laptop supply needs filtering ?

hi, a friend of mine has setup a conference listeneing using VOIP .

he has the pre amp out connected straight up to the laptop . he has the mains adapter plugged in to the laptop. but there is a hum coming thru ,, when he disconnects the mains supply for the laptop , and the laptop runs on its own batteries , the hum goes away ....

would the laptop supply need some sort of filtering ? thanks,

mark

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mark krawczuk
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This is usually because the laptop power supply does not provide an earth connection and there is significant leakage current.

I'v sometimes found it painfull to have a laptop in my lap while wearing shorts due to leakage current.

A little bit of 50/60hz current can pass through the capacitance between the mains side and the DC side in the power supply.

I have a theory that this is more of a problem in countrys with

230/240volt mains and where the neutral of the mains supply is usually earthed at the local substation. You appear to be in australia which has 240volt mains.

I believe the USA usually has an earthed center tap on the distribution transformers that supply houses so the mains only swings 55Volts rms above/below earth potential compared to the live conductor being 240V rms (360v peak) above earth in other systems.

240volt systems are of course vastly superior to 110volt systems in other ways such as brownouts being unheard of, fewer electrical fires because faults are more likely to trip breakers, fewer distributions transformers per household etc.

There are a few approches to the humm problem. I'd try an isolating transformer because I have a few around and also because I'd use equipment with balanced audio inputs and ouputs that is less prone to this sort of problem.

Audio isolating transformers are sometimes known as "DI box's" to sound engineers.

Earthing the laptop might cure the problem. I'd use a mains plug with a 1000ohm resistor in it and a D connector to get a connection to the laptop chassis.

Bob

Reply to
Bob

Let me guess. It's a Dell Lattidue C600 laptop?

There was a nasty design error in the microphone amplifier that would couple internal 3.3VDC power supply noise into the mic amp through the power supply. I don't recall if it was affected by the external power supply. There are mods around the net somewhere that show where to break the PS line and add a filter cap and series resistor. However, the easy fix is to get an external USB sound dongle on eBay and use it instead of the internal audio device.

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Jeff Liebermann

Happens with pretty much all of them, to some extent. I found that the only reliable method is to hang an audio balun transformer between the sound card and the pre-amp. Radio Shack has those, or at least they used to. Else check the local automotive place if they also sell car stereos.

Oh, and don't necessarily expect store clerks to understand what a balun is. Sometimes they didn't even know they carry them. "I'm afraid we don't have that" ... "I looked on the web and your part number is

123xyz-gizmo" ... "Oh." ... a few clicks on the keyboard ... "Oops, yep, here they are. How many do you need?"
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Regards, Joerg

http://www.analogconsultants.com/
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Joerg

Most laptops are grounded through the supply earth, so you likely have ground loop. An audio isolation transformer will take care of it, for example this one, from Wal*Mart of all places:

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Best regards, Spehro Pefhany

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Spehro Pefhany

Or RadioShack if closer:

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Even comes with a 1/8" adapter, usually.

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Regards, Joerg

http://www.analogconsultants.com/
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Joerg

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