I am sure that my microphone is biass in this circuit,beaucuse befure this circuit,I test Electronic stethoscope circuit , I could hear my sounds in 8 ohms speaker,but there were so much noise that I couldn t hear heartsounds . after that I test electronic stethoscope 2 , I couldn t hear my sounds(I mean my speaking sounds) and I couldn t hear heartsounds too but I put the stethoscopehead on my neck and I could hear some frequencies and when I rub stethoscope I coud hear it sounds,so I thing my problem is mic,(it has 2 wire) :'( ??? :( maybe there is problem beetwin mic and stethoscope head,Should I past and press them togather ? This is very urgently,I should finish it next week. thanks.