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Old 12th May 2013, 07:03 AM   #11
rjm is offline rjm  Japan
Richard Murdey
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Join Date: May 2004
Location: Kyoto
Blog Entries: 103
I'm having similar trouble with a USB bus powered sound card. Switching noise from the USB power gets into the recorded signal, mostly at around 25 kHz, but also at 400 kHz, 1 kHz, and harmonics thereof.

For the record, I don't have the problem when the Onkyo SE-u33 is plugged into the USB ports of my desktop computer, but all the various notebooks and netbooks I've tried have more-or-less the same issue.

Also, if I used 16/44.1 bit recording the problem would not really show up, since all the stuff >20kHz is caught in the brick wall filter and the rest of the noise baseline is much higher from the 16 bit LSB dithering.

However I want to record in 24/96 and I want to use my notebook, so...

So far I've attacked the problem by connecting the card to the notebook via an external USB hub that has a 1.35/3.5mm DC power jack. Right now its powered by a little switching wall wart, so the switching noise is still present (but less noise overall than with the notebook USB bus power). Next I plan to replace the wall wart with a linear or battery supply.

The chart below shows the FFT spectrum of the baseline noise, with the input volume at zero (purple) and in the normal position (green), with the phono stage attached. The phono stage output noise covers up the switching noise in the audio band, but the strong signal at 25 kHz gets through. It's probably so low as to be nothing to worry about, but it bugs me all the same.
Attached Images
File Type: png u33 ext FFT.png (34.3 KB, 85 views)
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Last edited by rjm; 12th May 2013 at 07:12 AM.
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