Buffer Circuit For Radio Shack Piezo

[LarsXI]

I'm making a dirt cheap radio shack piezo pickup for my dirt cheap strat copy. I've been searching for a buffer circuit that I can make so I can blend the piezo with a single 6.5ohm pickup without impedance problems. I've found several potential schematics, but I'm unsure if they'll serve the purpose.

One of them:

http://www.till.com/articles/GuitarPreamp/

Another:

http://www.cafewalter.com/cafewalter/fetpre/pzp1_project.htm

Ideally, I'd like to be able to blend the piezo signal and the magnetic signal with a 500k blend pot I have on hand.

Any insight would be greatly appreciated,

Thanks for your time

[psw]

Hey Lars...a very similar question to the one just posted by you on the sustainer thread. The cafe walter circuit looks interesting and if you understand his explanation a bit, you will have a good idea about "loading effects".

It is unlikely that you have a magnetic pickup of 6.5 ohms...more like 6.5K ohms or 65,000 ohms. However, this is not even close to the impedance of a piezo. Hence, while a piezo can be used on it's own, to match impedances and especially share signals into a guitar amp without negative and unwanted loading effects you need to have electronics that have even higher input impedances....op-amp and transistor circuits can provide inputs that are virtually infinite, or limited by the design of the circuit!

So in both the sustainer project (where a pickup signal is shared between an amp that runs the driver and the amp that makes the noise...ie the guitar amp) or in a piezo circuit, especially where you intend to mix it with a completely different input resistance (or impedance) preamps or buffers (preamps with no gain but impedance matching) are essential.

Also in relation to your question about the LM386 in the sustainer project, these kinds of circuits could be used in place of the fetzer part of the circuit in the design discussed, or indeed the fetzer could be used in this application.

I notice that the piezo preamp described was designed for a bass though...so not sure if this will make a big difference should you choose this thing. The tillman is a long established simple design. Op-amp circuits could also be used or you might find a kit that is a basic preamp that would at least work. Which sounds "best"...well, they all will work and this depends on a lot of things.

A lot of variations in response can be achieved by the location of the piezo or using multiple piezos. A search will reveal more designs and I know there is another at DIYguitarist that could be of interest and a thread here as it was developed...

I have a project that I am working on in this area, so I will be following your results!

As for the use of a 500k pot, often much lower 10k-25k are used in active circuits and you will likely need to put the entire guitar through the circuitry in order to mix or blend. As such a pot appropriate to the circuit is normally best and often, a dual gang pot to vary the two circuits if a mix control is required.

So...again, just about any preamp circuit will "work" to alleviate the effects of loading that will be caused by piezos and things like the sustainer circuit. There are hundreds of designs and approaches, but don't discount what you can do by locating the piezo itself, I put one in the neck pocket of a guitar once with very nice results and no fancy preamps or filters...kind of "woody" with no bridge noise, but a little handling noise if fretted high on the neck. Often they are glued to the sustain block on strats...again, search out the various projects people have completed right here at PG...

Also, beware making too many projects at once. The sustainer would work with a piezo, but be sure that you have one working perfect before trying to combine the two...troubleshooting will be a nightmare!

pete

[LarsXI]

Ah. The impedance stuff has really been tripping me up today. I'm sorry for bothering you with it, and I'm very grateful.

I'm going to try various FET based circuits, and I'll be more than happy to share once I get something working well. So far, a lot of DIY piezo ideas for basses are floating around, but guitar piezo designs tend to be for acoustics without any magnetic pickups.

hopefully I'll have something in the next couple of days.

Thanks again

[crafty]

Look up mixers. You basically combine either dual preamps or dual inputs into one preamp and use trimpots to adjust the gain on either side.