Misc design notes
C13's value has been decided according to an amplitude/whiteness trade-off ; and C9 has been added to prevent power supply noise or whine to leak through the circuit (and be amplified 15 times by IC4a).
C13 is a tantalum cap because ceramic caps are prone to microphonics. A prototype was built with a ceramic cap, and given the huge gain of the circuit, the module acted kind of like Ears – an unwanted Easter Egg!