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Noise on my speakers when laptop is AC powered
PaJaRo
post Oct 12 2008, 22:59
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Hello,
I'm using a laptop with an Edirol UA-1Ex usb sound card, when my computer is plugged to AC I hear some noise on the speakers, I thought it could be due the unbalanced outputs, so I tried to put the cables far from the AC adapter, but the noise stills. Moreover when I do something cpu intensive tasks I hear more noises (while cpu is working hard). If the AC adapter is unplugged I don't hear any noises.
I want to know if using a balanced outputs will solve the problem, or any other suggestion.
Thanks in advance.
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mixminus1
post Oct 13 2008, 00:08
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I come across this quite frequently at the conference center where I work as we see many laptops on a daily basis. Roseval is right - the root cause is noise in the switching power supply, and as such lifting the AC ground on the laptop and/or destination device rarely fixes the problem (and, as Phoyo's response implies, introduces the possibility of electric shock in the case of equipment malfunction).

I've been able to completely eliminate the noise every time it's occurred by using isolation transformers between the laptop's (or USB audio interface's) audio output and the input of the powered speakers/mixer/etc.

Note that you don't need balanced connections to use isolation transformers - with unbalanced connections, just make sure that the cold (-) leg is shorted to ground on both the input and output of the transformer and then engage its ground lift switch.

I use Whirlwind's ISO-2 stereo isolation boxes, and they work and sound great, but they're a bit spendy from a consumer audio standpoint (~US$130). The cheapest ($65) stereo isolation transformer I've seen is the Ebtech H32PKG, which has 1/4" inputs and outputs (RCA-to-1/4" adapters are very easy to find).

The only caveat is that you often get what you pay for with transformers. I've never used the Ebtech, so I can't say whether or not it would have an audible effect on the sound (low-end roll-off being the most common artifact of inexpensive transformers), but that is a possibility...


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PaJaRo
post Oct 13 2008, 01:30
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QUOTE (mixminus1 @ Oct 13 2008, 01:08) *
I use Whirlwind's ISO-2 stereo isolation boxes, and they work and sound great, but they're a bit spendy from a consumer audio standpoint (~US$130).



I'll try to buy the Whirlwind (better pay a little bit more than waste my money).
Two more questions:
1) Can my speakers be damaged if I still using them with that noise?
2) when should i use ground lift button?

Thanks for the info! wink.gif

This post has been edited by PaJaRo: Oct 13 2008, 01:34
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mixminus1
post Oct 13 2008, 02:17
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QUOTE (PaJaRo @ Oct 12 2008, 17:30) *
I'll try to buy the Whirlwind (better pay a little bit more than waste my money).
Two more questions:
1) Can my speakers be damaged if I still using them with that noise?

No, it's just like any other sound that your speakers reproduce. There is often some DC offset present with such noise, but since the vast majority of modern amplifiers are AC-coupled (i.e. they block DC on their inputs), even a large amount (hundreds of millivolts) of DC offset would be eliminated at the amplifier's input.

QUOTE
2) when should i use ground lift button?

Whenever you hear the noise. smile.gif The whole point of the isolation transformers is to achieve complete physical isolation between your laptop and the speakers (or the amplifier driving the speakers). Since you will have solid grounds on each side of the transformer (through the power supply of your laptop on the input, and the power supply of the amplifier/powered speakers on the output), the ground does not need to be carried through. About the only time you need to connect the input and output grounds is if you're isolating a microphone or guitar pickup, i.e. sources that don't have their own electrical ground.

Edit: One other thing...to get from your Edirol interface into the ISO-2, you'll need RCA-to-1/4" adapters, as I mentioned. However, the ISO-2 only has XLR outputs, so you'll need appropriate adapters to get from XLR to whatever the inputs of your amplifier are (although you mentioned balanced connections, so perhaps your amplifier/powered speakers are already capable of that?). Remember, if the inputs of your amplifier are unbalanced, you need to make sure the XLR-to-whatever adapters short the cold (-) leg (pin 3 on an XLR) to ground (pin 1 on an XLR).

This post has been edited by mixminus1: Oct 13 2008, 02:23


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