Missing post on hum and noise

Jay Kahrs · 25th October 2002

I copied the text and emailed to question to Mike Rivers because it was a little beyond my scope. Now I can't find the original post!
Anyway, here's the original question.

I have a XLR patchbay in my control room (an extra bedroom) running via a snake through the walls to the drum room (a room in the basement). There's an XLR patchbay in the basement as well. The length of the snake is probably about 60 feet and it goes through the walls of the house.

When I take a line level audio signal and send it from upstairs to downstairs, there is a low humming noise (and higher-pitched buzz) that is being sent as well. I tried taking the line level signal and running it through a passive direct box upstairs (so I'm sending a mic level signal downstairs), but the hum/buzz is still there. I've tested these particular jacks with an ohmmeter and it appears everything runs through it just fine...

I recall reading some advice on XLR patchbays where you can wire up one of the pins on one of the sides of the patchbay together... Unfortunately, I can't find where I read that and don't remember the specifics. Any suggestions on combating hum/buzz?

Thanks!

Jay Kahrs · 25th October 2002

I think he's talking about the "one end only" or "telescoping ground" connection scheme. This is where the shield around a balanced cable pair is connected to ground (pin 1) on one end and left open on the other end. This serves to break one ground loop path, and sometimes does the trick. It worked better in the days of analog system, when there was always an audible noise floor, than it does in today's digital systems, which we expect to be dead quiet even at unreasonable system gains. It's worth a try.

A couple of questions, though. Since he mentioned a line level signal, he must be sending something down that snake that's powered from the AC line. The buzz could easily be caused by a differnce between neutral and ground potential at both ends of the cable. A good way to check this out is to run a big orange extension cord upstairs and power whatever active equipment is up there from the same receptacle as the control room. If that fixes the problem, the real solution is going to involve some electrical wiring work.

Are the inputs and outputs truly balanced on both ends? If not, then there's bound to be some hum pickup in the cable - that's the low frequency hum. A good test for this is to disconnect everything on both ends except one cable path. Connect a mic at the upstairs end and connect the downstairs end to a mic preamp. This will be balanced all the way (assuming no wiring errors in the patchbays) and should be quiet. A transformerless condenser mic is probably the best test tool for this since, if there's a strong hum field up there, a dynamic mic element or transformer could be picking up the hum just like a guitar
pickup. That's not a problem with the wiring between upstairs and
downstairs, that's just a problem with the mic in the room. Get
someone to walk around the room with the mic and see if the hum
changes.

Problems like this almost never have a single, guaranteed-to-work
solution. They have to be solved methodically, starting with the
simplest connection (a direct mic to preamp through the cable) and start building things up until the problem appears. Then you know where it is and you fix it.

--
I'm really Mike Rivers

-------
If you need Mikes email address drop me a line. MooseAudio@aol.com

Jay Kahrs · 26th October 2002

Quote:

Originally posted by posterchild
Ooops, sorry! I had deleted the post, as I had posted the same question on another forum and received a bunch of ideas for things to test for... Thanks for responding, though!!!

Well, don't do that! I thought that maybe I had deleted the post by accident or something. But, OTOH it does kind of bug me when I vist other boards and see the same question posted by the same person in 15 different places. It also gets really confusing and I don't like confusing. Anyway, hope that helps ya out a bit. Feel free to come back and ask more. If it's beyond me I usually know someone who can answer it.

Jules · 26th October 2002

Waz goin on? Have I missed the action?!!!

Jay Kahrs · 26th October 2002

Maybe, but you saw plenty of action at the barbershop so I wouldn't worry about it.
grggt

Jay Kahrs · 28th October 2002

It happens. I always try to use a cable checker for that kind of stuff. Keep in mind that if you swap pins 2 & 3 you'll swap phase and that won't show up on a cable checker. That's the one I usually pull if I'm making cables and not spending a lot of time checking the connections.

25th October 2002	#2
Jay Kahrs One with big hooves Joined: May 2002 Location: Earth, NYC metro Posts: 5,899 Thread Starter	Mikes Response I think he's talking about the "one end only" or "telescoping ground" connection scheme. This is where the shield around a balanced cable pair is connected to ground (pin 1) on one end and left open on the other end. This serves to break one ground loop path, and sometimes does the trick. It worked better in the days of analog system, when there was always an audible noise floor, than it does in today's digital systems, which we expect to be dead quiet even at unreasonable system gains. It's worth a try. A couple of questions, though. Since he mentioned a line level signal, he must be sending something down that snake that's powered from the AC line. The buzz could easily be caused by a differnce between neutral and ground potential at both ends of the cable. A good way to check this out is to run a big orange extension cord upstairs and power whatever active equipment is up there from the same receptacle as the control room. If that fixes the problem, the real solution is going to involve some electrical wiring work. Are the inputs and outputs truly balanced on both ends? If not, then there's bound to be some hum pickup in the cable - that's the low frequency hum. A good test for this is to disconnect everything on both ends except one cable path. Connect a mic at the upstairs end and connect the downstairs end to a mic preamp. This will be balanced all the way (assuming no wiring errors in the patchbays) and should be quiet. A transformerless condenser mic is probably the best test tool for this since, if there's a strong hum field up there, a dynamic mic element or transformer could be picking up the hum just like a guitar pickup. That's not a problem with the wiring between upstairs and downstairs, that's just a problem with the mic in the room. Get someone to walk around the room with the mic and see if the hum changes. Problems like this almost never have a single, guaranteed-to-work solution. They have to be solved methodically, starting with the simplest connection (a direct mic to preamp through the cable) and start building things up until the problem appears. Then you know where it is and you fix it. -- I'm really Mike Rivers ------- If you need Mikes email address drop me a line. MooseAudio@aol.com

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26th October 2002	#4
Jules Gearslutz.com admin Joined: Apr 2002 Location: A Yank in London, UK Posts: 17,808 1 Review written	Waz goin on? Have I missed the action?!!!

28th October 2002	#6
Jay Kahrs One with big hooves Joined: May 2002 Location: Earth, NYC metro Posts: 5,899 Thread Starter	It happens. I always try to use a cable checker for that kind of stuff. Keep in mind that if you swap pins 2 & 3 you'll swap phase and that won't show up on a cable checker. That's the one I usually pull if I'm making cables and not spending a lot of time checking the connections.