Piezo + electromagnetic preamp

[steven_first_2001]

Hello guys! Hey I was hopping to get a hand with a project I have been toying with. I built an electric cello with electromagnetic pickups as well as piezo bridge pickups and I need to mix the two signals together and put them out to a common mono output. The piezo's put out high voltage (0-10v) but no current and the electromagnetic's put out a few mA but no voltage. The two don't like to be mixed without being amped first. Can someone please help me out with a small 9v powered preamp based off of either a 741 or an 386? I would like to bring the voltage of the piezo down and the current up and then bring the voltage up on the electromagnetic pickup and then mix the amp's outputs (adjustable). Up till now I just voltage divided the piezo and the mixed the pickups together with 2 pots but that has many issues because of different impedances. I just want the output to be able to be hocked to a guitar effects system. Thanks!

 

[steven_first_2001]

No thoughts anyone? Little guidance at least?

 

[Nigel Goodwin]

I would suggest you build a simple two channel mixer, feed the electromagnetic input to it via an opamp amplifier, with an input impedance about 1Mohm, and set the gain to be suitable. Feed the piezo via a higher input impedance, 2M2, 4M7, or 10Mohm - either use a suitable opamp, or if the output is truely as high as you say, use a high value series resistor to feed a lower input impedance opamp (making an attenuator). Using the two mixer level controls, you can mix the proportions from each pickup to get the sound you want - or you could use a single pan/balance pot to fade between them instead.

Obvious thing! - don't even think of using a 741, use something a little more modern and better spec, like the TL072 series.