Piezo pickups again - what input impedance?

I am modifying a Raven Labs PMB-1 for three (yes, three) piezo pickups - one on the pickup/inst channel, one on the mic channel, one on an extra preamp board that I built. I'm also designing my own 3-input (or 4-input) mixer for installation in an electro-acoustic hurdy-gurdy. I've read that piezos need a really high input impedance, like 5-10M. I've also read about the advantages of using a FET input stage, and the PMB-1 just uses op-amps. Which way should I turn here? Your comments are appreciated. I can post the circuit if people are interested.

Alden

Not all piezo pickups need the super-high impedance, and I've heard designers say that some piezos sound better with a lower (say 1-2Mohm) impedance. But there are definite improvements on a fair number of pickups with an impedance in the 8Mohm range.

Of course there's a tradeoff, since higher impedances will tend to pick up more noise and lose more treble from cable capacitance.

Maybe you could put in a switch, or an internal jumper?

Yes, in my experience most piezos will sound a lot better with a higher impedance. As to the opamps, well, it depends on the opamp. What chips does this box use?

Peace,
Paul

The Raven Lab pmb1 uses FET op amps if i remember correctly, i managed to gain a wee bit of information about the chip numbers(which seem to have been rubbed off most of the ic's in the units) and think i still have it somewhere if you need it. You can also increase the gain by adding a 10k resistor across two solder pads. Raven are closing down and are not very keen with service info. Have you seen the new Aura system made by Fishman its supposed to make a piezo sound more like a miced guitar and has different images for different guitars, seems quitye promising. The new Mama Bear unit is available soon and is supposed to be the bees knees!
Vikki :grin: