !

Can you help me to find this product please ? Thanks !
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Old 18th June 2013, 06:19 PM   #2
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That is not a piezo microphone. It is a piezo alarm ... a very loud whistle!
You will need a piezo microphone and a preamp to provide gain control and a low output impedance.
Rather than a piezo element, you may want to use a condenser microphone element as they produce better fidelity and they are quite inexpensive.
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Old 18th June 2013, 06:22 PM   #3
RipX is offline RipX  France
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Hi, thanks for this advise !
I want something pretty good but cheap, can you explain your idea with links of condenser micro element and a thread/pdf for build the preamp please ?

Thanks
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Old 18th June 2013, 06:27 PM   #4
kevinkr is offline kevinkr  United States
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You'd generally use piezo either in an under bridge transducer or as individual under saddle transducers.. You can also use them in the body of the guitar. Note that acoustical pick up with a mic on the guitar can be tricky to handle due to feedback..

You may be able to modify that piezo buzzer for use as a body pick up by removing it from the housing and mounting directly on the body. Is this an acoustic or solid body guitar?
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Old 18th June 2013, 07:17 PM   #5
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What will the preamp be driving into? If it's a further preamp with gain control and tone, you could probably just run the output of that buffer into a pot (replace the 10k resistor by a 10k pot to ground, wire the wiper to the output jack) If it's driving a power amp you're going to need more gain, and probably tone controls (I've never heard a pickup that sounds decent without equalisation).
A piezo is normally a contact mic; it takes the vibrations from the body of a guitar, or sometimes the bridge. I used to recuperate from crystal record player cartriges, and they didn't sound any worse than things like the Barcus Berry hotdot, but placement is critical – a couple of centimetres difference, and you get a whole different set of resonances. The electret gathers air vibrations, so the sound is more accurate – but if you mount it in the sound hole, you get what a guitar sounds like in the sound hole, which requires some treatment before it's anything like what a guitar sounds like from anywhere you might listen to it normally. The electret will feed back earlier, but has it's own preamp incorporated, so really only needs a potentiometer and wiring up (unless, as I've previously said, it's driving straight into a power amp).

Et tiens, il est parti. And I see someone types faster than me, though with many of the same opinions.

The problem I always had was how to mount the battery without damaging the instrument; I just hate having to take all the strings off to change it. I finished up putting XLRs on the instruments and phantom powering them from a box on the back of the stage (which also held the equaliser). Overkill, probably.

The standard on/off switch (which is horrible but does work, usually) is a three pole jack socket on the instrument, so plugging in a standard two connector guitar lead shorts between ring and sleeve, and makes the ground connection. Thus, unplugging the guitar removes your battery drain. You can even buy jack sockets with make connectors on them as well as break – or, if the instrument's going to spend the entire set plugged up and ready, for just one number used, it's easy enough to add a discrete switch. Less messy than the volume control, anyway. Perhaps a rechargeable battery while it's on the stand so you never need to open it up.

Piezos are capacitive (almost perfectly). Contact mics are frequently boomy (not the bridge ones, the panel ones). Changing the input impedance changes the low frequency response -bingo, equalisation.

As placement in the instrument is so critical, it's generally a good idea to set everything up outside the instrument, with an amplification system, and try sticking the mic ti different points with chewing gum (all right, bluetac). I can do diagrams if absolutely necessary, but I'm no artist.

Last edited by chrispenycate; 18th June 2013 at 07:19 PM.
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Old 18th June 2013, 08:04 PM   #6
RipX is offline RipX  France
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Hi guys, thanks for all !

It's an acoustic ukulele. i'm not a pro but I have a folk with a preamp, just one buton for gain, all integrated in the guitar. I plug the cable to a guitar amp and it's working fine ! I want this ...
I think "You may be able to modify that piezo buzzer for use as a body pick up by removing it from the housing and mounting directly on the body." it's a good idea and "unplugging the guitar removes your battery drain" it's a good idea

What type of material can work for me ?

Thanks
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Old 18th June 2013, 09:52 PM   #7
Struth is offline Struth  Canada
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Hi Guys

As stated, peizo pickups are capacitive. You just need a jfet buffer to capture the full tone, then place a volume pot after it.

If you've tried the raw peizo into a standard guitar amp input or anything else, you'll get a weak signal that has no bass at all. This can lead you to the mistaken impression that a lot of gain and bass boost is needed. However, all that is required is a very high input impedance >10M. If you really need gain, then use a jfet input opamp. These pull more idle current than a single jfet unless you use a low-power version, but then the sound quality is much poorer.

There is an acoustic preamp project in "Tonnes of Tone" (TOT) on my site. This project was also published in Guitarist magazine in the UK back around 2004.

Have fun
Kevin O'Connor
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Old 19th June 2013, 04:42 PM   #8
RipX is offline RipX  France
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Hi and thanks ! Sorry but I have not the budget for this preamp !
So, to make it simple :

- What mic I need ? Piezo or condenser mic or something cheap like this ?
- And, have you a preamp design or how to build for this mic ?

Finally, I think http://www.diyguitarist.com/PDF_File...iezoPreamp.pdf is not so bad ... but I like a pot for gain control and a system for stop the battery drain when I unplug the cble !
I think it's not very difficult to find for a pro like you guys

Thanks
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Old 19th June 2013, 04:51 PM   #9
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Quote:
Originally Posted by RipX View Post
Finally, I think http://www.diyguitarist.com/PDF_File...iezoPreamp.pdf is not so bad ... but I like a pot for gain control and a system for stop the battery drain when I unplug the cble !
I think it's not very difficult to find for a pro like you guys
It's a bit of a bizarre circuit?, the switched gain at the input halves it's input impedance, and only has a very minimum effect. Also there's the strange comment to change R4 to 220K for bass guitar? - crazy!!

Simply remove the input switching and 4.7M resistors, and stick a volume pot on the output to make it adjustable volume - personally I think 22K for R4 sounds too high, and I'd make it lower whilst checking the source voltage (aiming for around 4.5V).
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Old 19th June 2013, 04:51 PM   #10
Struth is offline Struth  Canada
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Hi Guys

The peizo buffer costs less than a dollar, even with a volume pot on its output.

Use a TRS jack - tip,ring,sleeve, aka 3-circuit jack. Sleeve ties to circuit ground, tip to volume pot wiper, ring to battery negative. When you insert a mono 1/4" plug, the ring and sleeve are shorted and the circuit is powered. Unplug and the battery disconnects.

Micorphones that mount on the instrument are probably about the same price or more than a peizo pickup for under the bridge. A mic preamp is more expensive than the peizo pre as the mic requires significant signal boosting to even be as high as an instrument pick up - about 30db of gain. I don't see how this is less expensive than the other, except for possible labour savings if you have to pay someone to mount the peizo.

Have fun
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