Technical question - Electrical hum on KH sound system.

[On the huh]

At our Kingdom Hall, there has always been a electrical hum coming through the speakers.  It is normally very quiet, only can really hear it when it is quiet, and then you have to listen for it, but when the meeting is going it is not noticeable at all.

 

Recently however, we have modernized the system - well, connected a laptop.  At first it was just for videos, but now using it for music, recording and KHConf.  But the hum seems to have became louder and more noticeable, even during the meeting since the laptop was connected to the sound system.

 

Nobody has commented on the hum yet, but it is there, and I know if it is not sorted, we could be forced to going back to using the old equipment.

 

So the question is, what can we do about the hum?

[Stavro]

How did you hook up the laptop sound to the system? Also, what model is your mixer, and which input ports are you using for laptop audio?

Edited November 5, 2015 by Stavro

[Qapla]

Is your system using 70v line for the speakers

[nanaaddo]

Our hall also experiences similar hum but only when we are running on the backup generator. Recently though, we, at the sounds department have to contend with high pitch hissing sounds when the volume is adjusted slightly up. We may soon have to replace our entire sound system. It performed well today with about 3 to 4 hissing sounds and a little echo. About the hum on your side, maybe you can try changing the connectors (the phono or audio cables that connects the sound playing devices to the main amplifier) and mic cables too. Hope you get a quick and lasting solution for a clearer meeting.

[On the huh]

We used a Behringer UCA-222 usb unit to connect the laptop to the mixer.  It is connected using the old CD input, and tape recorder outputs, both are line inputs/outputs.

The mixer is a Jape mixer,   And it is a 100-volt speaker system (Uk/Europe system)

[Qapla]

Since a laptop usually uses an external power supply it is possible to pick up some hum from that.

You may need to check that the amp and mixer are grounded

If you are using a phone patch that gets it's input from the speaker line it may need an independent ground.

If your Mikes are low impedance or use an XLR connection you should make sure all of them are properly wired by pin location and that no wires are touching.

Also, if the Mike wires and the speaker wires are running any distance together you can get hum.

[shali]

I really love all this technical talk! LOL

[Hartley]

Hum in audio systems is generally caused by having multiple ground points. Try disconnecting all inputs to your amp and turning up the volume. If the hum remains the problem is with the amp or speakers. Check that the amp output is from the 100v (70v) line and each of the speakers is connected via an matching transformer. With the power plug of the amp disconnected, and using a multimeter,you should not be able to measure any continuity to ground from any point in the sound system.

If the hum is not present after disconnecting the inputs, plug the inputs in 1 at a time and recheck.

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[On the huh]

Thanks for the suggestions.  Will check the connections, and make sure things are "grounded" (Isn't being grounded a form of punishment? , we call it "Earthed" over here )

[vern]

Might check to see if any lines running power can be seperated away from the line feeds to the speakers; or the speaker feeds can be removed from being near the power lines;amps;transformers.  They can pick up EMF and create "noise".  The grounding suggestions also attempt to fix the causes of "noise".  You could also check that the kingdom hall ground is actually working well and check that its good.  The lighting might cause "noise"  if its the old magnetic ballist types.

So many reasons probly...  Hope you find a solution.

[On the huh]

Since most of the recommendation concerned the grounding and connection, and after during some research, I put a ground loop isolator between the amp and the laptop.  It has helped to reduce the hum.  So thats good.  Now we can enjoy using our "modern" sound system.

 

Nobody complained about it, and only a few mentioned that they could hear it.  Most didn't, even when I spoke about it, they said they hadn't noticed.  So either they have poor hearing, or I have very good hearing and pick up on these things.

 

Now we know we are in line for a new KH very soon, I don't think I'll do anymore to the sound system.

[Áine]

We had this problem. .ours happened when the roving mike was used, the over head ceiling speakers were the cause.

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[On the huh]

One time we got some wireless headset to use on the platform, for demos and sisters student talks.  They were ok for a while, but then sometimes a loud electric crackle sound would come through the sound system.  We traced it back to the new wireless roving mikes.  If the mikes were left on, the two conflicted. 

Later on, we also found out that most sisters hated wearing the headsets, so we gave them up.