Grounding Issue!

  • Thread starter Thread starter Farno
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Farno

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Hey,

I'm getting this hum with the Lexicon Alpha ASIO. It's a USB sound interface that is basically a outboard sound card for recording. However, I have always been getting this hum when recording my guitar.

I did some testing and determined what it is. The problem is, I don't know what to do!

Ok so here is what's going on:

I have a Line 6 Spider Jam Amp that I am using the headphone jack to go out. Then with a long instrument cable, I'm going into the ASIO Interface. From that, it's going into the computer via USB. My speakers are also coming out of the sound interface.


I tested to see if my amp was giving off the sound directly by using some headphones. It wasn't which is good!

I tested my cable going into the amp from my guitar. All good there (nothing unusual).

I tested the cable from my amp to the ASIO Interface. Nothing wrong,

Lastly, I tested the ASIO Interface with nothing plugged into it. No humming at all so the speakers are fine and the device has no problems with it's output.


I noticed that the humming is the same sound as when you don't hold the guitar strings on your guitar (hence your guitar is not grounded). This is how I determined it was a grounding issue.

So, you know all the things that do work and the problem I'm having. I want to stop this humming.

Hopefully you can help and thanks in advance!
 
One thing that is easy to try that sometimes works......Try plugging your amp/interface etc...all into the same power strip or outlets. Basically you are trying to get all your gear on the same A/C circuit. Occasionally if you plug into separate circuits, you can introduce noise.
 
Farno said:
I have a Line 6 Spider Jam Amp that I am using the headphone jack to go out. Then with a long instrument cable, I'm going into the ASIO Interface. From that, it's going into the computer via USB. My speakers are also coming out of the sound interface.
Click to expand...

So you are running a mono cord from a stereo headphone jack into a balanced 1/4 input on your interface? You also have a ground loop created by 2 different paths to ground [computer+amp]. You need to lift the ground between the amp and the interface. A direct box with a ground lift switch would be just the ticket.
 
Both the Amp and comouter are plugged into the same circuit. For that direct box with a lift switch, what would be good? I want to be able to record a bass to (if it makes a difference).
 
Farno said:
Both the Amp and comouter are plugged into the same circuit.
Click to expand...

Both of them use a 3 prong power cord which means they each have their own path to ground. When connected together via the interface you have then created a ground loop.
 
http://www.maplin.co.uk/Module.aspx?ModuleNo=33172

This or similar will break the earth loop. They are available on the net, prob' RadShak, prob' cheaper too!

DO NOT! Under any circumstances lift mains earths. I know you only have the 115V pussy stuff as against our proper230V, but it will still kill ya!

Dave.
 
Yeah actually the amp was plugged into a different outlet. Still humming.

That ground loop insulator thing, how am I supposed to use it? I'm using a 1/4 instrument cable!

But yeah, I think that's the right idea.
 
Farno said:
Yeah actually the amp was plugged into a different outlet. Still humming.
Click to expand...

It doesn't make a difference whether the units are plugged into the same or different outlets. You still have 2 paths to ground which results in a ground loop.


"Direct (DI) boxes as ground loop problem solvers

Direct boxes are devices that convert unbalanced instrument-level (or line-level) outputs into balanced, microphone-level inputs. They are used heavily in large, arena-size sound reinforcement systems where the keyboards, etc., are located far away from the mixer. Impedance differences between the keyboard and mixer prohibit one from running cables directly out of the bass amp of keyboard and to the mixer.

To use the Direct Box, one takes the guitar amp's output (it usually has one on the back), and, using a 1/4" patch cable, plug the other end into the direct box input. Make sure that the switch (if it has one) labeled "Speaker/Instrument" is on "Instrument." Plug a normal XLR cable into the output end and the other into your cabling which goes to the mixer.

If the system is grounded at bothe ends and ground potentials start flowing, you'll get a ground loop. Ground loops can be identified by a low hum (60Hz in the US and 50 Hz in Europe) through the sound system. First place to check: direct box. Direct box usually has a switch labeled "Ground Lift" for solving this type of problem. The ground lift switch will lift the ground (safely) and the hum should stop. If the switch isn't there, use a special ground lift cable on the XLR cable. Do not, by any means, disconnect the grounding pin of the AC cord."
 
Ok. I'll do it that way. My amp has an output like that in the back. How much should all the cable's cost? Well, I'll check it out and post what I think I need. (It's 100% possible that I choose the wrong stuff).
 
I would like if I may to clarify some terms as to what is or should properly be called a "DI" box.

In its simplest and most commom form it consists of a TS jack socket connected thru' a transformer to an XLR. The transformer will have a stepdown ratio of 8:1 to perhaps 12:1 and reduce the signal voltage in the same proportion. The impedance of the guitars electronics (for this is the major use of such a box) will be reduced by a factor of the square of the turns ratio, thus, as has been said a low level (mic) low impedance balanced signal can be fed to a remote mic pre amp.

The device in my link is NOT such a device. The transformers are 1:1 and perform no voltage or impedance transformation. Another, better but more costly unit is the ART Cleanbox 2. What both of these do do is break the shield path from one piece of gear to the next whilst preserving the wanted signal.

Both the Maplin and the ART boxes "balance" the signal but that is not of much importance since they usually feed unbalanced inputs as the Maplin devices' connectors make plain.

There are other variations of things called "DI" boxes but it should be borne in mind what the specific function of each actually is.

With regard to balanced operation. Earth currents in earthed gear connected together (again!) by a shield are unavoidable but IF the input is correctly designed the hum current will be "rejected" as indeed will all other interfering signals.

Where many poorly designed balanced inputs fall down is because of poor common mode rejection*. This can be mitigated by breaking shields but it is not good practice (from an RFI point of view) and in fact contrary to EC directives over here.

Final note!: The term "isolator" has led to some confusion in that these devices offer some form of mains level safety protection. THEY DO NOT! There are signal transformers made to do this but they are not found in any DI boxes AFAIK. In any event audio connectors are not mains rated.

If the kit goes live one side of the box, chances are so will you on the other!

Dave * See Jensen Transformers and Rane electronics papers.
 
Ok, thanks for clearing up some stuff!

The Clean Box 2 looks good.
 
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