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Author Topic: 60hz hum on monitors but not mains?  (Read 7788 times)

Aaron Maurer

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60hz hum on monitors but not mains?
« on: June 10, 2016, 12:08:51 PM »

So earlier this week I had an outdoor show that was moved inside due to weather. Small room seats 100 very tight. I pulled power from the wall outlets stage right and left. The stage right only had mains and stage left had mains and 3 monitors. System is all powdered speakers for reference. The mains either side no issues. The monitors using the same power source as stage left mains had a terrible hum. Due to the circumstances I had no time to work things out as the show was already pushed an hour behind schedule. I am pretty sure the FOH position was on a different circuit then the mains and monitors for reference. So my question is why the buzz in the monitors and not the mains? All three monitors on different aux sends were effected but not a sound "peep" from the mains? 
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Mike Sokol

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Re: 60hz hum on monitors but not mains?
« Reply #1 on: June 10, 2016, 12:37:58 PM »

So my question is why the buzz in the monitors and not the mains? All three monitors on different aux sends were effected but not a sound "peep" from the mains?
I've done a lot of testing with this and give you a few hints. First of all, different brands of speakers will exhibit more or less hum/buzz than others, mostly due to the "pin-1" problem. That is, some monitor wedges like Mackie will hum and buzz with very little ground loop current, down around 100 mA which can be caused by as little as 1/10 of the volt ground differential voltage. While others won't have the pin-1 problem and you can throw 5 amperes of ground loop current at them and they won't hum a bit. I know this because I've set up the experiment where I inject a variable amount of ground loop current into the shield of the speaker's XLR line. Again, some speaker brands hum like crazy with any ground loop current at all (your active monitors?) while some won't make a peep (your FOH main speakers?).

This is also why I keep a pair of Ebtech hum eliminator the XLR/TRS jacks. I can eliminate all hum and buzz from the monitors 99% of the time by simply putting an audio isolation transformer in series with the XLR cable. If you want to spend more money, get the isolation transformers from Whirlwind or Jensen. But the key is to break the ground loop path with a isolation transformer. And yes, sometimes you can get away with a pin-1 lift, but I generally don't have time during gigs to troubleshoot the details.

I will note that testing with a clamp-ammeter around the outside of the XLR cable is a great way to quantify just how much ground loop current is flowing in the shield path. I've found that there's typically around 1 ampere of current per volt of EGC ground wire differential. So 500 mA is likely caused by 1/2 volts or so differential voltage between two EGC connections.   

Keith Broughton

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Re: 60hz hum on monitors but not mains?
« Reply #2 on: June 10, 2016, 12:50:46 PM »

Isolation transformers are your friend.

http://www.rapcohorizon.com/p-373-isoblox.aspx
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Jonathan Johnson

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Re: 60hz hum on monitors but not mains?
« Reply #3 on: June 10, 2016, 02:14:13 PM »

What Mike says is good advice. I'll add that lifting the safety ground on the power cord (i.e., with a "cheater plug" or breaking off the ground pin) is not an acceptable practice. Lifting the ground on the signal interconnect is generally not hazardous.

Connecting the power to a GFCI won't fix a hum problem, but it will provide a secondary layer of shock protection if the safety ground is compromised (like where someone broke off the ground pin to "fix" a hum problem).
« Last Edit: June 10, 2016, 02:21:22 PM by Jonathan Johnson »
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Aaron Maurer

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Re: 60hz hum on monitors but not mains?
« Reply #4 on: June 10, 2016, 10:40:11 PM »

Thanks for the responses. Learn something everytime I get on here.
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Mike Sokol

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Re: 60hz hum on monitors but not mains?
« Reply #5 on: June 11, 2016, 07:27:03 PM »

Thanks for the responses. Learn something everytime I get on here.

I have one of these on my analog drive rack: http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/specialty-interface-solutions/iso-8-line-level-isolator-and-balancer

I just wire up the L-R console outputs to channels 7 and 8 and the monitor sends to channels 1 thru 6 on the return snake. Then you can hook active mains and monitors to any grounded power without ground loop hum, no matter how much ground loop differential voltage there's between outlets.

On a side note, I just did an X32-Rack and S16 install at a church using a CAT-5 STP interconnect cable. But instead of bonding the shield on both ends, I lifted the shield on the RJ45/Ehtercon connector on the rack side. I have ZERO hum with this configuration, feeding a Crown amp up at the stage area. I think that's the correct way to run AES-50 between gear connected to different power outlets. Remember, I'm not talking about hum injected in the digital audio stream (that won't happen). It seems like the shield of the CAT-5 STP connection can inject ground loop current into the input of the analog amplifiers on stage. At least that's my hypothesis at this time.

Kevin Maxwell

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Re: 60hz hum on monitors but not mains?
« Reply #6 on: June 12, 2016, 12:18:26 PM »

I have one of these on my analog drive rack: http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/specialty-interface-solutions/iso-8-line-level-isolator-and-balancer

I just wire up the L-R console outputs to channels 7 and 8 and the monitor sends to channels 1 thru 6 on the return snake. Then you can hook active mains and monitors to any grounded power without ground loop hum, no matter how much ground loop differential voltage there's between outlets.

On a side note, I just did an X32-Rack and S16 install at a church using a CAT-5 STP interconnect cable. But instead of bonding the shield on both ends, I lifted the shield on the RJ45/Ehtercon connector on the rack side. I have ZERO hum with this configuration, feeding a Crown amp up at the stage area. I think that's the correct way to run AES-50 between gear connected to different power outlets. Remember, I'm not talking about hum injected in the digital audio stream (that won't happen). It seems like the shield of the CAT-5 STP connection can inject ground loop current into the input of the analog amplifiers on stage. At least that's my hypothesis at this time.

I have read on the Behringer forums or possible in the mega thread on Sound forums that the shielded Cat cable will not create a hum problem, but if you lift one end you might still have the static discharge problem. So I understand you want both ends of the shield connected to the housing of the Ethercon connectors, contrary to the common shielding practices.
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Jean-Pierre Coetzee

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Re: 60hz hum on monitors but not mains?
« Reply #7 on: June 13, 2016, 01:22:20 PM »

I have read on the Behringer forums or possible in the mega thread on Sound forums that the shielded Cat cable will not create a hum problem, but if you lift one end you might still have the static discharge problem. So I understand you want both ends of the shield connected to the housing of the Ethercon connectors, contrary to the common shielding practices.

From what I know common shielding practice for Audio systems is both sides terminated? That way the shield isn't acting as an antenna
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Kevin Graf

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Re: 60hz hum on monitors but not mains?
« Reply #8 on: June 13, 2016, 03:16:18 PM »

For balanced interconnect systems, different experts have different opinions:
a] Some say always connect the shield at both ends.
b] Some say that under some circumstances don't connect the shield at the receive end.
c] Some say to use a hybrid connection at the receive end. The hybrid is a small capacitor.
d] Some may say to never connect the shield at the receive end. (not sure if any say this)

***********************
But in any case, a shield connected to a chassis (at one end or both ends) does not act as a chassis.
Second but, an XLR connector with a pin #1 problem will act as an antenna.

Another afterthought:
This only applies to AC powered units at both ends.
Mics and battery powered units should always have their shields connected.
« Last Edit: June 13, 2016, 03:26:35 PM by Kevin Graf »
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Mike Sokol

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Re: 60hz hum on monitors but not mains?
« Reply #9 on: June 14, 2016, 09:31:32 AM »

For balanced interconnect systems, different experts have different opinions:
a] Some say always connect the shield at both ends.
b] Some say that under some circumstances don't connect the shield at the receive end.
c] Some say to use a hybrid connection at the receive end. The hybrid is a small capacitor.
d] Some may say to never connect the shield at the receive end. (not sure if any say this)

***********************
But in any case, a shield connected to a chassis (at one end or both ends) does not act as a chassis.
Second but, an XLR connector with a pin #1 problem will act as an antenna.

Another afterthought:
This only applies to AC powered units at both ends.
Mics and battery powered units should always have their shields connected.

This is the pin-1 ground lift that's provided by most all DI boxes. What I teach is that you want ONE and ONLY ONE ground (actually BOND) applied to an interconnecting XLR cable. So if you have an AC powered backline amp on stage, and you're feeding signal from it to a stagebox connected to a mixing console that's AC powered, then LIFT the ground on the DI box. However, if you have an acoustic guitar on stage plugged into a Direct Box without any AC powered stage amp, then DON'T LIFT the DI ground (pin-1). This ensures that the acoustic guitar has its internal electronics bonded to the XLR shield, which reduces RF pickup and demodulation. 

Under no circumstances should you lift the EGC (Safety) Ground pin on the AC power plug of a stage amplifier to stop ground loop hum. That opens up a whole bunch of shock and equipment destruction possibilities.

ProSoundWeb Community

Re: 60hz hum on monitors but not mains?
« Reply #9 on: June 14, 2016, 09:31:32 AM »


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