ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2  All   Go Down

Author Topic: 60hz hum on monitors but not mains?  (Read 4102 times)

Aaron Maurer

  • Full Member
  • ***
  • Offline Offline
  • Posts: 206
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? 
Logged

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3342
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
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.   
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Keith Broughton

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3031
  • Toronto
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
Logged
I don't care enough to be apathetic

Jonathan Johnson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 2961
  • Southwest Washington (state, not DC)
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 »
Logged
Stop confusing the issue with facts and logic!

Aaron Maurer

  • Full Member
  • ***
  • Offline Offline
  • Posts: 206
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.
Logged

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3342
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
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.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org

Kevin Maxwell

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1138
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.
Logged

Jean-Pierre Coetzee

  • Classic LAB
  • Hero Member
  • *
  • Offline Offline
  • Posts: 748
  • Gauteng, South Africa
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
Logged
Audio Technician
Word & Life Church

"If you want "loud", then run a piece of sheet metal through a table saw------

If you want "watts"-then plug in a toaster"
- Ivan Beaver

Kevin Graf

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 298
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 »
Logged
Speedskater

Mike Sokol

  • Moderator
  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3342
  • Lead instructor for the No~Shock~Zone
    • No~Shock~Zone Electrical Safety
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.
Logged
Mike Sokol
mike@noshockzone.org
www.NoShockZone.org
Pages: [1] 2  All   Go Up
 


Page created in 0.118 seconds with 23 queries.