Your shielded cavity has ground loops

[stevewf]

I'm imagining a hollow globe-shaped object, made of non-conductive material, maybe a sphere like mom & dad's Earth globe back in the 'burbs home I grew up in.

Now I take a thin -- infinitely thin -- wire and lay it in a circle around, say, the equator. Let's say that represents a ground wire in my guitar. It's a ground loop.

Now I take another wire and lay it in a different circle on the surface of that globe. Another ground loop.

I add enough wires so that the entire surface of the globe is covered with ground loops (an infinite number of them, as it would require an infinite number of wires to accomplish this).

My guitar cavity not only introduces ground loops, but it's made of them.

People warn against creating ground loops. Heck, even recently, I myself advised the use of a star-shaped grounding pattern in a person's guitar. But why caution against ground loops when we're standing in the middle of one? Are ground loops in passive guitars a real concern? Do the same rules apply to passive and active guitar wiring?

[newey]

The concern for ground loops is the potential to introduce noise into our guitar signal. A ground loop that is contained within your shielding is not in the signal path, and hence cannot induce noise into your signal.

Even ground loops that are in the signal path are of minimal concern in a guitar cavity as they are typically too short of a loop to induce any significant noise. But in your example, the loops are completely irrelevant.

[sumgai]

Steve, even in my fevered state of reduced mental capacity, I really have to question just where you came up with your hypothesis about how ground loops are constructed.  Did you read this theory somewhere, and if so where (links, please), or did you dream it up all on your own?

newey gives you the barest clue as to what a ground loop entails, but I suspect that you'd benefit from a more detailed (and nuanced) discussion on this topic.



sumgai

p.s.  Gawd, I hope I'm not being trolled!

[JohnH]

We used to worry about such loops here on GN and GN2.

But the usual ground-loop issue between units of audio kit is related to equipment with different grounds, which have very slight voltage differences between them due to stray currents, that then get amplified. They are usually fixed by lifting one ground, or using isolatated/balanced connections.

A guitar has a single ground and there can't be a ground loop.

But the shielding does act as a cage (a Faraday cage? ) around the pickup electronics. I had a shielded cavity, and a shielded pick guard. Due to the wiring, I could have the pg connected to the amp as I assembled them together. While separated, there was a slight, normal amount of hum. But as the cavity and pg shielding touched and connected, there was a crackle, and then much quieter as the cage was completed!

[stevewf]

Apr 5, 2024 13:33:30 GMT -5 sumgai said:

Steve, even in my fevered state of reduced mental capacity, I really have to question just where you came up with your hypothesis about how ground loops are constructed.  Did you read this theory somewhere, and if so where (links, please), or did you dream it up all on your own?

newey gives you the barest clue as to what a ground loop entails, but I suspect that you'd benefit from a more detailed (and nuanced) discussion on this topic.



sumgai

p.s.  Gawd, I hope I'm not being trolled!

I was extrapolating from places like Fralin's website or myriad others that warn against ground loops inside the guitar. JohnH points out (correctly, I think), that ground loops really only happen across multiple mains, but there are others who think it can happen inside the guitar, and that it can be fixed.

So if my definition is flawed because it's not about multiple mains, then that's why.

If there's a flaw in the extrapolating from one such Fralin-esque ground loop to an infinite number of them and equating that with a cavity shield, I don't see it. 

[newey]

If there's a flaw in the extrapolating from one such Fralin-esque ground loop to an infinite number of them and equating that with a cavity shield, I don't see it.

Since it's all just one big interconnected shield, it's not an infinite number of loops, it's just one big one. Fralin isn't be the first manufacturer to go astray on this subject, and they probably won't be the last. In the majority of electrical wiring, such as inside your amplifier, or in any stereo, radio, TV, etc., ground loops do matter. But not in your guitar cavity.

[sumgai]

Steve,


One of the few things I have left in my noggin is a well-battered sense of being a classically trained Electrical Engineer. To that end, please forgive me if I sound defensive, that's not my intent here. I've probably forgotten more than I can imagine that I ever knew in the first place, I started so long ago.... about 1957 or '58, the memory ain't quite what it used to be.

Enough of that. What matters here is why I'm about to make a bald statement - Fralin, and for all I can tell, nearly all other pickup makers have never seen the inside of an Engineering School. In short, they're very good hobbyist-grade tinkerers. Chris Kinman is one of the few exceptions I can think off the top of my head. This easily explains newey 's correct assertion that he (Lindy Fralin) isn't the first one to get it wrong, that in fact he's just repeating "legend and lore" as if it were fact, without so much as a nod at a sustainable reference to back up his assertions.

I used to rail against this kind of thing, thinking that I was gonna set the world straight, but for all my barking and whining, nothing ever improved. It took me more than 60 years to learn that lesson, you can make of it what you will, free of charge.


So, to the jousting yard.

What is a ground loop?

Good question. Long and short, it's defined as more than one pathway for current to travel, with the specific destination of returning to the current's source, aka 'ground'. We'll some specific examples in short while.

So what kinds of ground loops play a part in our music production (and reproduction)? Within a guitar, there's no significant current flow across any of the wiring, so any loops are going be undetectably small.  But outside of the axe?  As intimated by newey , we need to see significant current flow in order to observe ground loops. This can be seen at the 9v battery stage of pedals, etc. but it's more commonplace among items that plug into wall sockets... there's where can get some real power flowing!  Let's start here.

Imagine plugging in and powering up a multi-effects pedal, perhaps a Boss ME-80.  Now we want to do some serious looping, so we'll plug in and power up a Boss RC-300.  Cool. Now, imagine that somehow, when you turn them both on, they are silent.  Still cool.  But when you feed the -80 into the -300, you get a hum, even before you plug in  your guitar - that's a ground loop, working hard to annoy the crap out of you!.  The best way (meaning the safest way) to eliminate that hum is "lift the ground" between the two units.  There are lots of gizmos on the market to do this, but you can "cheat" by simply cutting the ground wire between the two units. What ground wire, you ask?  Why, the shielding surrounding the inner core on your guitar cable, the one doing duty as a signal carrier between Unit A and Unit B.  Trimming the cable's shielding back from the plug on one end will break that ground loop circuit, but there is a caveat - don't strip back any more shielding than you can help, because it's still doing the job of protecting the inner core from general noise.  The simple fact is, it only needs to be grounded at one end, not both.

So what's up with that, cutting one end's shielding cuts out the hum?  Because back at the power supply cords, the two units are connected to the same ground point at the wall socket.  This is a path that is supposed to carry the power ground, but as luck would have it, it also acts to carry the signal ground. And BINGO!, there're our multiple paths I spoke of earlier - one via the guitar cord between the two units, and one via the power cords.  Cut one or the other, and the ground loop goes away, taking with it the bothersome hum.

I hope it's obvious that cutting a device's power cord ground is an unwise decision, right? Better to spend a few buck on a gizmo made to do that job, if you can't bear to cut up your guitar cables.

Any questions?

HTH





sumgai

[stevewf]

Thanks for the time taken to explain, sumgai. Much appreciated, and you find me convinced.

Actually, maybe that original post was a bit of trolling, though not meant to trap anyone here. I'm among those who are not concerned with ground loops inside a passive guitar (I know even less about actives, so I'll steer clear for now). That OP was meant either to lead to the debunking of the myth by way of devil's advocate... or meant to educate me otherwise. For me, GN2 is a good place for learning and debunking.

That big industry names like Fralin, who, regardless of skill at making pickups, propagates the myth, is a sign of what the crusader is up against. So I don't blame anyone for giving up the battle cry. I think I'm about to do that, also, as yes, it's like cursing the wind.

On a tangent , I've used a shielded jack cable and lifted one end of its shielding (well, not really lifted, but clipped short and shrink wrapped without connecting it). Not for fear of any ground loop, just because it's easier to install!

[sumgai]

Apr 7, 2024 14:51:33 GMT -5 stevewf said:

On a tangent , I've used a shielded jack cable and lifted one end of its shielding (well, not really lifted, but clipped short and shrink wrapped without connecting it). Not for fear of any ground loop, just because it's easier to install!

That's a valid way to go. The only time one can't do this is when plugging his/her guitar into the first "box" outside of said guitar - there you need to have both conductors intact for, again, obvious reasons.

If all of the pedals in your signal chain are battery powered, then the chances of a ground loop are much lower. Might happen, should one pedal perhaps have a positive ground circuit instead of the more common negative ground style of wiring, but even there, it's most likely that any noise will be small. But if noise does crop up, try the clipped shield technique first, before resorting to cash-eating gizmos.

HTH

Oh, and you're welcome!






sumgai