hurdy gurdy piezos - help!

[alwig]

Hi -I need some help here! After several requests from customers to fit piezo hotspots to my hurdy gurdies, I've decided to try and sort it out.
Basically, I want to use 3 piezos. Two to go through a blend pot and then that signal to be one channel and the 3rd piezo to be the other.
So, here's the problem - how do I actually do the wiring on the blend pot and also the 3 pin XLR to get a stereo output? I really could do with a diagram or schematic. I've already tried using the blend pot wiring diagram on StewMac website, but when I connect up the XLR it seems I only get a mono output. Also, I want the blend pot to work like the balance pot on a hifi, but it doesn't seem to do that.
As you may have gathered, I might be a hurdy gurdy maker but I'm a total novice when it comes to this sort of thing.
An idiots guide would be much appreciated!
Thanks.....

[Rob Mercure]

Alwig,

We can always use more hurdy gurdy makers - fascinating instrument, I've got a acquaintance in E. TN who makes them and the amount of labor is somewhat amazing.

Anyhoo, piezo PUs want to "see" a truly high input impedance - in the order of ten mega ohms or so - before they're "happy. And I'll admit that I'm not much of a fan of the sound of piezos and this sound only sounds worse with an impedance mismatch.

So I'd recommend that you craft some sort of buffer/preamp for each PU and then blend these outputs. They're lotsa piezo buffer schemos on the net most of which seem to use a single FET (field effect transistor). These truly simple circuits - parts count of about 1/2 dozen components each - allow the piezo to "see" a sufficiently high input impedance while having an output impedance of a few hundred ohms to a couple of kilo ohms. If you were to cobble up a buffer for each piezo you could then passively mix the ouputs (isolation resistors) of two of these to create one channel and then use the other buffer output for your second channel.

But this assumes that the XLR input you're feeding is "looking for" a stereo signal and not a balanced single channel input - if you just feed a stereo signal into the low-Z input of a stage mixer you'll wind up with some funky "sum/difference" signals. Now one of the good "side effects" of using a buffer is that you can then feed either a high or low Z input as long as you've got the right connectors (and signal levels) so you might consider feeding the ouput from a couple of 1/4" jacks or two separate XLRs jacks to ensure that your customer mixer/amp is compatible with a stereo signal and not trying to process two different signals as either "half" of a balanced input. This would also provide the customer with some signal processing flexibility if they wanted to - for example - send one signal through the "house" PA mix while sending the second signal through a modelling guitar amp with a zillion effects on it <grin>.

Hope this helps.

Rob

[Satamax]

Alwig, have you tried asking on the MIMF?

[alwig]

Hi Rob - I understand about the preamp/buffer - good idea. But would i need a 9v supply to each of them? I'm sure I've seen an electro acoustic gurdy that had preamps, but I can't remember seeing a bank of batteries! As I'm not really sure what to look for on the net, can you recommend a simple system that you may have come across? Or do you know where I could get someone to make them up for me?
I've got a feeling that the stereo bit is going to make this idea way too complicated for me, and if I end up with a balanced single channel, then that'll be fine.
I'm beginning to wonder if I've inadvertently strayed into a parallel universe with this whole idea!#Thanks for your help
Mike (alwig)

[alwig]

Yes, but some time ago and didn't get very far. Very technical response to the question if I remember right, so gave up. However this little problem does seem to have risen to the surface again!
Cheers
Mike

[Satamax]

Alwig, check the preamp in the mimf library! And also polyvinylidene pickups PUTW on teh mimf for the first lead.

[Enzo]

No reason at all your little preamp can't be run from one battery. Think of it as a small very simple three input mixer.