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OT: Help with Audio Problem...?

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Old 11-20-2002, 08:56 PM
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I have a Sonic Impact A3D audio card, basically just a decent sound card in my computer. I am running RCA type cable to my home stereo amplifier inputs. The sound card is driven by an application called MusicMatch Jukebox, just an MP3 player. When the signal gets to the home stereo amplifier there is a 60 cycle hum. I tried feeding the amplifier with a handheld Sony walkman and the 60 cycle hum went away. I tried one other computer using a different type of sound card and the hum continues. If a laptop running on battery is used there is no hum. I have tried relocating the sound card lower in the case and placing a metal shield over it. Also unplugged all other device on the computers power circuit and tried plugging it into the same circuit as the amp to eliminate a ground loop.

Cable in use is RCA cable to COAX to RCA cable. Yes this is a goofy arrangement but for the distance and walls I have to pass through this is the only way. When I temporarily relocated the computer to the same room as the home system and ran a RCA cable that was only 20 feet long the hum was still there so itís not the cable.

Is there any device active or passive that can cut the 60 cycle out of the RCA cable? If I get this system rigged properly I may just put the laptop in the boat with and inverter. The interface for selecting and changing MP3ís on the computer is so much better. Bought a top of the line MP3/DVD changer and the remote has more buttons than a triple installation. What a pain! I would rather just use the computer.

Tim T.
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Old 11-20-2002, 09:12 PM
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When you plug the laptop in does the hum return? Have you tried an isolator? I'm not sure that would work.
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Old 11-20-2002, 09:59 PM
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Yes the noise returns when laptop is running on AC. Tried flipping the AC input cord on the amp and removing the ground wire on the computer. Removed shield on output end of audio cable. Looks like I need to run the computer off of a transformer?
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Old 11-21-2002, 01:12 AM
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A couple suggestions to try:

Does your computer have two or three audio jack connectors? If three: one input for MIC and two outputs, one for headphone and one for line out. If you have a line out use that. Also plug in a set of headphones in the headphone jack (first disconnect from stereo) and see if you get the humming. Which input on the stereo are you using? How high is the volume set on the PC?

Steve
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Old 11-21-2002, 09:10 AM
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The problem is the two units are developing a different groud plane and that is causing your hum. SO there are a few things you can do to probably eliminate the problem. First is if your laptop has a three prong plug then get one of those Three prong to two prong plug addaptors like you need if you life in a really old house. The other is to get a ground loope Isolator for the RCA's. You can also try cutting the ground shield on the RCA at the Stereo end of the RCA. That should also get rid of the hum. Hope that helps ya out. Chances are there is a problem with your computers moter board ground that is shoing itself with the hum.

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