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Even though I'm a total beginner at electronics, I've decided to build a 6-channel piezo pickup buffer/mixer unit, for a stringed instrument. It should fit into a typically sized electronics cavity, and be powered by a 9 volt battery. Gain boost is not necessary, but I'll take it, if possible :-)

One of my questions, is how to best sum 6 of these circuits together.

enter image description here

Before summing the outputs together, I was thinking of adding 50k trimpots, to allow for level adjustments. Ideally, the summing/mixing section, would also be jfet based (j201).

I've built the stratoblaster circuit before, and was wondering, if that could be used as a mixing stage? enter image description here

All very naive assumptions, I know :-)

So, any suggestions would be greatly appreciated. Thank you.

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  • \$\begingroup\$ The first thing that comes to my mind is to use an OP Amp, a summing operational amplifier. "The Summing (Operational) Amplifier - Electronics Tutorials": electronics-tutorials.ws/opamp/opamp_4.html. Cheers. \$\endgroup\$ Commented Jul 18, 2020 at 13:18
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    \$\begingroup\$ Thanks for the response. Is an op amp based circuit better than a jfet one? \$\endgroup\$ Commented Jul 18, 2020 at 14:54
  • \$\begingroup\$ #bankschreiner, actually my limited experience of using OpAmp is LM386 audio amplifier, and LM310 OpAmp comparator. I also played with general purpose, small signal MOSFET 2N7000. I think 2N5457 JFET should be good for audio applications, though I have the feeling that JFET sounds old fashioned, and is not so popular as MOSFET, especially power MOSFET is so common nowadays. Anyway, I think @analogsystemsrf's advice is good. \$\endgroup\$ Commented Jul 19, 2020 at 12:46
  • \$\begingroup\$ You are welcome. Have a great project! Cheers. \$\endgroup\$ Commented Jul 20, 2020 at 1:14

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Almost all modern opamps are CMOSFET input, which will provide the same picoAmpere input currents that JFETs achieve.

To select an opamp, determine if you need

  • Rail_to_Rail inputs (such as 0v to +3.3v)

  • Rail_to_Rail output (often spec'd as "within 0.01v of rail, for 10Kohm load)

  • unity_gain stable (needed if you only use for Gain = +1) and for 100pF Cload

  • of adequate slew_rate ( 2 volt peak_peak output at 20,000Hz needs more than 120,000 volts/second , or 0.12 v/microsecond). So pick the default of 0.5v/uS , or faster.

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