Laptop Causing Audio Interference

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Laptop Causing Audio Interference

Post by jtcoops »

Hi All

When my laptop is connected to a power socket whilst being hooked into the desk, a weird intereference seems to start coming through the desk affecting the master out. It starts off gradually as a white noise and then increases in volume over time to include other electrical modulation sounds that change depending on what the hard disc is doing.

It is the same if it is connected using the internal sound card or through a Focusrite Saffire via Firewire.

If it is completely isolated from the desk there is no problem, but I've started doing sessions with clients linked in over Skype, with a mix from the desk feeding the saffire and into the laptop (the studio PC is completely isolated from the internet).

On battery power it is fine, but the battery runs out after an hour or so and as soon as it goes back on charge it becomes unusable due to the interference.

Any idea how I can cure this? Would a mains filter for the laptop do the trick? Its starting to get really annoying!

Signal chain is laptop out (direct or firewire) to line in on a Soundcraft Ghost. The laptop is on its own 13 plug but part of the same ring.
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Re: Laptop Causing Audio Interference

Post by Agamemnon »

A common problem, there are probably other threads here covering the topic - it's caused by ground loops and/or PC switched-mode power suppplies.
Solution: take PC output to desk mic-input via a decent DI box (I use a Radial) to isolate it and provide ground-lift if necessary.
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Re: Laptop Causing Audio Interference

Post by ef37a »

Mains filter? No.

DI box? Not really but what might be needed is a 1:1 transformer isolator such as the Art Cleanbox 2 but transformers will always degrade audio somewhat so best avoided if possible.

The best solution is to buy a laptop power supply that does not have an earth (cannot see for the life of me why ANY of them have one anyway!)

Are the lines to the monitors balanced? That could help as would lifting the earth on the monitor AUDIO feeds NOT mains!

Dave.
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Re: Laptop Causing Audio Interference

Post by TrickyDicky »

Is this a power cable kettle lead without an earth??

So it would cure the noise to my old PC laptop power plus help the OP?

Browser URL:
EUROPEAN EURO KETTLE CABLE LEADS MAINS POWER 2PIN

Am I on the right lines with a shaver point adaptor for a UK mains plug?
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Re: Laptop Causing Audio Interference

Post by ef37a »

TrickyDicky wrote:Is this a power cable kettle lead without an earth??

So it would cure the noise to my old PC laptop power plus help the OP?

Browser URL:
EUROPEAN EURO KETTLE CABLE LEADS MAINS POWER 2PIN

Am I on the right lines with a shaver point adaptor for a UK mains plug?

NO!! Do NOT replace 3pin mains leads with 2 pin versions*! In anycase that seems to be a eroplug with side contact earth.
What is required is a new power supply with a 2 pin "figure eight" mains input or one with a 2core flat mains lead.
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Re: Laptop Causing Audio Interference

Post by ef37a »

*Continued because of the hiding problem.
I admit that the risk of internal breakdown and subsequent shock from a laptop PSU (or any other decently made such supply) is vanishingly small but the warning MUST stand. NEVER lift mains earths.
There also seems to be no reports of any other problems from people with earth free supplies so I say again, why make most of them with an earth?

Dave.
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Re: Laptop Causing Audio Interference

Post by Folderol »

ef37a wrote: There also seems to be no reports of any other problems from people with earth free supplies so I say again, why make most of them with an earth?

Dave.

CYA and fear of lawyers :frown:
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Re: Laptop Causing Audio Interference

Post by narcoman »

ef37a wrote:*Continued because of the hiding problem.
I admit that the risk of internal breakdown and subsequent shock from a laptop PSU (or any other decently made such supply) is vanishingly small but the warning MUST stand. NEVER lift mains earths.
There also seems to be no reports of any other problems from people with earth free supplies so I say again, why make most of them with an earth?

Dave.

Earth free supplies? I've only ever seen them in labs.
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Re: Laptop Causing Audio Interference

Post by ef37a »

"Earth free supplies? I've only ever seen them in labs." No, not that kind of "earth free" Narcoman. The paradox of mains safety is not that kit is earthed but that one side of the incoming mains is also tied to earth, neutral. If you use an isolating transformer as we did in TV workshops, there is no longer a mains reffered to earth so copping live even if your feet are grounded or you are touching an earthed metal object will not kill you.
But for all sorts of other reasons, isolated mains* is not a practical or safe practice for "domestic" situations.

I refer merely to power supples that are class 2 insulation and therefore do not REQUIRE a mains earth. NB. most wall rats have plastic earth pins!

*Another variant is balanced mains, again not intrinsically safe under all conditions, there is a thread somewhere.

Dave.
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