Other suggestion, can you power the board with a 9 volt battery?

I could try this. It draws about 150 mA of current... 95% of that is the screen.

 

Disconnect the screen, maybe that's what causing the noise?

 

Has this been tried?

 

Which layer is the ground plane? Is it red or blue?

 

Same thing with a battery.

 

Which layer is the ground plane? Is it red or blue?

GP on both top and bottom.
Top is Red. Bottom is Blue.

 

All the digital stuff is on the lower part of the board (JST connector and 5v regulator, etc). We separated the GP there to separate the digital and analog grounds. They eventually come together at the DC jack, star pattern.

 

Suggestion from post #43

 

Are you using the DIL package or the SOIC package? The SOIC package is missing the link between pin 4 and the ground plane.

 

Has this been tried?
View attachment 348911

Sorry, haven't tried that yet. That'd be a LPF at 16Hz and cutting out some of the input signal content, right?

Disconnect the screen, maybe that's what causing the noise?

FWIW, I did just try disconnecting the ESP32 by unplugging the JST cable. Noise is still there.

 

Suggestion from post #43
View attachment 348914

OK, I'll go work on this.

 

Or maybe like this:

 

I have C1 removed. Which way do you prefer?

 

Sorry, haven't tried that yet. That'd be a LPF at 16Hz and cutting out some of the input signal content, right?

According to AI:
The highest frequency a guitar can produce typically ranges from about 5 kHz to 15 kHz, with some harmonics and overtones potentially reaching up to 15 kHz, according to Neural DSP. However, the most common and practical frequency range for a guitar is generally considered to be from around 80 Hz to 5 kHz.

 

I have C1 removed. Which way do you prefer?

Post #52.

 

According to AI:
The highest frequency a guitar can produce typically ranges from about 5 kHz to 15 kHz, with some harmonics and overtones potentially reaching up to 15 kHz, according to Neural DSP. However, the most common and practical frequency range for a guitar is generally considered to be from around 80 Hz to 5 kHz.

Ok, agreed that a guitar wouldn't be creating something as high as 16Hz on its own. Something like a 4.7k or 7.5k paired with a 1nF would theoretically keep the audible hearing range untouched. Well, I can't hear past 16Hz anymore anyway.

 

If there's no signal on the non-inverting input, then putting a filter on that input isn't going to help.

 

If there's no signal on the non-inverting input, then putting a filter on that input isn't going to help.

The scope lead might be filtering the noise signal. It's worth a try.

 

What make is your TL072? Back in the 1990's when I used to work in professional audio, we always insisted in Motorola or Texas TL072s, because the SGS-Thomson ones were not unity-gain stable.

 

The scope lead might be filtering the noise signal. It's worth a try.

If that is the case, then put the second channel probe on the output and see if the signal disappears.