ground loop help?

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midge

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Hi

I'm having problems with out live PA - I know this is a studio forum but you people are the experts with this sort of stuff hence why I'm posting here!

We have powered RCF speakers that run out of a Spirit Folio notepad mixer. As far as i can tell this equipment is alright.

We recently dumped our minidisc players in favour of laptops. Each laptop has an Edirol UA1A usb audio output interface for clean sound.

I just plugged in the PA to practice today and as sook as I plugged in the RCA lead to one of the laptops there was a huge hum/buzz through the speaker. I checked all the leads - fine. I tried through a totally different power source and there was still no difference.

A wierd thing is that even when I turn off the powered speaker, I can still hear a hum/buzz (although reduced) through the speaker - bizzarre!!!

One laptop has a worse hum than the other, but they are both annacceptable. We've used the laptops for three weekends and had no problem up until now.

As soon as I unplug either the audio lead or the power lead from the laptop, the hum disappears and things like cd player and mics are fine throught the speaker.

I don't know much about ground loops, so if you could use layman's terms to help me out, that would be fantastic.

Cheers People!!!

Shaun Thomson.
 
So the minimal setup that would give the hum is: laptop to edirol USB-thingy to mixer to speaker?

If it's really a ground loop problem, you can solve it by disconnecting the ground at a certain point. Try disconnecting the ground of your RCA cable. First on one line, then on both, and see if this helps. If this doesn't help, then it's not a groundloop problem.

If it IS a groundloop problem, try making special cables, or put a DI in between...
 
Hum sucks :)

But its easily fixed.

The first order of business is to determine what is causing the hum - equipment, wiring or external devices.

With everything connected as is, turn off the lights in the room. If the hum goes away, you know its any flourescents, dimmers, etc. If not, continue along...

Unplug all the inputs from the mixer, and leave the mixer and the powered speakers connected. Hum? If so, then its more than likely a faulty power supply or a cabling issue between. Cabling issues are easier to resolve - you simply replace them. If that doesn't solve it, well, it might be more complicated.

If you have no hum in our speakers at this point, then plug in each device one at a time until you get the hum. Then you know what device is a problem and you can trouble shoot further.

I'm with Roel that it very well might be the laptop and the external converter - its not all that uncommon. If it is, you'll need to isolate the ground between the converter and the mixer, and you can do this with a transformer, or a simple op-amp circuit if you are up for some fun soldering / project building. The transformer option is the easiest way to go - solder four wires to the transformer with a simple patchbay and you're good to go.

Let us know how you make out!

BTW, I recently spent two weekends going through this, only because I have a ton of gear, and I found that my Akai XE8 drum module had a short in its ground internal to the unit, which grounded itself to the aluminum rack, which in turn was... well... grounded :) So thats where my hum came from.

Of course the XE8 was at the bottom right of the rack area, so it was one of the last units I tested :(

Oh well :)

midge said:
Hi

I'm having problems with out live PA - I know this is a studio forum but you people are the experts with this sort of stuff hence why I'm posting here!

We have powered RCF speakers that run out of a Spirit Folio notepad mixer. As far as i can tell this equipment is alright.

We recently dumped our minidisc players in favour of laptops. Each laptop has an Edirol UA1A usb audio output interface for clean sound.

I just plugged in the PA to practice today and as sook as I plugged in the RCA lead to one of the laptops there was a huge hum/buzz through the speaker. I checked all the leads - fine. I tried through a totally different power source and there was still no difference.

A wierd thing is that even when I turn off the powered speaker, I can still hear a hum/buzz (although reduced) through the speaker - bizzarre!!!

One laptop has a worse hum than the other, but they are both annacceptable. We've used the laptops for three weekends and had no problem up until now.

As soon as I unplug either the audio lead or the power lead from the laptop, the hum disappears and things like cd player and mics are fine throught the speaker.

I don't know much about ground loops, so if you could use layman's terms to help me out, that would be fantastic.

Cheers People!!!

Shaun Thomson.
Click to expand...
 
Does your laptop (or anything else you're using) have either a "wall wart" or a "line lump", both otherwise known as an external power transformer? If so, where are they physically in respect to any audio cables? These transformers are almost exclusively either poorly or NOT shielded, and can cause immense amounts of hum, even with shielded cables. Try moving any of the above as far AWAY from your audio cables as possible while listening.

If that doesn't work, try using a transformer direct box between the audio interface and the mixer. Plus, all the above - troubleshooting is finding all the things it's NOT - if you get lucky, it's the FIRST one but usually it's more like Frederic's Akai XE8 drum module... Steve
 
Hi

I tried putting a DI with a ground-lift between the laptop and mixer, but with no difference.

Roland Australia's tech guy said to try putting the laptop on a different power circuit to the rest of the gear. If the hum disappears then it's an isolating transformer that I need. So I plugged the laptop into next-door's power while leaving the mixer on our power - still no difference!!!

I then tried putting the gear through a different mixer - exactly the same problem.

It's strange that it's only started happening in the last few days - it suggests something has gone wrong. But it's not the mixer or the speaker. How the hell is the laptop doing this???

Any clues - sorry to keep bugging you but I don't know who else to ask!

Cheers - Shuan Thomson.
 
Seems like it really is no ground loop problem...

If the roland has it with different mixers too, it has to be the roland. I don't know the thing, so I cannot help you there...

Don't bother asking. That's what forums are for!
 
Did you try moving the laptop's power transformer further away from the audio cables?

Will the laptop's power plug hook up backward (reverse polarity)?

If so, did you try it both ways?

Does the hum change if you swap left/right speakers?

If you pull down the fader for the input from the laptop, does it make a difference in the hum?

The fact that the speaker still hums with the speaker turned off is more than bizzare, considering that they're POWERED...

Please answer ALL the above questions and maybe something will jump out... Steve
 
Hi Knightfly

Nothing else is at fault - I tried connecting the laptop directly to the powered speaker input, and the same hum appeared. I don't think the speaker is at fault, as I've tried both speakers separately and with different cables - same thing, hummmmmmmmmmmmmm!!!!

It's driving me mad - the only thing I can think of is a faulty power supply.

Let me know if anything comes to mind.

Cheers!!! Shaun
 
Have you tried headphones with the laptop? On AC and on batteries?

When you said removing the power from the laptop stopped the hum, did you mean no power to the laptop, or did you mean running on batteries?

By your comments, do you mean that you tried each and every point I made in the previous post? If so, was there ANY change?

I'm like you, I'm finding it hard to believe that two different laptops both screwed up in the same way overnight. Isn't there any ONE THING that is common to everything that hasn't been checked/thought of/replaced? running out of ideas here... Steve
 
OK

Yep, I tried moving the transformer away from the mixer - no change.

Not sure what you mean about reversing the lappy's power.

No change to hum when speakers are changed.

When the fader was pulled down it made no change to the hum.

I don't think anything else is at fault - I eliminated everything bar the lappy and speaker by connecting the two directly together and still getting hum. I pretty much eliminated the speaker by using our other one and by swapping speaker cables - still no change.

By no power I meant I ran the lappy on batteries - hum disappeared.

I couldn't hear any hum when using headphones on the mixer.

Help!!!!
 
By no power I meant I ran the lappy on batteries - hum disappeared.
Click to expand...

The answer is right there.

Replace the power supply for the laptop.

If that doesn't cut it, borrow a laptop from a friend and see if the hum continues. If not, buy/rent/trade laptops.
 
I think Frederic may be right - maybe those particular supplies are just crap.

What I meant by reversing the power, is this: if both blades of the power connector that plugs into the wall outlet are the SAME width. sometimes you can lessen hum by plugging the unit into the wall with the blades SWAPPED relative to the receptacle's slots.

I'm more inclined to agree with Frederic though... Steve
 
Hi Guys

I went to the gig last night and the hum disappeared!

i'll be plugging the gear in to check it out today.

Shaun
 
When you had the stuff plugged in at home, was it ALL plugged into the SAME outlet? INCLUDING the powered speakers? IF not, try plugging stuff into the EXACT SAME place you had it and check the hum, then move the stuff so it's all in the SAME outlet, if you have to get a power strip... STeve
 
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