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Old 8th February 2005, 03:05 AM   #1
diyAudio Member
Join Date: Dec 2004
Location: Perth, WA
Question USB Sound Card

Hi there,

I have just made a USB Sound card using a MAX4299 Stereo Amp and a PCM2900E Audio Codec. It didn't work properly at first, and whenever a sound was played a loud crackling sound would play, no matter what the actual output was supposed to be.

After spending ages error-checking and using an oscilloscope to work out where the problem was, I got the card to work perfectly but only if I connected the analog ground of the board to the ground of the oscilloscope. As soon as a I disconnected the oscilloscope ground, all I got was the loud noise again.

Does anyone have any idea why it would only work when attached to the oscilloscope ground, and if there is anything I can do to fix it?

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Old 12th February 2005, 06:23 AM   #2
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wes-ninja250's Avatar
Join Date: Feb 2005
Location: Inverary, ON
You obviously have a grounding problem.

I take it you have amplified speakers. Are they by any chance grounded to the same ground as the scope? (I'll bet they are).

If so, I would immediately check the connector where they plug into the sound card, and the wiring to the speakers.

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Old 12th February 2005, 09:28 AM   #3
d3imlay is offline d3imlay  United States
diyAudio Member
Join Date: Sep 2004
Location: Ohio
I presume you're using the +5v usb power for the board. Does the analog ground and power supply share the same ground wire? What happens if you use an external power supply?

For a different application i bought and used an isolated hub from these people.
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Old 16th February 2005, 06:33 AM   #4
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Join Date: Dec 2004
Location: Perth, WA
Default Thanks

Thanks for the replies, turns out I had a 10R resistor in between analog and digital ground and that was causing the problem!

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