ground loop problem caused by distributing audio into midi pitch tracker and usb sound card.
audio in ---> audio transformer --> midi pitch tracker
             ---> audio out

links
http://www.tkk.fi/Misc/Electronics/circuits/audio_isolator.html
http://nl.farnell.com/jsp/search/productdetail.jsp?SKU=1610409
http://www.monacor.de/typo3/index.php?id=84&L=1&artid=1868&spr=EN&typ=full

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for Richard Graham