Piezo Preamps Question

[Akula]

Hello,

I'm looking into building a DIY piezo bridge for a project of mine, and I've become aware that I'll need a preamp, to prevent the classic squawky thin-sounding piezo situation. So I did a little googling, and after a very long search (first result), I found this web page here: http://www.scotthelmke.com/Mint-box-buffer.html

My plan is to use the circuit explained there: http://www.scotthelmke.com/Mint-box-buffer-schematic.jpg

But my bridge will have six piezo transducers, one underneath each saddle, and I'm planning to build a single preamp for each piezo. This will allow me to set the gain of each string's sound individually. I showed my plans to my dad, and he wondered what would happen when all the preamps are just connected together, such as this: http://img29.imageshack.us/img29/7379/piezopreamps.jpg (sorry for the huge size...)

Any blindingly obvious error I've overlooked? 6 piezo's -> 6 preamps -> combined into 1 volume pot -> output...

Cheers,

Akula

[WezV]

if that pre-amp design only has two gain settings are you really going to be able to balance the strings with it?

To me it makes more sense to have the single pre-amp and the guitar set-up well so it produces an even string response... or you need to incorporate a gain knob instead of the jumper switch so you really do have a accurate control over the gain of each string

[Akula]

Indeed... instead of those two resistors and the jumper switch I'll have a large value preset potentiometer. Y'know, the little blue ones you set with a screwdriver. My reasons for not having a single pre-amp are because I'm not confident in my ability to make such a good DIY piezo bridge so I can set up each string well, and also because I'd like to play around with hexaphonic pickups and stuff later on. But thank you for your advice.

Another concern is how I'm going to fit six of these circuits into my guitar, without effectively making it hollow...

[Donovan]

Indeed... instead of those two resistors and the jumper switch I'll have a large value preset potentiometer. Y'know, the little blue ones you set with a screwdriver. My reasons for not having a single pre-amp are because I'm not confident in my ability to make such a good DIY piezo bridge so I can set up each string well, and also because I'd like to play around with hexaphonic pickups and stuff later on. But thank you for your advice.

Another concern is how I'm going to fit six of these circuits into my guitar, without effectively making it hollow...

You're going to have to spend a lot of time on the layout to accomplish what you want, unless you go with a surface mount build or buy a premade circuit.

Why not build a simple mixer, mix 6 down to 1 using the minipots you suggested, then preamp the output signal. You can then accomplish what you want with less components.

Check out the schematics on this page. This is how I'd do it.

Of course, this does mess with your idea of hex MIDI stuff later, but I don't think there is another way around the space thing for component count reasons.

[Akula]

That does sound a lot more sensible...

I'm thinking I'll go and make a single preamp, then play around with a simple mixer circuit or two. I'll probably end up making six preamps anyways, even if just for comparison.

I'll let you know how it goes.

[billm90]

What kind of guitar are you doing this for?

I have made a **** load of DIY piezos. Let me help save you some time.

A tap test will only tell you if the piezo works.

Make each piezo replacable. Make it similar to the RMC pickups.

I am going to make each one out of a small pieze of metal, and glue the piezo to it.

I made mine in the past on classical guitars, with a piezo on each string, but I epoxied each piezo to the whole saddle. Usually one or two strings were low on output. I would not find out till it was all together, so I would build a whole new set. I did this several times and to this day I am still finding pieces of plastic or wood covered in epoxy with 7-12 wires hanging out of it. All my failed attempts.

If you end up with low output piezo's, mixing it down wont be the best route. just fix it right (make new ones), then you can mix down the rest as you please for balance.

Next. running passive mixing. My fishman bridge with piezos says to use a 5meg pot. I dont have one, never seen one. I run it straight to the jack. they really knew what they were doing because it is pretty loud as a passive piezo. They say performance will be lowered with a lower value pot. I had issues when I tried a 100K pot, but also had other complicated stuff going on with it. so I am unsure.

If you try mixing the piezos before a preamp, run a test to see how it sound striaght to the jack, and then through a pot. I am actually going to do this pretty soon as I am gearing up for a ton of nylon's going to electric with diy piezos.

Another few things I would like to break down a little more scientifically but have not had time.

1. Does the amount of wire/solder used effect tone/output?

2. Does rigid mounting like in epoxy help, or should it be attached at one small point, where it can move semi freely?

3. 0n a classical bridge, is mounting in front, middle, or behind the saddle work better?

These questions arise as I have pretty much done something similar to all of these and had different results. which make me more aware now when I make these things, how the results vary.

I should make a guitar that has a 1/8 jack on top of it so I can just change out saddles done every which way and see what the results are.

[Akula]

I'm using a stratocaster copy. My plan is to cut some piezos down to size (about 5mm square), and wedge one underneath each saddle, in between the two screws and the bridge. This is pretty easily replaceable, just detune the string, lift the saddle, and slide the piezo out and replace.

I've run a quick and dirty test, and it works pretty well, it picks up the sound of that string and hardly any of the other strings. This is exactly what I want - the ability to individually set the volume of each string's piezo.

Out of interest, anyone know where to get cheap piezos? I can see myself getting through many of these things. I was planning to buy from Rapid Electronics, but paying £6.50 postage for £1.30 of tiny flat components is absolutely ridiculous. I bought a few from Maplins, but they're four times the price of those from Rapid Electronics. Any hints?

[billm90]

I have done the few under the strat saddles. I had mine on a popsicle stick to not damage them. I was not really happy with them. I was going to try and solder some under the saddles. I have the vintage type.

As for buying those, I went on ebay and bought 500 of them for 30.00 or something like that.

[weaponepsilon]

Out of interest, anyone know where to get cheap piezos? I can see myself getting through many of these things. I was planning to buy from Rapid Electronics, but paying £6.50 postage for £1.30 of tiny flat components is absolutely ridiculous. I bought a few from Maplins, but they're four times the price of those from Rapid Electronics. Any hints

You can get cheap piezo elements from headphone speakers. Everyone has band old headphones that you can tear apart.

Edited May 27, 2009 by weaponepsilon