PA Hum????

[howlingwolf487]

...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.

 

[NUSound]

most professional microphones will be balanced

All professional microphones will be balanced.

 

[FMEng]

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.

 

[museav]

"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.

 

[zmb]

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

 

[Morpheus]

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...

 

[museav]

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.