This is a common problem for speakers of southern Chinese dialects like Cantonese. See my web page "http://www.chass.utoronto.ca/french/as-sa/PEP2/phonetics/index.html" and click on "voiced/ unvoiced consonants." Then you can click on the sound files (recorded at 44,000 Hz, nice quality) and listen carefully, repeating, to hear these differences.
This should provide some practice.
Student or Learner