USB ground silences guitar noise

Dave Weir

Gold Supporting Member
Messages
1,388
I just wanted to make a quick recording. USB out of a Fender Mustang Mini to Audacity on my lap top. About as simple as it gets.
Humbucker guitar and I'm sitting directly in front of the lap top. Amp is 6" away. There is a fair amount of typical noise when hands are off the strings. I can deal with it, but I don't like it. As soon as I plug the USB into the computer, dead quite, even hands off. I pretty much have to lay the guitar on the keyboard to hear anything.
Unplug the USB, noise returns. Unplug the power cord from the Amp, so it's running on batteries, dead quite again.
So when the amp and guitar are not grounded at all, no noise, even thought the guitar is a foot away from the monitor. So where does this leave the notion of the ground draining off the RF noise?
Oh yeah. With the amp on batteries and USB plugged in, the noise is back.
The amps transformer is 12V, and the computer's is I think 19V or something strange. I'm not sure that either device is truly grounded. Maybe the USB grounds them together and this silences the RF?
Anybody know what's going on?
 

Dave Weir

Gold Supporting Member
Messages
1,388
The amp seems to be properly grounded. It has a 3 prong plug and then a few feet of wire then the transformer. The lap top only has a two prong plug and isn't grounded. Apparently the USB shell at both ends grounds the lap top to the amp and earth.
This knocks out pretty much all guitar noise, even on a mid 50's Epiphone single coil with no shielding and no string ground.
It doesn't need to go to a USB on the amp. Just a ground. You can even buy this thing:

USB Ground Cord (Cat. #A295-USB) ........... $8.95


The USB shell is connected to the ground prong, the other two are dummies.

Anyway, this did more to quiet my guitars than all the shielding I've ever installed combined. Makes me wonder if there is a way to properly ground some of the other junk around the house.
 

John Coloccia

Cold Supporting Member
Messages
9,584
I'm going to guess that the Fender Mustang Mini uses a transformer and thus is isolated. I'm also going to guess that you do not have a fully isolated laptop power supply, so when you plug in the USB the amp suddenly becomes ground referenced. I'll also guess that however it is configured, when you unplug the amp's power cord, for whatever reason it deliberately floats the amp's reference. Why? I don't know, but I have a feeling if you look at the power cord input jack, you might find two switches in there...one to disconnect the battery (pretty normal) and another to tie all the references together. My final guess is that when you're on battery, and the USB is plugged in, because they lift the amp's reference somewhere in there you end up with a ground loop, making all sorts of noise.

These are all just WAGs just based on what you're describing. I really don't know. Complete, wild tuckused guesses at 4:00 in the morning so I could be completely, 100% wrong. It's all pretty easy to test, though. :)
 
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Dave Weir

Gold Supporting Member
Messages
1,388
When plugged in, the amp seems to be at least partly grounded. I get very low resistance between the shell of the guitar cord and the ground of my power strip.
It's the Computer that seems to have no ground. Grounding the computer with the USB cable to the Mustang Mini or to any other ground point has the same effect: It eliminates all or nearly all of the guitar noise.
It's starting to make sense. I just didn't realize a computer would not be grounded, or that grounding it would knock down most of the EMI coming off the computer and hitting my guitar.

There are no external switches on the amp by the power cord. There seems to be something internal. If you unplug it with batteries in it, doesn't keep working. You have to turn it off and back on.

I don't think it's a ground loop when the amp and computer are connected and the amp on batteries. More like there is no ground.

I'm wondering if anyone else has noticed that grounding their computer has significantly knocked down guitar noise. Is this a well know phenomenon, and I'm just in the dark?
 
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Killed_by_Death

Senior Member
Messages
16,842
Hmm, I was having a problem with noise when using a Y cable to split my signal between my amp & my computer. That little do-hicky might have saved me $50, because I went out and bought a ABY switch with an isolation transformer to quiet the noise.
 

John Coloccia

Cold Supporting Member
Messages
9,584
When plugged in, the amp seems to be at least partly grounded. I get very low resistance between the shell of the guitar cord and the ground of my power strip.
It's the Computer that seems to have no ground. Grounding the computer with the USB cable to the Mustang Mini or to any other ground point has the same effect: It eliminates all or nearly all of the guitar noise.
It's starting to make sense. I just didn't realize a computer would not be grounded, or that grounding it would knock down most of the EMI coming off the computer and hitting my guitar.

There are no external switches on the amp by the power cord. There seems to be something internal. If you unplug it with batteries in it, doesn't keep working. You have to turn it off and back on.

I don't think it's a ground loop when the amp and computer are connected and the amp on batteries. More like there is no ground.

I'm wondering if anyone else has noticed that grounding their computer has significantly knocked down guitar noise. Is this a well know phenomenon, and I'm just in the dark?
I wouldn't have guessed that...isolated computer and non-isolated amp power supply. I assumed the opposite! But yeah, it's all about the grounds. :)

The switches I was talking about are internal to the power jack on the amp. Sorry I didn't make that clear. When the power cord is plugged in, it's standard to switch the batteries out. Your pedals will do the same thing.
 




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