Do you mean the noise goes away, or you hear the noise when an audio signal is present?
On Teensy 3.2 the noise increases when audio is present, and I now know why (and fixed it).
Could it be this issue? I have a fix in my
fork
https://forum.pjrc.com/threads/4577...e-distortion-in-input_adc-cpp-input_adcs-cpp
One way to test is take your audio output right off the input to the ADC, bypassing the whole software side of things. Then you can hear what it is like right at the input of the ADC. This will help you know if your op amp circuit is getting noise or if it's something inside the Teensy after the ADC.
It's good to separate these things in functional blocks and test down through the system. Something common in the DIY stompboxes community is an
Audio Probe
[EDIT] I do recommend using the diodes for input protection. The only change to the circuit is put a 47 ohm resistor feeding the ADC input with a 47n to 150n capacitor to ground at the ADC input. The parasitic capacitance in the diode works as a nice low impedance to inject switching noise on the ADC input. The capacitor will shunt this to ground and the resistor gives it some impedance to work against. This also serves as a basic anti-aliasing filter.
Here is what I am doing for a guitar input. My circuit is optimized to guitar signal levels, and it is hard limited between 0V and 1.2V. I noticed wrap-around when the ADC is overdriven -- in fact the ADC makes noise when it gets signal between 1.2V and 3.3V (another possible source of your problem -- maybe you are overdriving the 1.2V range).
I decided to use a precision voltage reference to clamp the voltage just shy of 1.2V, and used the discrete "op-amp" circuit to give me decently linear operation right down to the 0V rail. The second harmonic measured in SPICE simulation is around -80 dB for a 0.5Vpp output and the remaining harmonics are in the noise floor (<90 dB). For a 1Vpp output the first harmonic is about -60dB and the final visible harmonic before dropping into the -90 dB floor is the 6th. I am expecting the nice mix of even harmonics will not sound bad on guitar as it approaches the limit and it is on-par with typical stompbox effects.
I have added several stages of ground-earth capacitors for anti-aliasing. This comes to about -6dB at 20kHz, intentionally starting to roll off at around 14 kHz since this is not too hard to correct with a digital filter.
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