# PA Hum????



## emac (Mar 23, 2010)

Ok so here is my problem...


when I do events at my school and at other venues I plug everything in to my Peavey RQ 200 and then send the outs to a mix of different PAs to run the sound. Usually I have at least one mic running into the board and one or two computer inputs. Non of the cable I use are balanced.

When I run this type of set up I usually come across a hum from my speakers which I want to eliminate.

I talked to a person that is supposed to be knoladgable with this type of stuff and he said to plug everything into a power conditioner and run balanced cables.

Will this help? 

and where should I run balanced cables? from the mics to the board? from my laptop to the board? from the board to the PAs? from the PAs to the speakers????


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## derekleffew (Mar 23, 2010)

By "PAs" do you mean amplifiers? PA usually refers to a loudspeaker array or cluster, or to the entire *p*ublic *a*ddress system.


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## emac (Mar 23, 2010)

derekleffew said:


> By "PAs" do you mean amplifiers? PA usually refers to a loudspeaker array or cluster, or to the entire *p*ublic *a*ddress system.




I am referring to a PA system.... AKA A package of 2 speakers and a powered mixer.....


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## Footer (Mar 23, 2010)

There are two basic ways to keep from getting a hum or "ground loop". Usually the hum occurs at 60hz (low bass hum), which just happens to be the same frequency AC power uses. 

One way is to make sure you have clean power coming in. A power conditioner will help with this. Even more so clean power will help. If a sound system and a lighting system are on the same transformer or the same distro this can cause havoc in a sound system. Same thing goes for HVAC systems or gymnasium/florescent lights. The best practice is to always use a power conditioner and always make sure the console, amplifier, and other gear are all hookup up to the same leg of power. It is pretty common to see a snake ran with a AC line alongside just to keep everything on the same power. 

Along with the power issues, you want to make sure all of your gear uses a common ground. Balanced cables such as 1/4" TRS and XLR cables are the way to do this. All input cable should be properly shielded. The third connector (pin 1 or sleeve) in a balanced connector ensures every piece of gear shares a common ground and therefore any electrical annoyance in the system is equalized through out the entire system. 

Literally one bad cable that is not properly grounded can cause a hum in a multi million dollar sound system.


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## emac (Mar 23, 2010)

Footer said:


> Along with the power issues, you want to make sure all of your gear uses a common ground. Balanced cables such as 1/4" TRS and XLR cables are the way to do this. All input cable should be properly shielded. The third connector (pin 1 or sleeve) in a balanced connector ensures every piece of gear shares a common ground and therefore any electrical annoyance in the system is equalized through out the entire system.




By all input cables you mean everything going into the mixer??? or???


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## Footer (Mar 23, 2010)

Tape/CD/ipod cables do not need to be balanced, though its nice if they can be (only professional CD players have balanced outputs). Microphones do need to be balanced. Lines going out of the mixer into the amplifier do need to be balanced. Lines going from amp to the speakers do not need to be balanced.


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## emac (Mar 23, 2010)

Footer said:


> Tape/CD/ipod cables do not need to be balanced, though its nice if they can be (only professional CD players have balanced outputs). Microphones do need to be balanced. Lines going out of the mixer into the amplifier do need to be balanced. Lines going from amp to the speakers do not need to be balanced.



This is SUPER help full and clears a lot of things up...

Thanks Footer


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## NUSound (Mar 23, 2010)

Footer said:


> Usually the hum occurs at 60hz (low bass hum), which just happens to be the same frequency AC power uses.



I'm not sure if it's what you were getting at, but this certainly isn't coincidence. AC power in the US cycles at 60Hz, so effectively a 60Hz sine wave is being added to your system.


Footer said:


> One way is to make sure you have clean power coming in. A power conditioner will help with this.




Footer said:


> The best practice is to always use a power conditioner and always make sure the console, amplifier, and other gear are all hookup up to the same leg of power. It is pretty common to see a snake ran with a AC line alongside just to keep everything on the same power.



The power conditioner does little to help here other than acting as a glorified power strip which ensures that a common ground has been made. And, although you want them on the same circuit, amplifiers should not be powered through a conditioner. The inrush current pulled by multiple amplifiers can often trip the breaker in the conditioner and the switches in them are generally not designed for this amount of current inrush. Not to mention that most of them are breakered at 15 amps, which prevents you from using the full 20 available in most outlets.




Footer said:


> Even more so clean power will help. If a sound system and a lighting system are on the same transformer or the same distro this can cause havoc in a sound system. Same thing goes for HVAC systems or gymnasium/florescent lights. The best practice is to always use a power conditioner and always make sure the console, amplifier, and other gear are all hookup up to the same leg of power. It is pretty common to see a snake ran with a AC line alongside just to keep everything on the same power.



A power conditioner cannot 'clean' the EM interference generated by these sorts of devices. This is what balanced connections are for, and, in extreme cases, what quad-core cabling is for.


Footer said:


> Along with the power issues, you want to make sure all of your gear uses a common ground. Balanced cables such as 1/4" TRS and XLR cables are the way to do this. All input cable should be properly shielded. The third connector (pin 1 or sleeve) in a balanced connector ensures every piece of gear shares a common ground and therefore any electrical annoyance in the system is equalized through out the entire system.



Actually, balanced cables *increase* the probability of having a ground-loop (This does NOT mean that they should be avoided. Any cable run over ~15 ft should be balanced) as can be seen in the diagram here: 

Ground loop basics

The connection of the chasis' creates a second path to ground, or a 'loop.' Thus, ground loop. 

Couple of troubleshooting ideas:

Start by connecting everything possible to the same outlet via power-strips or preferably quad-boxes (or a power conditioner for that matter). If you can get an extension cord that runs from the same circuit as the 'other' PA's, even better.


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## NUSound (Mar 23, 2010)

After re-reading the OP, my guess is that most of the problem is coming from the laptop power supplies. Run the laptops off battery and see if you still have the hum. If not, you need an isolation transformer between the laptops and the mixer. If the hum is still present, it's likely a ground loop.


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## Morpheus (Mar 23, 2010)

NUSound said:


> A power conditioner cannot 'clean' the EM interference generated by these sorts of devices.


Uhhh......


huh? Power conditioners were designed exactly for this reason - to _condition_ the incoming power... Just look at Furman's LiFT technology.
Also, see Why do I need a power conditioner


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## Anonymous067 (Mar 23, 2010)

derekleffew said:


> By "PAs" do you mean amplifiers? PA usually refers to a loudspeaker array or cluster, or to the entire *p*ublic *a*ddress system.



Not to hijack...but why are the amplifier and pa links red and go to "create new article"?? I don't think that link will help the OP.


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## Morpheus (Mar 23, 2010)

Blah067 said:


> Not to hijack...but why are the amplifier and pa links red and go to "create new article"?? I don't think that link will help the OP.


Yea, I've noticed the automatic wiki links don't always work too well... it's actually kinda funny, tbh...


Uh... back on topic....
It might help to go through items one-by-one to find the "source" of the hum... start with only cable to house system, then your mixer, then one mic, another, computer, etc. etc.... this should help find the hum...


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## Footer (Mar 23, 2010)

Blah067 said:


> Not to hijack...but why are the amplifier and pa links red and go to "create new article"?? I don't think that link will help the OP.



Thats Derek's way of saying.... write the article on that subject.


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## arshan (Mar 23, 2010)

NUSound said:


> Actually, balanced cables *increase* the probability of having a ground-loop (This does NOT mean that they should be avoided. Any cable run over ~15 ft should be balanced) as can be seen in the diagram here:



To throw my hat in here, as far as I understand, balanced cables shouldn't help *or* hurt a ground loop situation. Both balanced and unbalanced cables (say TRS vs. TS) carry a ground line which is what has the potential of creating a ground loop. Or am I forgetting something?

As far as this all relates to the original poster's problem:
1. Balanced lines for anything line level and below may well help with interference. (personally, I try to use balanced line for everything except amp-speaker runs whenever possible)

2. If a ground loop specifically is the problem, another thing to try (*after* actually trying to assure everything is on the same ground) would be ground lifting some of the cabling either through ground lifted adapters, re-soldering some of your cable, or some audio gear has on-board switching for ground lifting.


-Arshan


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## Morpheus (Mar 24, 2010)

arshan said:


> To throw my hat in here, as far as I understand, balanced cables shouldn't help *or* hurt a ground loop situation. Both balanced and unbalanced cables (say TRS vs. TS) carry a ground line which is what has the potential of creating a ground loop. Or am I forgetting something?
> 
> -Arshan


"Unbalenced" cables usually short (combine) the signal '-' and the ground, while "balenced" cables have a seperate, dedicated line for it (usually the shielding around the twisted pair)


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## SHARYNF (Mar 24, 2010)

The whole discussion of Power conditioners makes for very interesting reading, On the pro side of things, most folks think many of the claims made are suspect. You might want to look at

PSW Sound Reinforcement Forums: LAB: The Classic Live Audio Board => Surgex vs Furman SMP

Most of the time you do not have the option of using balanced cables to interconnect the pa's If you DO them by all means use them, but at the same time I would say that there are two possible issues that are causing your problems

One is grounds that are not all the same, you could test this by trying to run all the systems off the same outlet. Older buildings can have all sort of outlet ground issues

The second one that is very common is Laptop power supply issues. if you disconnect the laptop and the hum goes away then you have found your problem. There are a lot of discussions re what to do if that is the case, some of us are willing to take the risk and try to remove the AC ground ONLY on the laptop power supply (NEVER EVER ON THE REST OF THE PA SYSTEM)

Hum and buzz keeps us busy

Sharyn


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## techfreek (Mar 24, 2010)

emac said:


> Usually I have at least one mic running into the board and one or two computer inputs.



You said you have one or two computer inputs. How are they hooked up? Do you have any sort of a DI box between the computer and the board? From my experience you will get a hum if you do not have a DI Box (with the switch set to ground).


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## Morpheus (Mar 24, 2010)

SHARYNF said:


> The whole discussion of Power conditioners makes for very interesting reading, On the pro side of things, most folks think many of the claims made are suspect. You might want to look at
> 
> PSW Sound Reinforcement Forums: LAB: The Classic Live Audio Board => Surgex vs Furman SMP



I wondered about that... it's an interesting read.

If the marketing (of Furman) is indeed as bad as those people make it out to be, it would certainly be of interest to me...
If, however, the products DO work as advertised, and people just want to think they're that important by buying more expensive gear, well, that's also of note to me...

To sum up, I'll take it with a grain of salt, but is it iodized or natural sea salt?


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## NUSound (Mar 24, 2010)

Morpheus said:


> To sum up, I'll take it with a grain of salt, but is it iodized or natural sea salt?



Haha, let's agree to disagree on this? I simply cannot bring myself to believe that a 'magic box' makes everything in my system work better. That having been said, all of my processing racks have power conditioners in them. Why? They are an incredibly convenient source of outlets, lights, and may even do some good in terms of fidelity (honestly can't tell though). 

At the end of the day though, I firmly believe that a Power Conditioner is NOT the solution to the OP's problem and I don't want him running off and spending money on something he doesn't need.


P.S: I never trust a manufacturers website to tell me why I need their product.


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## SHARYNF (Mar 24, 2010)

I think most people buy the Furman units as a rack mounted strip with some lights and a meter reading in a lot of situations. I for one also use the tripplite power units that basically level the ac voltage via a transformer switching system. I also agree that this is NOT likely to be a problem solved via any sort of strip. 

It could be a bad cable, poor ground, or laptop power supply


Sharyn


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## howlingwolf487 (Mar 24, 2010)

emac said:


> ...I plug everything in to my Peavey RQ 200 and then send the outs to a mix of different PAs to run the sound.


So, are you saying:

1. You run the output of your mixer into the inputs of another mixer?

OR

2. You run your mixer's outputs straight into a power amplifier/powered speaker?

If you run into another mixer, make sure you use a TRS-TRS cable to run into the Line Inputs. That will be more suited to the signal level you have coming from your mixer.


> Usually I have at least one mic running into the board and one or two computer inputs. Non of the cable I use are balanced.


Okay, well most professional microphones have balanced outputs. You should be fine there. (HOWEVER, if they are a dynamic microphone they CAN still work if only 2 out of the 3 legs are connected (for example: if Pin 3 is disconnected, but Pins 2 and 1 are left intact, you will still get signal - it'll be 6dB quieter and might sound weird, but it will be there.))

Always try to balance whatever you can in your system. I, personally, use a Whirlwind PCDI for the task of interfacing computers, DVD players, etc. with balanced equipment. Your mixer has purpose-built stereo inputs for unbalanced, consumer-level gear. Use them, but keep the cable runs as short as possible.


> When I run this type of set up I usually come across a hum from my speakers which I want to eliminate.
> 
> I talked to a person that is supposed to be knoladgable with this type of stuff and he said to plug everything into a power conditioner and run balanced cables.
> 
> Will this help?


Well, the hum could come from any number of devices - not even ones directly connected to your system. I suggest doing what some of the other posters said and start by plugging only your mixer into the other system, be it a mixer of power amplifiers. Then, work your way back, adding one device at a time until you find the one (or two, three, etc.) that is/are the culprit(s).

Balanced cabling will definitely help - and it gives you more range to work with between your mixer and the devices plugged into it AND the devices it is plugged into, also.

A power condition probably won't do too much as far as "filtering" goes (some might...it's a touchy subject, as you can tell) but it WILL keep all of the gear plugged into it at the same ground potential (which will eliminate the hum). If you get a good one, your voltage will be stabilized and will allow for any equipment that doesn't tolerate voltage fluctuations well to operate normally.


> and where should I run balanced cables? from the mics to the board? from my laptop to the board? from the board to the PAs? from the PAs to the speakers????



As said before, most professional microphones will be balanced, so you'll most likely be fine there. I already mentioned a device to balance the computer lines, so that should help also.

Your mixer's main outputs are balanced. Use a TRS cable to run from the main outputs of your mixer to the line inputs of whatever device is hooked to the power amps (even the amps themselves). Your mixer's monitor output (and FX output) are NOT balanced, so any monitor sends you use will be unbalanced. Also, the FX send will be post-fader, so fader level changes will affect the level of that channel in that send.


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## NUSound (Mar 24, 2010)

howlingwolf487 said:


> *most* professional microphones will be balanced



*All* professional microphones will be balanced.


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## FMEng (Mar 25, 2010)

Footer said:


> One way is to make sure you have clean power coming in. A power conditioner will help with this. Even more so clean power will help.



Please let us end the myth that any kind of power conditioner, surge protection, or UPS will clean up low frequency hums that are caused by ground loops and un-balanced audio lines. Never in my 30 year audio career have I ever needed a power conditioner to cure a noise issue. Furman power stuff is an example of great marketing that mostly fixes non-problems. 

I have run whole radio studios on the dirtiest possible, harmonically horrible, generator power, to the point that the windings in the power transformers in the gear made very audible buzzing, and yet all of the audio was still perfectly clean. Most equipment has good enough filtering and regulation built in to not need external help.

If the noise is high frequency buzz or hash from sources like lighting dimmers or RF interference, then maybe a power conditioner might help. That type of problem is relatively rare. I'd say 95% of all noise issues are ground loops or un-balanced connections, or a combination of both. If you live in lightning land, and want some surge protection, then I can't argue with that.

Now, back to the matter at hand. Using balanced connections between equipment that is physically separated by a significant distance will solve most hum problems. Occasionally, a stubborn ground loop will need to be fixed by dropping the shield connection at one end of the cable. This works only for line level, not microphones. If you need to modify a cable to drop the shield, make sure it is plainly marked. 

There are XLR to XLR adapters available for "ground lift," which opens the shield (pin 1) connection. I know ProCo makes them, and probably Whirlwind.

In really severe cases, such as running lines between buildings, or hundreds of feet, then using a good quality audio transformer can work miracles. A transformer provides for better common-mode rejection and isolation than is possible with active balanced inputs and outputs. 

Transformers are also an excellent way to make an un-balanced input or output balanced.


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## museav (Mar 28, 2010)

Morpheus said:


> "Unbalenced" cables usually short (combine) the signal '-' and the ground, while "balenced" cables have a seperate, dedicated line for it (usually the shielding around the twisted pair)


 No, there is no "-" in an unbalanced line or signal, only the signal and a ground reference. A balanced line has two signal lines, each with equal impedance to ground, along with the ground reference. For audio a balanced line is typically run into a differential input, which along with the balanced impedance to ground provides noise cancellation, while a balanced output typically puts a polarity inverted (symmetrical) version of the output signal on the second "-" connection, this is not really related to noise reduction but along with the differential input does have other benefits.

On the original issue, it might help to first narrow down if the hum seems related to a particular source or connection. If unplugging a source gets rid of the problem that really helps narrow things down.


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## zmb (Mar 28, 2010)

Also check that all connections are firmly connected. A TRS connector partially inserted will usually cause a hum.


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## Morpheus (Mar 29, 2010)

museav said:


> No, there is no "-" in an unbalanced line or signal, only the signal and a ground reference. A balanced line has two signal lines, each with equal impedance to ground, along with the ground reference. For audio a balanced line is typically run into a differential input, which along with the balanced impedance to ground provides noise cancellation, while a balanced output typically puts a polarity inverted (symmetrical) version of the output signal on the second "-" connection, this is not really related to noise reduction but along with the differential input does have other benefits.
> 
> On the original issue, it might help to first narrow down if the hum seems related to a particular source or connection. If unplugging a source gets rid of the problem that really helps narrow things down.



Ah. Well, it seems we both mean the same thing, you just state it more accurately...


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## museav (Mar 29, 2010)

Morpheus said:


> Ah. Well, it seems we both mean the same thing, you just state it more accurately...


I agree that the same thing was intended. I really wanted to highlight that the "-" and "+" connections are more indicative of an output that uses signal symmetry, the "-" output could have no signal on it and the circuit would still be balanced. And that the noise resistance is gained by the use of a balanced circuit in conjunction with a differential input. Used differently, such as with the two signals added rather than subtracted, a balanced line could actually increase any induced noise.


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