Hello,
I am using the Teensy 4.1 + Audio Shield as the signal processor for a custom guitar fx unit. It's been working great except for that I can't figure out how to get the noise under control. So I kept stripping back my design until I got to a pretty basic minimum and found that the noise is probably being introduced via the power going to the Teensy.
That is the schematic. The two OPA-2134 spots in the schematic are two channels of a single chip. For testing, "Audio In" just connects directly to "Audio Out." This roughly creates a unity gain system. It works as expected. I can turn the circuit on and off and not much changes. There is a tiny bit of noise introduced but fairly acceptable for a guitar setup. This is with nothing connected to +VAA -- i.e. the Teensy is not connected in any way.
Now if I connect +VAA to the Teensy 5V pin and G to ground. I upload an empty script to the Teensy that does nothing except print "Hello World" in the setup. The circuit behaves roughly the same and not much noise is introduced.
That's the vero layout. Now I upload a new script that does a lot of processing in each cycle. Suddenly, a lot of noise at roughly 700 Hz gets introduced. You can hear it in the attached audio file. Note again that the audio is not going through the Teensy/Audio Shield at all. The Teensy is only connected via the 5v and G pins and the audio is all analog signal unrelated except for power supply at this point. The Teensy has no other pins connected to anything at this point -- not even USB is connected.
Zip file with audio sample of the noise: View attachment Pedal noise.zip
The specific change that introduces the noise is adding computations to each clock cycle. Is it possible that CPU noise is getting into the power line and creating noise in the original analog circuit? You can see in the circuit that I tried some decoupling capacitors. Does anybody have any thoughts on what could be causing this noise and how I can remove it?
Thank you!
-Kenji
I am using the Teensy 4.1 + Audio Shield as the signal processor for a custom guitar fx unit. It's been working great except for that I can't figure out how to get the noise under control. So I kept stripping back my design until I got to a pretty basic minimum and found that the noise is probably being introduced via the power going to the Teensy.
That is the schematic. The two OPA-2134 spots in the schematic are two channels of a single chip. For testing, "Audio In" just connects directly to "Audio Out." This roughly creates a unity gain system. It works as expected. I can turn the circuit on and off and not much changes. There is a tiny bit of noise introduced but fairly acceptable for a guitar setup. This is with nothing connected to +VAA -- i.e. the Teensy is not connected in any way.
Now if I connect +VAA to the Teensy 5V pin and G to ground. I upload an empty script to the Teensy that does nothing except print "Hello World" in the setup. The circuit behaves roughly the same and not much noise is introduced.
That's the vero layout. Now I upload a new script that does a lot of processing in each cycle. Suddenly, a lot of noise at roughly 700 Hz gets introduced. You can hear it in the attached audio file. Note again that the audio is not going through the Teensy/Audio Shield at all. The Teensy is only connected via the 5v and G pins and the audio is all analog signal unrelated except for power supply at this point. The Teensy has no other pins connected to anything at this point -- not even USB is connected.
Zip file with audio sample of the noise: View attachment Pedal noise.zip
The specific change that introduces the noise is adding computations to each clock cycle. Is it possible that CPU noise is getting into the power line and creating noise in the original analog circuit? You can see in the circuit that I tried some decoupling capacitors. Does anybody have any thoughts on what could be causing this noise and how I can remove it?
Thank you!
-Kenji