elenore19
Slowing becoming un-noob.
Wow, fantastic! I'll definitely have a look. My guitar only has one volume and one tone. So only 2 pots. But it still should be similar, right? Yeah. Alright. I'll hopefully figure this out.
Thanks man!
Thanks man!
Gear_Junky
New member
i believe that when you have shielding, you should NOT connect backs of pots with ground wires. they should be grounded by the shielding (just by virtue of touching it) and they are. these wires are ground loops now. may or may not be a problem (noise-wise) but certainly unnecessary. It's all described in the articles on shielding. If your shielding material (foil or paint) is not itself grounded (by soldering a wire to the foil or to a screw/washer, which is driven into the wood so as to touch the shield) then it does no shielding.
Gear_Junky
New member
yeah, i know what you mean. they also often put foil on cavity covers, but that foil is not connected to anything. a bridge to nowhere.
oh, but it is the right way - any metal part of the pot (except lugs) is ground. That's why you don't need those black (in your case blue and red) wires that connect pots and then go to ground. I think.
See here:
http://www.guitarnuts.com/wiring/shielding/shield3.php
It's for a strat, but logic can easily be applied to any other guitar. I don't own strats and I was able to use it no prob.
Here's a pic he uses:
the grey background represents the shield, which is a conductor, it grounds everything that touches it.
That article is VERY well written and illustrated and gets results. I've done this to a friend's strat, a bass and my own les-paul type guitar. Worth a read.
Light
New member
In theory, that's not a bad idea, but in practice it really doesn't make a difference. I've never seen a ground loop in a guitar that caused any problems.
Light
"Cowards can never be moral."
M.K. Gandhi
W
Whale Bone
New member
I've wondered some about this potential problem (which I don't bother to eliminate in my basses), but if I were to address it, I would instead leave the sturdy and robust connection of the pot casing ground wires and insulate the pots from the shielding with nylon washers. I'd use a star ground or depend on the output jack for the connection to the shielding.
Gear_Junky
New member
good points, guys, you got me rethinking the whole thing. i think i'll do just that with my next shielding project.
elenore, let us know what you figure out.
elenore, let us know what you figure out.
dgatwood
is out. Leave a message.
That would be because "ground loop" is a misnomer. Every PCB-based electronic circuit in the world contains at least one looped ground around the perimeter of every board If a loop of grounds mattered, every device in the world would hum. If somebody tells you not to connect grounds between pots in a guitar, it is a pretty sure sign that he/she doesn't understand what causes ground-related hum, and that IMHO any other shielding advice from that person is suspect....
The so-called "ground loop" is really caused by two (generally AC-powered) devices having a shielded, unbalanced signal cable between them and one of the devices having a high resistance between its signal ground and the earth ground. As long as all the devices have a low resistance to ground, the vast majority of the signal flows out through the earth ground because that has lower resistance than flowing through the audio cable shield. If a device has a high resistance to earth, then any noise that device generates or otherwise picks up takes the new least-resistance path to ground through the audio cable shield into a device with a lower ground resistance, which ends up leaking into your signal as it passes through the shield.
It should be noted that you can only break the loop by either lifting the signal ground or lifting the earth mains ground on the device with the low resistance path to ground. If you lift the earth mains ground on the device with the high resistance path to ground, it won't do any good because the shortest path is still through the audio cable.
Well, a ground "loop" can also be caused by a few other things like leakage across high impedance (unbalanced) input transformers if the ground is poor, etc., but those are the sort of issues that would only get worse if you lifted a mains ground somewhere....
The point is that it isn't the loop that causes the problem. It is the device with a high resistance between the signal ground and the mains earth ground. The worst offenders are devices with no mains ground pin at all, including devices with wall wart power supplies and devices with two-prong power cords. If you have even one device that matches one one those descriptions, that's where a star grounding setup comes in handy.
Indeed, merely creating a secondary low resistance path between the devices (such as a 10 gauge wire connected between the grounds on an audio cable) often clears up the problem sufficiently because it creates a much lower resistance path to ground that doesn't flow through the audio cable. That said, it will generally work better if you feed this ground bus directly into the third grounding prong on a power cord.
In the guitar case, the shielding is equivalent to the shielding on an audio cable. Adding grounds externally as a bypass would then make the sound imperceptibly better if the guitar were somehow producing hum... with the caveat that induction inside the guitar will be utterly dwarfed by induction between the shield and the signal conductor inside the guitar cable itself. Adding grounds internally as a bypass won't do much. About the only way it would make things worse is if you happened to run it parallel to a signal line (and closer than the shield), and even then, it is likely to be dwarfed by what's going on inside the guitar cable. And again, all that is predicated upon the guitar's shield and induction being the primary source of the hum, which it almost never is. The source is almost always the coils in the pickups, i.e. the hum is in the signal line to begin with, not in the shield. To the extent that hum is coming through the shield from the outside into the coils, however, lowering the resistance of the shield by adding grounding points can only be a good thing because it provides a better path to ground than traveling through the shield a greater distance to ground, thus reducing the amount of induction off the shielding material.
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Gear_Junky
New member
nice, i'm living and learning 
That explains why some pedals cause noise.
what about guitars with passive pickups, which don't have a power source?
generalized theory is great, but i only have 1 application for this and would like to know specifically. Thanks.
That explains why some pedals cause noise.what about guitars with passive pickups, which don't have a power source?
generalized theory is great, but i only have 1 application for this and would like to know specifically. Thanks.
mshilarious
Banned
OK, it's just about impossible for any guitar to have a ground loop, because the guitar is only connected to one device. It can happen through the player into a microphone or something like that, but I am just talking about the guitar. Pretend it's sitting on an insulated stand, just sitting there, buzzing.
So this isn't ground current, it's induced interference. This is what people don't seem to understand about noise and guitars. The reason your guitar is noisy is because there is interference floating in the air, waiting to be picked up by an antenna--like your guitar! If it were a ground loop, shielding wouldn't matter. Why do we shield guitars, anyway? Because we are trying to cause EMI to be rejected.
So why are guitars still noisy? Because the shielding is never ideal. That's true for most electronics, including microphones, but guitars are an especially bad example because of their compromised construction.
Take your average SM58 in comparison: it has a coil too, and works on a sorta similar operating principle to a passive guitar pickup. The SM58 has a fully shielded humbucking coil with a balanced output. A guitar might have a humbucker, it might not. That humbucker might be shielded; it might not. The quality of the control cavity shielding may vary. Guitars are generally not balanced output, because guitar amps aren't balanced input. The shielding on the guitar is common with signal ground, combine that with the input topology of your average guitar amp, and you're doomed! DOOMED!
In contrast, if your SM58 ever makes so much as a peep of buzz, you know right away that something has broken. And the output of the SM58 is about 30dB (or more) lower than your guitar! Which means it probably has interference rejection that is 60dB or better than your guitar.
All of this has been known for 50 years, but nobody feels like doing anything about it. So instead, we get three or four threads a week asking about guitars and ground loops . . .
Gear_Junky
New member
that's hilarious, mshilarious! DOOMED! 
yeah, i know whatcha mean - how come we still don't have XLR instrument cables. Occasionally you'll see an acoustic with an XLR output, but that's about it (cuz you don't "need" an acoustic amp).
well, that's how things be. it took way longer than it should have for computers to stop coming standard with 1.44" FDD.
I do love me my pickup covers, tho! my guitar with Dream 180's is about as quiet in SC as it is in HB.
DOOMED! yeah, i know whatcha mean - how come we still don't have XLR instrument cables. Occasionally you'll see an acoustic with an XLR output, but that's about it (cuz you don't "need" an acoustic amp).
well, that's how things be. it took way longer than it should have for computers to stop coming standard with 1.44" FDD.
I do love me my pickup covers, tho! my guitar with Dream 180's is about as quiet in SC as it is in HB.
dgatwood
is out. Leave a message.
It's worth pointing out that the giant metal grille around it serves as an EMI shield. Some people might not know that. In effect, the entire body of the mic acts like a faraday cage with no leaks until you get up into... probably the hundreds of GHz range, but I'm estimating off the top of my head and out of the back of my @$$ here.
For those folks who don't know what this means, if you have a metal box (grounded or otherwise) and it has no openings, you are reducing the amount of EMI that passes through it. What you may not know is that when you have openings (e.g. a mesh), the size of the opening determines the frequencies that can pass through the opening.
Thus, the metal pop filter on your microphone not only diffuses wind, but also serves as ancillary shielding for the capsule. On condenser mics, this becomes deafeningly obvious if you ever take off the front filter. Hummmmmmmmmmm.
Of course, there's a catch. 60 Hz hum requires a sizable gap to pass through the opening---like measured in thousands of kilometers large---but it also is very hard to shield against because the amount of metal you need to completely block such low frequency energy is very high. A single skin depth at 60 Hz in copper is more than a third of an inch. An ideal shield, therefore, would be at least that thick in copper, or about a tenth of an inch of iron.
This is why nobody builds guitars that are properly shielded. That's a lot of metal. Of course, because the calculation for gaps is in thousands of kilometers, you should be able to build a properly shielded guitar by using such a heavy body cavity shield and adding a thick, grounded metal cross brace across above the strings so that you can't see the pickups when you look straight down on the guitar. The gap that the strings pass through beneath the brace should be small enough that you would have a perfect faraday cage around your pickups. In theory. Anybody want a fun hacking project?
mshilarious
Banned
This is why nobody builds guitars that are properly shielded. That's a lot of metal. Of course, because the calculation for gaps is in thousands of kilometers, you should be able to build a properly shielded guitar by using such a heavy body cavity shield and adding a thick, grounded metal cross brace across above the strings so that you can't see the pickups when you look straight down on the guitar. The gap that the strings pass through beneath the brace should be small enough that you would have a perfect faraday cage around your pickups. In theory. Anybody want a fun hacking project?
The strings don't need to be shielded because they are part of the shield
And the pickups can be shielded too.Anyway, people think of hum as 60Hz, but it's not that fundamental frequency that's usually troubling, it's all of its harmonics in the 1kHz-4kHz range. Still, your post points out why shielding is only a partial solution--humbucking and balanced transmission are equally important if not more so. In fact, I can build a fully balanced microphone circuit that doesn't need to be shielded at all.
elenore19
Slowing becoming un-noob.
FIXED! (Pics)
Alright, so I found this cable that was coming out of the side of the "electronics area." It wasn't connected to anything.
...
So I figured..aha! There's the problem.
What is this cable? I have no idea. You guys probably know. (hopefully?)
So I started making contact with a bunch of other spots with this cable, some spots fixing it, and others making an even worse hiss. So I found a spot where it seemed like it fixed it the most. I Soldered it to that spot, and it's now fixed.
So there you have it. Thanks again for all the help guys. If you have any questions let me know.
Alright, so I found this cable that was coming out of the side of the "electronics area." It wasn't connected to anything.
...
So I figured..aha! There's the problem.
What is this cable? I have no idea. You guys probably know. (hopefully?
)So I started making contact with a bunch of other spots with this cable, some spots fixing it, and others making an even worse hiss. So I found a spot where it seemed like it fixed it the most. I Soldered it to that spot, and it's now fixed.
So there you have it. Thanks again for all the help guys. If you have any questions let me know.
dgatwood
is out. Leave a message.
Alright, so I found this cable that was coming out of the side of the "electronics area." It wasn't connected to anything.
...
So I figured..aha! There's the problem.
What is this cable? I have no idea. You guys probably know. (hopefully?
I think that wire grounds the strings/bridge.... And yeah, without that wire grounded, things will hum pretty badly.
I always thought it would be fun to create an RF active guitar amplifier. Put a different signal up in the 2.4 GHz open band on each string. Use the existing pickups as a receiver. Then use an FM receiver so the doppler effect from the string vibrating back and forth produces amplitude variation in the output. That way, your pickups can be completely unshielded and it won't matter....
Anfontan
Banned by eurt
Alright, so I found this cable that was coming out of the side of the "electronics area." It wasn't connected to anything.
...
So I figured..aha! There's the problem.
What is this cable? I have no idea. You guys probably know. (hopefully?
So I started making contact with a bunch of other spots with this cable, some spots fixing it, and others making an even worse hiss. So I found a spot where it seemed like it fixed it the most. I Soldered it to that spot, and it's now fixed.
So there you have it. Thanks again for all the help guys. If you have any questions let me know.
Congrats dude! When you can troubleshoot and fix guitar problems like you did it's a big step in musical education.
That would be a ground cable that came lose-a hiss in the circuit is a tell-tale sign.
You did just fine!