Electronic Stethoscope Audio Feedback Problem (Analog)

Hello everyone, I am in my final semester of an electronics technology program and I am building an electronic stethoscope for my project. I found a circuit (by audioguru) and modified it a bit. I incorporated more digital aspects so that more analysis can take place. Anyways, I build my prototype and right now I seem to have a large feedback. My speaker portrays a loud buzzing noise, but you can still hear a signal when you tap on the scope chestpiece (which the microphone is directly behind). I have had some ideas such as using an op amp with a higher CMRR (I am using a 90db version now). But I need more backround before I attempt my troubleshooting again. A thought for this was using my micro to delay the sound to get rid of the noise? I am not too sure how to approach this but any help would be awesome! I will post a circuit tomorrow when I am at school. Thanks.

you have a bad ground somewhere in the first amp section
hence the buzzing

post the schematic lets see

VSMVDD said:

you have a bad ground somewhere in the first amp section
hence the buzzing

post the schematic lets see

Click to expand...

This could be possible but I have checked the connections numerous times and the powers and grounds seem to be quite secure. Here is the circuit I am working with. If you have any questions regarding the circuit please ask. Thanks

"loud buzzing noise", this could be from the microproc - have you tried unplugging the PIC? . The other thing is that any hash on Vcc gets fed straight into the input of the opamp via the 4.7uF cap. Try putting a 1k ohm in series with R1 and VCC and decouple this point to earth via a 100uF cap.
Frank

You know chuckey I did find what my problems were. First of all, your right about putting a cap between Vcc and ground AND Vee and ground. It was my noisey power supply getting fed into my gain circuit. And it was at this point I was doing only the analog circuit, nothing to do with the PIC. That will be for my next thread when I try to program haha