Ground loop?

[simonrichie]

Hey all,

I've been Googling this issue but I have yet to find a simple answer.

I built a Z.Vex Wooly Mammoth. It hums like crazy.

In terms of grounding does it matter which component is wired to which component? Or does it just have to go to ground and that's it? For example, if Lug 3 of Pot 1 is supposed to go to ground, can I solder that to the input sleeve, output sleeve, an open stripboard hole on the same track as the ground wire, etc.?

-SR

[electrosonic]

http://www.diystompboxes.com/smfforum/index.php?topic=100148.0

RG's post at the bottom of page one is worth reading...

Andrew.

[simonrichie]

Thanks. Very informative, though I don't think I understand certain things.

(Please bear with me. I'm new but learning fast.)

He says the input jack should be the only thing touching the enclosure, but isn't it the case that the output jack, pots, and switches are touching the enclosure as well, metal-to-metal, in most pedals?

And about the input and output jacks not being connected at the sleeves . . . that goes against all the diagrams I have seen, from TagBoardEffects, Beavis, etc., regarding off-board wiring.

Ditto the stereo jack "input-power switching trick."

I know he's talking about what's theoretically the best way to do things, but . . . man, I think he just added to my confusion.

In regard to this pedal, the hum drops considerably once I back off the "Wool" a hair or two, and almost completely once I get "Wool" halfway down.

Anyone have any ideas what could be the issue here?

Also the "Pinch" knob has little effect. I read the Z.Vex description of what it should do, but what mine seems to be doing is compressing the signal and lowering the volume.

-SR

[GibsonGM]

It's not always necessary to have only the input jack grounded (MUCH more important on amplifiers and the like), but it's good practice to only have the one.   You can get insulated jacks for the output rather than the ones we usually see, they have a plastic body.

Yup, the article is all about BEST practice, and we often get away with much less than that level of perfection    But it's well worth knowing where you're 'cheating'...in a high-gain circuit, this can make a LOT of difference...and in many of the pedals we build, it's ok to skimp a little.

What are you using to power your pedal?  A battery will give the cleanest power for testing, so you can see if the problem is inside or with the power supply...many power supplies are very noisy and need better filtering before audio use.  If that is what you're using, then yes, you have a problem, ha ha.

Your description kinda leads me to think that maybe you don't have something grounded that should be, and that you're just amplifying the hum already there at low level by cranking the "wool" knob.    Just a theory....you could try an audio probe to see if there's a starting point for the noise, and that might get you near to where the problem is.

http://www.diystompboxes.com/pedals/debug.html

[PRR]

> He says the input jack should be the only thing touching the enclosure, but isn't it the case that the output jack, pots, and switches are touching the enclosure as well, metal-to-metal, in most pedals?

The pot-shells go to case, but the pot-guts do not. This is fine.

In a large system it may be best to have ONE point going to case. In little 3x5 pedals we can usually nail all the jacks solid to the case.

> It hums like crazy.

Something is not grounded at all.

-or-

Power supply is nasty.

[simonrichie]

Using a One-Spot. I tried alligator-clipping the chassis and then touching all ground spots. Hum didn't quit. Still working on it.

One more thing: all those "Show your build" photos on the D*A*M forum--lots of folks don't connect the ground to the output jack. What's up with that? Necessary? Unnecessary? Does it matter? Brad from Creepy Fingers and John from Basic Audio (if I'm remembering correctly) are who I'm talking about. Is it a battery/non-battery thing? A negative ground thing?

[GibsonGM]

It's a thing to simply reduce the chance of having a....ground loop, LOL.    The irony of this stuff is...if the contact from jack to box (where it IS grounded) ever fails, you will have UNGODLY noise, worse than what you're reporting.

I choose to ground in AND output jacks, just for this reason.  I never rely solely on the case; had too many occurrences that weren't to my liking that way.   

Gotta ask...the One Spot doesn't hum when used with other pedals, right?  And you know your cords are good? 

I'd audio probe and try to see where this bug starts, if you can't find anything obvious like a broken jack wire.  If everything that is supposed to be grounded IS, you shouldn't have hum.  Not even from a ground loop.   Sounds like an assembly error....we all do it at times!

[davent]

On John's builds at least, he uses lock washers, inner toothed lockwashes on his jacks which if the the jack is tightened down sufficiently dig into, imbed into the enclosure and jack giving an airless electrical connection. The output jack is connected to ground through the enclosure by means of the ground connection of the input jack.
dave

[GibsonGM]

Not to say it "doesn't" work or "couldn't" work, just a preference      After all, the main ground in every amp you build IS connected to chassis exactly how you describe it, Dave.    It's the movement of the jack that bugs me...works loose one way or another.

[davent]

Hi Mike, I was just trying to clarrify the 'What's up with those pedals?' for Simon. For amps i isolated the jacks and run seperate ground wires from them to the amp's grounding bus or star.

I'm comfortable locked away in my room with using the single ground point in a pedal... that is until mayhem ensues.

Take care!
dave

[GibsonGM]

Hi Mike, I was just trying to clarrify the 'What's up with those pedals?' for Simon. For amps i isolated the jacks and run seperate ground wires from them to the amp's grounding bus or star.

I'm comfortable locked away in my room with using the single ground point in a pedal... that is until mayhem ensues.

Take care!
dave

     I hear ya!    Everyone should hear about the toothed washers; those are the key to that kind of construction.    I used to use normal flat ones, and would be surprised to hear that lovely "BLAAAAAAAAAAAAAA" coming from my amp when someone in the crowd stepped on my jack while dancing around, ha ha. 

It's ALL good, when done right.