Capacitor to Ground at Input

[YourGuitarist]

Hello,

I am working on a pedal right now where I am troubleshooting 60 cycle hum when my hand touches the guitar strings and the other hand touches a ground surface. So in short, I think there is a ground loop forming at the input of my pedal. My input section is shown below.

I have seen variants of input where they have a capacitor in parallel with what would be R2; usually on the order of picofarads.

I have also seen variants that employ a series resistor either between C1 and the virtual ground or leading into the following op-amp stage.

I would assume the resistor is simply to limit current but am ultimately unsure. I have no idea what the capacitor does as the corner frequency I simulate is typically outside of the audio range.

So are these above measures possibly related to ground loop protection? If not, what is their purpose AND what are some common culprits of hum at a pedal input.

FTR, I am testing on an unhoused PCB with non PCB mount pots.

Any and all thoughts appreciated.

 

[jwin615]

Establish a ground plane with your jacks and PCB first.
You can float the output but I would try it with and without the output grounded.
Also make sure your amp and PCB are on the same electrical circuit.
And that all cables involved are solid. Even the "good" cables go bad.
If your guitar is floating, weird stuff happens.
This seems simple, because it is. Just want you to alleviate outlying issues before deep diving.

 

[YourGuitarist]

My PCB is a two layer with ground occupying the back. I noticed that if I connect my gnd pin of power to the gnd pin of the power supply unit, much of the noise goes away.

Does a lot of noise tend to be handled upon mounting to an enclosure? While I've done a number of pcbs, I've not progressed to housing them yet.

Btw I'm running my pedal direct into a computer. If I unplug the power to the computer, the noise tends to quiet down.

 

[jwin615]

My PCB is a two layer with ground occupying the back. I noticed that if I connect my gnd pin of power to the gnd pin of the power supply unit, much of the noise goes away.

Does a lot of noise tend to be handled upon mounting to an enclosure? While I've done a number of pcbs, I've not progressed to housing them yet.

Btw I'm running my pedal direct into a computer. If I unplug the power to the computer, the noise tends to quiet down.

You should establish a common ground. Input to power supply is pretty much mandatory in non-isolated circuits.
Outputs should generally be grounded by the subsequent input.
Inputs should have a direct and short connection to ground.
If you make that connection and noise improves, that was you major issue.

The enclosure acts as a emf/RF shield. Common emf from lights, etc will be sent tonground via the enclosure. This is, if you ensure solid ground connection to the enclosure. If you have metal to metal contact(clean drill holes) from your input jack sleeve to the enclosure, that is usually enough.

I'm not sure how you hear things through a computer that is turned off so can't comment.

Hope this helps.

 

[YourGuitarist]

Its not turned off, it's a laptop, the charger is what I'm referring to.

Well my signal gnd at the input is a direct connection to the pcb gnd. Same goes for the power inputs (9V and GND).

I see that I am be being unclear. I had connected the 9V and GND pins of my pedal to a traditional regulated wall supply.


The noise subsided when I simultaneously jumped that GND pin to a dedicated lab style power supply earth. This is what I meant.

 

[Feral Feline]

Hello,

...
I have seen variants of input where they have a capacitor in parallel with what would be R2; usually on the order of picofarads.

...I have no idea what the capacitor does as the corner frequency I simulate is typically outside of the audio range.
,,,

From what I've read, the picofarad capacitor helps cut radio-interference and high-frequency hash from your signal.
Even if you can't hear it, that hi-freq is mucking up your signal so why include it?

 

[YourGuitarist]

Yeah I suspected that might be the case, there are instances where HF information can cause systems to oscillate or become unstable. But usually the filter cap is embedded in the active stage if that were the case.