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I've built a few guitar pedals using op-amps, transistors, etc. (with supporting resistors & capacitors of course), and I've generally 3D printed the housing for the components for custom designs. With these circuits, there is generally static and sometimes radio stations being picked up and amplified. On the pedals and circuits on my full metal test rig, however, I almost never have any interference and noise problems. My best guess is that the metal acts as a Faraday cage and blocks most, if not all, interference that the plastic boxes instead let through. I've thought of different ways to shield my 3D printed pedals with aluminum foil or other metal, but I'm sure there is a way I'm not thinking of. How can RLC audio circuits be shielded in a plastic 3D printed pedal box?


Edit:

enter image description here

The legendary Marshall 100W Super Lead adapted for use as a distortion stompbox is one of the schematics (of many). Schematic from Runoffgroove.com.

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  • \$\begingroup\$ Please post the schematics, and how you built the circuit. It might be better to have the PCB properly designed, or add a few capacitors to block RF, instead of working around the issues with shielding. \$\endgroup\$ – Justme May 16 at 23:13
  • \$\begingroup\$ you should consider using properly shielded cables. High gain circuits + long guitar cable? sounds like a big antenna attached to a an amplifier. \$\endgroup\$ – ppmbb May 16 at 23:18
  • \$\begingroup\$ @Justme, it's not limited to one circuit, I've seen it between multiple builds over a few months now. Which caps would you recommend and in what location? E.g. across the power supply, in the feedback path, etc... \$\endgroup\$ – nate May 16 at 23:33
  • \$\begingroup\$ @ppmbb, can you clarify the big antenna? Not sure exactly what you mean. The quarter inch cables I'm using are pretty solid coax, so I don't think it's the long cables \$\endgroup\$ – nate May 16 at 23:33
  • \$\begingroup\$ @nate the long guitar lead external to the pedal can act as an antenna. If your circuit has bad grounding from the input connector to the common ground point, it may be enough to receive radio. Usually the first thing is to shunt the RF at the input connector with a capacitor, but without seeing the schematics and how it is built, the problem could be anywhere. \$\endgroup\$ – Justme May 16 at 23:49
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There are metalized paints for EMI shielding but they tend to not be cheap. You would coat the interior of your enclosure with this and tie it to your system ground.

Otherwise if the design files are available,it won't be hard to work out what is going wrong.

I design amplifiers that get mounted next to automotive engines, so guarding against external noise is my specialty.

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  • \$\begingroup\$ Awesome, thank you! I've linked a schematic that I've noticed the issue with. How would you implement a RF filter in that schematic or something similar? \$\endgroup\$ – nate May 17 at 0:18
  • \$\begingroup\$ @nate I'll need some time to analyse how to improve on it, but at a glance this thing looks down right horrible for external noise immunity, lots of high impedance nets into gain stages, all on perfboard.... \$\endgroup\$ – Reroute May 17 at 3:18
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    \$\begingroup\$ @nate First up, seems the 100K trim pots should actually be closer to 10K in value, as most of there range does not seem to be used, this will not be causing an issue, but likely makes it harder to setup, main issue is that gain potentiometer, I would take the signal from the 22n cap to its input, use a fixed say 220K resistor to ground, then have an op amp buffer that to a 10K pot for your gain control, with its center feeding the RC network, There is a lot more that can be done, but he seems the most significant \$\endgroup\$ – Reroute May 17 at 4:27
  • \$\begingroup\$ why not use metal enclosures like commercial stomp-boxes do? \$\endgroup\$ – Jasen May 17 at 12:39
  • \$\begingroup\$ He asked how he could shield a 3D printed enclosure, as you cannot practically print a low enough conductivity to be a meaningful shield, this option was the most practical. he then asked how he might alter his circuit to not need the sheild. \$\endgroup\$ – Reroute May 17 at 12:41

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