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I've been trying to implement the electrosmash Pedal Pi project using a Raspberry Pi 4 Model B, breadboards, and parts purchased from DigiKey.

For reference, here is the overall schematic of the build: enter image description here

So far, I've built the input stage, and the power supply stage which seem to work fine. My struggle currently is with the output stage. When I plug in the output jack into my amplifier, there is an incredibly loud hum emitting.

Here is what I have tried:

  • I disconnected PWM0 and PWM 1 from the raspberry pi. When I do that, there is still a loud noise, which leads me to believe that the noise isn't coming from the PWM pins.
  • I have plugged in my 1/4 inch jack into my amp and disconnected the other end. The amp did not create any hum at this point which leads me to believe that the issue is not with the amp.
  • I have tried changing out the op amp for a spare one I have, and the humming persists.
  • I have tried changing the C6 capacitor to an arbitrarily higher capacitor (I believe 0.1uF) and that did not change the humming.

I'm not quite sure how to debug this, and it's been quite a roadblock since I can't distinguish the output from the PWM pins through all the noise.

Further details:

The parts I am using for the circuit are:

My breadboard layout for the output stage looks like: Breadboard layout

where the output jack is on a separate PCB, following the schematic.

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  • \$\begingroup\$ Do you get the hum in both the "effects" and "Bypass" position of the switch? Is the ground terminal of the output jack connected to your circuit ground? \$\endgroup\$ Jan 9, 2021 at 23:53
  • \$\begingroup\$ I implemented this without the bypass switch, so that the Output Jack connects directly to the Fx out. I will try simulating bypass mode by using a wire to see if there's any hum, and let you know! The ground terminal of the output jack is indeed connected to circuit ground. \$\endgroup\$
    – Keith Choa
    Jan 9, 2021 at 23:57
  • \$\begingroup\$ Do you have a bypass cap near the opamp? \$\endgroup\$
    – Mattman944
    Jan 9, 2021 at 23:59
  • \$\begingroup\$ Hi Mattman, are you talking about the C8 cap? \$\endgroup\$
    – Keith Choa
    Jan 10, 2021 at 0:07
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    \$\begingroup\$ it might be due to the power supply you're using for the Pi. Try using a powerbank and see if the situation changes \$\endgroup\$
    – Kartman
    Jan 10, 2021 at 2:00

1 Answer 1

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I had made an incorrect assumption about the pins of the Jack (referenced here: https://www.digikey.ca/en/products/detail/switchcraft-inc/SN37A14B/1288988). I had assumed the pin orientation was parallel, assuming the furthermost pin from the base would be the "tip" or positive, and that all subsequent pins would be "ground". However, that proved to not be the case. After resurrecting my issue, the loud "hum" subsided. Thank you all for your kind suggestions! There is still a slight amount of noise remaining, which I am sure will be fixed by your suggestions. But the bulk of my issues have been fixed. Thank you!

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    \$\begingroup\$ Thanks for posting your findings. After a day or two you should be able to "accept" this answer to indicate to the system that the question is solved. \$\endgroup\$
    – Transistor
    Jan 10, 2021 at 9:36

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