Ground Loop Issue With USB Mixer

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tdrusk

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I have an Alesis Multimix 8 USB. It goes into my computer with USB. My current setup is:

Condenser microphone > Alesis Multimix 8 USB > Computer

I get a hum that I am 99% sure is due to a ground loop issue. I noticed that the problem occurs when I have the firewire plugged in. I tried to switch cables, and while a better cable did help, it did not fix the problem.

I have been fretting about this all day. I saw the Ebtech Hum X Voltage Hum Filter, which may look like what I need. I am just not sure if it will fix the problem or not. Note, my Alesis Multimix 8 USB does not have a ground prong on the power adapter.

Do you guys think that it would fix the problem? Thanks in advance.

If the USB cable is the issue, I am going to buy the Monster Cable Performance Digital USB 2.0 Audio Cable.
 
If you're sure it's a ground loop issue, you're probably right.

I'm almost 100% sure that neither of the products you linked will solve the problem.

Martin Walker did a column on this in Sound On Sound suggesting a DI box as a guaranteed solution. The problem is that a search on the SOS web site failed to yield a link and I can't quite remember on which cable to use the DI, but it would almost have to be the main output from your interface to the mixer.



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tdrusk said:
I have an Alesis Multimix 8 USB. It goes into my computer with USB. My current setup is:

Condenser microphone > Alesis Multimix 8 USB > Computer

I get a hum that I am 99% sure is due to a ground loop issue. I noticed that the problem occurs when I have the firewire plugged in. I tried to switch cables, and while a better cable did help, it did not fix the problem.

I have been fretting about this all day. I saw the Ebtech Hum X Voltage Hum Filter, which may look like what I need. I am just not sure if it will fix the problem or not. Note, my Alesis Multimix 8 USB does not have a ground prong on the power adapter.

Do you guys think that it would fix the problem? Thanks in advance.

If the USB cable is the issue, I am going to buy the Monster Cable Performance Digital USB 2.0 Audio Cable.
Click to expand...


I'm not sure if the hum that you were referring to is the same with what I have encountered with my phonic firewire mixer. I believe it is not the hum from the mixer's power supply as elctronic companies now a days use great filters for power supplies... Basically the hums that I was getting were from the electric guitars and other inputs perhaps including mic. I was able to get rid of it when I connected the body of the mixer to the ground( water pipe line ) no need for the ground prong I simply attached it to one of the screws of my mixer and that solved my hum problem. You might want to try it.
 
The loop is occuring through the grounding of the PC and the Multimix, and the ground conductor of the USB cable.

Try plugging the two things into the same wall outlet / extension lead.

You can also try cutting the ground on one of the devices, and in theory it should still be grounded through the other piece of equipment (over the USB cable). This isn't wholly recommended though.

If that doesn't sort it, one of those Hum-X devices attached to either the PC or the Multimix should work.

Oh, and don't get sucked in by the Monster Cabling marketing. Its all a load of BS :)
 
Thanks guys!

mattr said:
The loop is occuring through the grounding of the PC and the Multimix, and the ground conductor of the USB cable.

Try plugging the two things into the same wall outlet / extension lead.

You can also try cutting the ground on one of the devices, and in theory it should still be grounded through the other piece of equipment (over the USB cable). This isn't wholly recommended though.

If that doesn't sort it, one of those Hum-X devices attached to either the PC or the Multimix should work.

Oh, and don't get sucked in by the Monster Cabling marketing. Its all a load of BS :)
Click to expand...

I started thinking, "Maybe since my mixer is meant for this, there may be another problem." Since the mixer does not have a grounding pin, connects to the computer, and is meant for this, the computer seems to be the culprit. I am going to run a 2 prong extension chord(that eliminates the ground pin) from my computer to the wall. If that fixes it, the Hum-X should fix it. Right?
 
If you don't have two pieces of gear connected to building ground, it's not a ground loop. Period. Your mixer uses a wall wart; that is effectively an isolation transformer, and its case has to be double insulated so no safety ground connection is required. Your computer's chassis is connected to safety ground--do NOT defeat that connection.

Hum can be caused by an antenna picking up 60Hz noise being broadcast by your house's wiring. The question is, what is the antenna? Ground loops can be that antenna, but there has to be a loop! Another example of an antenna is a guitar pickup. People say all the time that a guitar plugged into an amp that hums is because of a ground loop. That's not possible, because there is only one connection to the wall--the amp. No, it's because a guitar pickup makes a pretty good antenna, especially a single-coil.

OK, back to your mixer. My question is how is the mixer's chassis grounded? It should be grounded to USB ground at a single point, which hopefully is also its power ground reference (the wall wart gives it floating AC, which it would need to rectify and reference to some ground point). If there is some problem there, then the mixer isn't properly shielded, and the signal traces inside the mixer could be the antenna that picks up the hum.

Or the mixer could simply have poor power filtration. Remember that the power coming at the mixer is 60Hz AC, it needs to filter that quite well to eliminate any hint of hum or buzz in its DC supply.

Finally, the condenser mic itself could be poorly shielded, or have some defect in its manufacture (poor pin 1 to chassis connection, etc.) that causes the mic to be the source of the hum.

I think more troubleshooting is called for before you buy anything else. Do all mics hum? Try a dynamic mic, they are normally very very resistant to hum. Does the hum happen with all mic cables? All inputs to the mixer? How about when the mixer isn't plugged into the computer? How about the mixer plugged into the computer, but nothing connected/volume all the way off? And so forth. Try to isolate the conditions of the hum, that will tell you the source of the problem.
 
Try unplugging the laptop and running it off battery power.....
 
I "fixed" it.

After reading threads about this problem I decided to try the simple solution and just run my mixer into my Line 6 Toneport UX. So now my setup runs

Guitar/mic/etc input >> Out through Phones jack of mixer >> Line 6 Toneport UX input >> into the computer. The audio returns from the Toneport.

It is now crisp and clear like it should be.

Thanks for the tips guys. I have a feeling it was a grounding issue because many people in that thread were grounding it to their computer and it helped, but this route keeps it simple and accomplishes what I need.
 
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