Hi AudioGuru, Thanks about the valuable informations. I was jz tring to calculate each op-amp's gain (necessary for my project report). Currently learning to use ADC and ATMEL8051 for the interfacing purpose. Will post my results here for comments if there's a breakthrough. I will use a standard electronic stethoscope which is sold on the market and compare the signals obtained from it with the electronic stethoscope 2 posted here. Try to calculate the percent root-mean difference between these 2 and analysis the quality of the circuit.