This morning I soldered two new 3.3 volt LDO modules.

Plugged one in and I got the same result! 2.4Volts! Why?

Actually before I snapped the LDO module pcb it was working perfectly fine, the main problem was I were feeding it 3.3 Volts instead of 5 Volts. With the voltage drop you can never reach a 3.3 Volt output with a 3.3 Volt input... Stupid me. Anyway with that mistake cleared up, I can continue.

I plugged the LDO correctly to VUSB so it gets enough voltage to work properly now I should get a tone with 397Hz but I don't. Looking at the measurement after the pre-amp in the last log, it's clearly clipping the waveform at the pre-amp stage.

Back to basics again. To be brutally honest, I don't even know, how and why op-amps work. After clearing up some of the op-amp basics, I noticed I had the gain set at 2.8 instead of 2. So I replaced the 18k gain resistor with a 10k resistor and now it sounds better but not quite right.

I have to look at the waveform with my oscilloscope, but the project time has run out. So that will have to wait until tomorrow morning.