Fuzzy digital noise!

[davlan]

Having a bit of a prob. I am using a PCMIA Indigo sound card with my Dell laptop. With headphones plugged in this is an almost silent sound card. No back ground noise! However the problem arises when I am plugged into the mains as opposed to battery use.

With headphones plugged in everything is fine. However if I attach the output to a mixer or amp I get this dreadful digital background noise. It comes across as being whenever the computer does something, access hard drive, change something on screen, there is an digital audio equivalent sent down the audio cable to the amp.

Any ideas?

Dave

[john]

Hey Dave!

Great to see you back on the boards.

Hmmm - Digital cross talk. You can get a similar problem with audio and Midi leads.

Is the lead to your amp close to your power cable? It sounds like the Mains power circuitry is not well insulated from the digital signals within the computer. Solution: try and physically separate the power cable and the audio cable.

It could also be leakage directly through the mains (digital noise spills on to the laptop mains, which travels through the mains circuit to the mains input on your amp). The best solution to this is power supply filtering on the lap top and amp.

Cheers

John

[davlan]

So let me get this right.

The noise is travelling through the mains from computer to amp?

Where do I get a mains filter?

Cheers

Dave

[davlan]

Contacted Dell about this . they reckoned the problem is likely to originate in the AC power supply adapter and said if I could test it with another adapter they would then send out a replacement. Any ideas where I can get hold of a power adapter?

[john]

Hey Dave

Well, it can be a bit of hassle changing the power supply on your PC.

So Dell say, get another power supply, and then we'll send you one. Do they expect you to have spare ones lying about? And if you get one, why would you need one from them?

You can get 4 way gangs with power supply filtering at PC world. Of course, replacing the filter may reduce the need for it, but I don't think you can over filter the power supply!

Cheers

John

[john]

Hey

I forgot to say, the power supply to your amp should be filtered too. If you amp is vulnerable to mains noise (a lot are) then filtering it's supply will help.

You can spot this easily. Try getting someone to plug in something in another room and then turn it on. You'll probably get an audible click through your speakers.

Cheers

John

[john]

Hey

Did you get this sorted out?

Cheers

John

[davlan]

Solved. I have been doing a bit of research on the net and discovered that this is a fairly common problem especially with laptops. The noise is interference from the motherboard onto the audio caused by a ground loop. Technically I should send the computer back but as this will be a major hassle I have been able to resolve the issue perfectly by getting a ground loop isolater. I believe a DI box with an earth llift will acheive the same thing.

[john]

By ground loop isolator, you mean through the mains? i.e. noise from the mains is spilling onto your audio card through the main board power supply?

[davlan]

John , I'm not word perfect on the math and the physics so here is a quote from the manufacturer:

What Does It Do?

The Xitel Ground Loop Isolator is designed to eliminate the annoying hum that speakers give out due to imbalances in your electrical wiring. It works by breaking the earth between your computer or audio components and stereo so there’s no path for current from an electrical imbalance to flow. This prevents your speakers from humming! It’s a simple and effective way of eliminating ground loop issues without having to resort to an electrician.

Also here is the URL examining the problem (look under soundcard problems 11&12):

http://sound-on-sound2.infopop.net/2/OpenT...572&f=514099644

Dave