make a buffer balanced

i am still new to electronic and i probably have a stupid question..

i am trying to build a buffer for a piezo microphone following the project on this site: http://www.scotthelmke.com/Mint-box-buffer.html

but now i am wondering how to make the input of this circuit balanced, to connect two piezo mic in a coupled structure like this http://www.instructables.com/id/Balanced-piezo-contact-microphones/

is it possible to modify this circuit ? or do i need something new ?? thanks ! :)

If you take the outer connections from your balanced piezos and ignore the middle one, you can connect up to your buffer circuit just as if you had a single piezo.

simulate this circuit – Schematic created using CircuitLab

• In the article OP linked, they're gluing 2 piezos "back-to-back" to enable them to use differential output for noise suppression, so I don't think the SPDT is necessary. That said, I don't see any reason why the 2 mics can't bw wired in series, with 1 end dropped to gnd; as long as he's not wanting to keep the output balanced. Feb 6 '16 at 3:07
• thanks for the help but i'm interested in the noise suppression, so one of the two signal must be inverted in the phase, and then summed. the simple sum of the two signal is not a solution for this, right ?
– Vinc
Feb 6 '16 at 10:59
• Tying the black wires together, then grounding a red wire suppresses noise from the piezos too. Black-to-red sums, black-to-black subtracts. If you remove the resistors & switch from brhans' diagram, then connect a buffer amp (op-amp, or FET/BJT based buffer) to the other wire, it becomes essentially identical to the op-amp circuit. Either method can accomplish pretty much the same result for you; the main difference is resistor sizing & placement (other differences only really apply to circuits with more differential inputs, like if you were trying to run several of these mics from one PCB) Feb 6 '16 at 15:05
• The resistors & switch are in the OP's original circuit for selecting gain. Feb 6 '16 at 16:15
• i have tried all these solutions and they work as expected ! thank you all for the support and the patience !
– Vinc
Feb 12 '16 at 10:20

If you check Wikipedia's article on "differential amplifier," and check out this writeup as well, I think they'll get you well on your way to understanding the type lf circuit you'll need to get that microphone pair working for you.

• the inverting op-amp works seems what i need, as it invert one signal and then sum it to the other; this is what i need for a balanced mic. but, as the op-amp has a hi-z input and a low-z output, could i need only this to buffer the signal ? or i must use an op-amp, and then put the summed signal in the buffer ?
– Vinc
Feb 6 '16 at 11:02
• The hi-z input and low-z output of an op amp are used in many circuits as a buffer amp (whether differential input, or single input). Dependinb on what you're planning to connect after the amp, it may be all you need, or you may want to add a second op amp after the first in "voltage follower" configuration ("voltage follower" is also mentioned in the Wikipedia op amp article, for more information). Feb 6 '16 at 12:11
• it seems really nice and simpler ! so if i want a buffered (hi to low z) and balanced (inverted and summed) in for the two piezo, and a unbalanced out i will need something like this, right ? en.wikipedia.org/wiki/Operational_amplifier_applications#/media/…
– Vinc
Feb 6 '16 at 14:41
• Yes. R1 & R2 determime balance & impedance (more precise sizing improves 'common mode rejection,' but that's only important for noise coming from EMI/RFI vs your wire), Rf determines the voltage gain, and Rg keeps a static charge from "hanging out" on the non-inverting input & 'smearing' your output (FET based op amps can have ridiculously high input impedance). Feb 6 '16 at 14:55
• perfect ! the FET's were inadeguate. so if i don't want to amplify the signal a lot i must scale the rf, or can i choose an op-amp with a low gain ? are there any ? in a lot of schematic i've seen a capacitor in series with the input, like the one the this forum. has it any advantage ? guitarnuts2.proboards.com/thread/5028/piezo-preamp-again last question, i promise! if i would to keep the signal balanced in the out too, have i to use two op-amp, one with an inverting and one not-inverting layout, only to reduce the impedance, letting the mixer xlr input sum them ?
– Vinc
Feb 6 '16 at 16:14