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Author Topic: 60 HZ hum, only in Backline Amps - on same power as Sound System  (Read 8225 times)

Jerome Casinger

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So I was stumped last night.  New venue, shady power to say the least, but had a dedicated distro for sound/lights.  I have been running my rig for 3 years now In the area and never had an issue. (QSC KW, X32).  Last night the Bass Amp and Guitar amp had a hardcore 60 hz buzz....flipped ground lifts and it got better but was still horrendous.  Moved them onto my power so we were running on the same circuit....no change to there amps. 

I had a slight hum in the PA, was odd, as I turned on a sub it was there, when I turned the other it disappeared.  When the gig was over (everything powered down, bar cleared out), you could hear the 60hz hum big time from an AC unit or Ice Machine or something big time, luckily the band heard it as well at that point and realized it wasn't my gear (I was a fill in sound guy, they didn't have the issue a month or so ago when they where there).

I ran through what I new how to do....could an appliance introduce "noise" somehow into the entire power grid?  Is there a way to check that?  Or is there anything I could have done.

John Roberts {JR}

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #1 on: July 04, 2015, 01:53:55 pm »

Guitar pickups sending high impedance instrument level signal around in a high magnetic hum field will generally pick up hum.

JR
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Mike Sokol

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #2 on: July 04, 2015, 02:09:42 pm »

Active or passive DI boxes? Lots of cheap active DI boxes don't have an isolated pin-1 so they can send 48-volt power across the pin-1 lift. Also, there are imported DI boxes with rather crappy transformers that don't do well at lifting grounds. If pulling out the XLR cable from the DI box stops the hum/buzz, then stick a mic in front of the offending amp and get on with the show. Sound check is no time for mental exercises. Just get it done and run...

Also, check to make sure your XLR cables don't have a pin-1 to shell bond. If they do and you're plugged into a DI with a metal XLR jack, that bond will be in parallel with the DI's ground lift and stop it from lifting properly.
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Jerome Casinger

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #3 on: July 05, 2015, 11:12:32 am »

No DI's on either.  The guitar cab was Mic'd, and the Bass amp had it built into the head but had the hum without anything plugged in.  When he would unplug the bass the hum was ridiculous.

Active or passive DI boxes? Lots of cheap active DI boxes don't have an isolated pin-1 so they can send 48-volt power across the pin-1 lift. Also, there are imported DI boxes with rather crappy transformers that don't do well at lifting grounds. If pulling out the XLR cable from the DI box stops the hum/buzz, then stick a mic in front of the offending amp and get on with the show. Sound check is no time for mental exercises. Just get it done and run...

Also, check to make sure your XLR cables don't have a pin-1 to shell bond. If they do and you're plugged into a DI with a metal XLR jack, that bond will be in parallel with the DI's ground lift and stop it from lifting properly.

Tim McCulloch

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #4 on: July 05, 2015, 11:30:25 am »

I'm suspicious of the distro or something plugged into it.

Stuff hums because something is grounded that shouldn't be, or isn't grounded that needs to be.

Could be defective back line amp, mis-wired extension cord or power drop box, or outlet on the distro.  Only takes 1 to create problems.
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Geoff Doane

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #5 on: July 05, 2015, 01:18:13 pm »

I'm suspicious of the distro or something plugged into it.


Ground and neutral swapped at some point, or just connected together, has been know to cause problems like that.

GTD
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Mike Sokol

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #6 on: July 05, 2015, 08:19:21 pm »

Ground and neutral swapped at some point, or just connected together, has been know to cause problems like that.

GTD
But that usually requires a ground-loop path, such as a DI to XLR connection, to make it hum. As mentioned previously, this sounds like magnetic coupling. I've heard this happen when the stage was built over top of a large conduit passing building power. Did you happen to notice if the hum changed when the guitar necks changed orientation on stage? When they line up with a power conduit they can hum, but when at a 90 degree angle the hum usually nulls out. Interesting problem...
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Jerome Casinger

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #7 on: July 06, 2015, 12:14:38 pm »

The GTR player said he noticed a difference in his positioning.  I should have went and investigated what it was in the back at the end of the night that was making the hum (annoying).  At that point it was 230am and my focus was elsewhere, but it was distinctly coming from the back of the bar.  Sounds like if I have to go back there I will be doing some more investigating prior.  I just didn't expect to walk into such an issue so didn't have enough time to do much more troubleshooting than switching power to different circuits.

But that usually requires a ground-loop path, such as a DI to XLR connection, to make it hum. As mentioned previously, this sounds like magnetic coupling. I've heard this happen when the stage was built over top of a large conduit passing building power. Did you happen to notice if the hum changed when the guitar necks changed orientation on stage? When they line up with a power conduit they can hum, but when at a 90 degree angle the hum usually nulls out. Interesting problem...

John Roberts {JR}

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #8 on: July 06, 2015, 01:58:52 pm »

The GTR player said he noticed a difference in his positioning.  I should have went and investigated what it was in the back at the end of the night that was making the hum (annoying).  At that point it was 230am and my focus was elsewhere, but it was distinctly coming from the back of the bar.  Sounds like if I have to go back there I will be doing some more investigating prior.  I just didn't expect to walk into such an issue so didn't have enough time to do much more troubleshooting than switching power to different circuits.
An old school neon light sign could make some noise...

If the position of the guitar player changes the noise it may not be related to conducted power noise, but the general noise field in the room. (I think I already suggested this in an earlier post).

JR
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Tim McCulloch

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #9 on: July 07, 2015, 10:15:11 am »

The GTR player said he noticed a difference in his positioning.  I should have went and investigated what it was in the back at the end of the night that was making the hum (annoying).  At that point it was 230am and my focus was elsewhere, but it was distinctly coming from the back of the bar.  Sounds like if I have to go back there I will be doing some more investigating prior.  I just didn't expect to walk into such an issue so didn't have enough time to do much more troubleshooting than switching power to different circuits.

If that's the case then I'll walk back my idea about a wiring problem or defective gear...  But with inductive interference the guitarists usually find out quickly that there is a positional aspect to it and helps with identifying the cause.
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"Practicing an art, no matter how well or badly, is a way to make your soul grow, for heaven's sake. Sing in the shower. Dance to the radio. Tell stories. Write a poem to a friend, even a lousy poem. Do it as well as you possible can. You will get an enormous reward. You will have created something."  - Kurt Vonnegut

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Re: 60 HZ hum, only in Backline Amps - on same power as Sound System
« Reply #9 on: July 07, 2015, 10:15:11 am »


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