The guitar is one signal source.
A function generator is another, using a precise square wave can reveal timing.
Changing a capacitor can be seen as a delay on an oscilloscope.
What is the first sign of instability in a jfet op amp?

 

Studying this now!

TI and Analog Devices have some very good design manuals:

https://www.analog.com/en/resources/technical-books/linear-circuit-design-handbook.html
https://www.ti.com/lit/an/sboa092b/sboa092b.pdf
https://pearl-hifi.com/06_Lit_Archi...Texas_Instruments/Op_Amps_for_Everyone_TI.pdf
They are better than anything you might find on YouTube.
I know about them because Iearned my trade before YouTube arrived!

By the way, some of Pearl Hifi's files may be illegal to download in your country because of copyright issues. UK and Canadian copyright laws permit fair use including "use for study". However, Op-Amps for everyone is public domain.

 

Many thanks to all who helped us with this issue and get this far on our tuner project..
@joulupukki and I are almost ready to launch. Join us night for a live stream event:

Here’s a teaser video of the product: