Following cause I wanna do custom guitar piezo electronics, eh and MIDI digitizing :)

You don't need to make an entire op-amp for that: I suggest you a J-Fet preamp and careful shielding:

http://www.rason.org/Projects/jfetamp/jfetamp.htm

Hello Vito,

Indeed I'm looking at discrete JFET amplification for the #Active scope probe with no dedicated power supply. While looking at differential amplifiers on google, I found a lot of audio circuits as well, including this one

I don't remember where I found it but with some enhancements (offset trimming, current sources instead of certain resistors) it makes a semi-decent op-amp that I can set to unity gain. That got my headgears spinning...

I won't try to reinvent something revolutionary, just remove the noise that keeps me from using my instruments for recording. I know that audio amplification is one of the golden applications of electronics and everything has been tried already :-D

If you have other F-FET suggestions, thank you for sending more links, I'll study them !

note for later: https://www.passdiy.com/project/amplifiers/diy-op-amps