Another wiring thread

[IsildursBane]

I mix at a church every now and then, and a few weeks ago, during a weekend arts festival (that I didn't attend) they had an issue w/ a ground hum when plugging the computer's audio line output into the console. Somebody in the audience later came up to the program director and claimed to know how to resolve the problem. This is the e-mail he sent (I'm mainly interested in the bolded parts):

 

 

Ground loops are most frequently the cause of the hum that is all too common in many sound systems, but not always. The first step in troubleshooting the source of the hum in a system is to disconnect all elements in the system, and to verify that the power amp connected to the speakers alone does not have a problem with excessive output ripple. This is done by disconnecting the inputs to the power amp and powering it up. If hum is still audible, it is most likely due to electrolytic capacitors in the power amp that have leaked electrolyte, lowering their capacity and increasing the ripple on the amplifiers power supply. If this seems to be the case, it can be verified by borrowing a home stereo receiver or amplifier and substituting it into the church system to see if the hum has been eliminated. The power rating on this setup may have inadequate power headroom for full reinforcement amplification, but it at least verifies the diagnosis. More likely, the hum will only recur when some component(s) in the sound system are reconnected. Occasionally, this type of ground loop interaction can be substantially reduced by reversing the polarity of the power plug on the hum inducing component. More likely, an isolation transformer will be needed.

 

 

To the first bolded part: I don't know the first thing about amp design. Can leaky caps cause this sort of noise?

 

To the second: I don't know much about wiring or electrical design (& the related safety features), but this strikes me as a really bad idea. Could one of you more knowledgeable electrician types educate me as to whether or not this truly is dangerous, why it is (not), and what could potentially happen?

 

For a number of reasons, we will not be taking him up on his offer of free corrective services. My response to the e-mail was an emphatic !NO! but, while I was able to convince the folks in charge to not allow it (they were skeptical to begin with), I'm not really confident in my own explanation & understanding.

 

-Dan.

[W. M. Hellinger]

Ground loops are simple (but can be difficult to fully eliminate if the system isn't engineered from the ground up). If there is one clear path to ground, and only one clear path to ground from any device, then there cannot be a gound loop.

 

Simply connecting a mixer to a crossover with a regular mic line will generally produce a ground loop.

[IsildursBane]

 

Ground loops are simple (but can be difficult to fully eliminate if the system isn't engineered from the ground up). If there is one clear path to ground, and only one clear path to ground from any device, then there cannot be a gound loop. Simply connecting a mixer to a crossover with a regular mic line will generally produce a ground loop.

 

 

I just told them to run the computer into a DI box first, then into the mixer. Problem solved.

 

I'm more interested to know what could happen if we swap the hot & neutral on the cordset.

 

-Dan.

[Craig Vecchione]

 

I just told them to run the computer into a DI box first, then into the mixer. Problem solved. I'm more interested to know what could happen if we swap the hot & neutral on the cordset. -Dan.

 

 

An isolation transformer would be a better solution, as the transformer is a 1:1 ratio, instead of a hi-Z to lo-Z as in a DI.

 

DO NOT swap the hot & neutral in the cordset. Grounded cordsets have polarized contacts (they can't be flipped, obviously) and ungrounded cordsets would also have polarity blades. If the amp has a polarity switch on the backplate, then it's obviously okay to use it. But don't swap the wires in a cordset. Someone servicing that unit will be expecting the wiring to conform to standard and you don't want to give them an unwelcome surprise.

[caseyjf]

 

Simply connecting a mixer to a crossover with a regular mic line will generally produce a ground loop.

 

 

Why is that?

[Keyrick]

 

Why is that?

 

 

Because, depending on the connector, the cable, and how the cable is terminated on the connector, a second path to ground can be created. Not all ground loops will cause hum.

 

Rick