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Author Topic: 60hz hum on monitors but not mains?  (Read 3942 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.   
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Mike Sokol
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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.
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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.
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Mike Sokol
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Jonathan Johnson

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Re: 60hz hum on monitors but not mains?
« Reply #10 on: June 14, 2016, 11:25:06 am »

Mike, what is considered best practice when one device has balanced outputs/inputs but has no factory-provided EGC (power cord ground) because it is "double insulated"?

How about unbalanced ins/outs?

(On second thought, how common is it to see balanced I/O on double-insulated equipment?)
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Kevin Graf

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Re: 60hz hum on monitors but not mains?
« Reply #11 on: June 14, 2016, 05:54:09 pm »

Remember a XLR balanced interconnect system has no ground connection, it only has a shield connection to the chassis. The fact that the Safety Ground/Protective Earth is also connected to the chassis has nothing to do with the balanced interconnect system. A correctly designed unit (one that does not have a pin 1 problem) should not experience any problems regardless of the number of balanced interconnects.
Tony Waldron, a UK large system expert (CADAC Electronics) recommends that all shields on all cables be connected, even in multi-building systems.
Maybe people should build a John Windt "Hummer" tester.

A double insulated enclosure should not be modified.
I would thin that low powered double-insulated equipment will become more and more popular.
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Mike Sokol

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Re: 60hz hum on monitors but not mains?
« Reply #12 on: June 14, 2016, 06:29:25 pm »

Remember a XLR balanced interconnect system has no ground connection, it only has a shield connection to the chassis. The fact that the Safety Ground/Protective Earth is also connected to the chassis has nothing to do with the balanced interconnect system.

JMS: I only said the word "ground" because it's typically called a ground lift by many manufacturers. It should be called a pin-1 "bond" but then nobody would know what I was talking about.

Quote
A correctly designed unit (one that does not have a pin 1 problem) should not experience any problems regardless of the number of balanced interconnects.

JMS: True, but what do we do about the many thousands (millions?) of active speakers and amps with the pin-1 problem. We can't just throw then all away.

Quote
Tony Waldron, a UK large system expert (CADAC Electronics) recommends that all shields on all cables be connected, even in multi-building systems.

JMS: I don't agree with that. Engineers in the UK don't have to deal with how bad our American grounding systems are. Much (maybe most) of our EGC (safety grounds) are contaminated with multi-point bonding to building steel. From what I've read, the UK doesn't have that problem. Hence their engineers don't think about it much.

Quote
Maybe people should build a John Windt "Hummer" tester.

JMS: Actually, the Mike Sokol version is much better. It can generate up to 3 volts and 40 amperes of ground loop differential voltage and current.

Quote
A double insulated enclosure should not be modified. I would think that low powered double-insulated equipment will become more and more popular.

JMS: True that. But I've run across one of these (a Roland cube guitar amp I think) that developed a line to chassis short, possibly from a nearby lighting strike. That's what I fear most about double-insulated appliances. However, a GFCI on that outlet should protect the meat puppets from electrocution.

BTW: Here's a pic of the business end of the Glo-Melt transformer I use in my Pin-1 hum injector. There's a huge amount of amperage available in many grounding systems, and this will easily produce 30 amperes of ground loop current. As I've noted before, my experiments show a typical ground loop current of around 1 ampere per volt of ground loop differential. I've measured between 2 and 5 volts difference between EGC grounds in American wiring, so that implies up to several amperes of current that an XLR shield has to pass if its bonded on both ends and there's a significant voltage between EGC connections.
« Last Edit: June 14, 2016, 10:00:10 pm by Mike Sokol »
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Mike Sokol
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