newbieNoah

Hey Adioguru - whoo hoo!! ;D Finally got this circuit working! Got new opamps and carefully replicated my wired circuit into a Visio diagram thinking it would help you, help me, find my problems... and that process pointed out a couple of mistakes I missed! Now I think I need to work on isolating the mic better in the stethoscope head because slight movements of it hurts my ears! A little background is in order here... I too am expecting my first child and my wife has been very supportive (and eager) for me to complete it. She has just started her 3rd trimester. I can hear my heartbeat easily but I can't hear hers. I'm sure her heartbeat is faster and of course our baby's would be faster yet. Didn't try long because of the loud friction noises from the steth head but I wonder if I should do as you suggested to Dazza in reply #144? Is 3.3 uF for C3 and C4 a good place to start like Dazza did or are there better values that you'd suggest? Thanks again for your patient help! :)

Hi again, I'm sure you're right about the opamps, adioguru. Have ordered some more. Soon as I get them and replace them I'll let you know what happens. :)

Hi Audioguru, I just posted a long reply with an updated diagram but either the server hasn't had time to update it or it got lost. Having a rough night here :P. Think its time for bed. My last comment was the suspicion of an opamp being blown which I see you have just suggested. I hope that is the case! I'll check tomorrow to see if my updated diagram was posted. And I'll check out those opamp pinouts you posted. Thanks, again. Talk to you soon.

Hi Audioguru, yes - you're right about that diagram. I made it too quickly. But I didn't wire it according to that diagram. For building the circuit I used a printout of your diagram (its much more clear) and I made this erroneous diagram after the fact. I thought it would provide a better visual aide in the debug process if I put the voltages on the diagram itself. So with that in mind can you tell what might be wrong? :-\

Hi Audioguru, thanks again for your patience. ::)

Hi Audioguru, well I have a much cleaner looking circuit on my breadboard now. And I removed the optional 741-LED section. I have the wiper of the pot connected as you described but something is wrong somewhere :-[. So I made a pic of my circuit showing the DC voltages hoping you'll help me debug it! file:///D:/stethoscope/Steth2-aaron.jpg

thanks for the quick response, Audioguru I made the correction but then there was a slow steady oscillation going on that leads me to believe I made other errors. So I dismantled it and am in the process of putting it back together with a much cleaner layout. I had long leads and wires poking out all over the place. Gotta take my time and do it right. Soon as its done I'm sure I'll have more questions. Thanks again! ;D

Hello Audioguru, I'm new to this forum (and most forums) so please bear with me. I've built your circuit for the modified (corrected) stethoscope but am having problems similar to Dazza's reply #132. That being I can hear my heartbeat faintly with the pot turned down low but I can't turn it up very far before I get loud noise then nothing. I checked the voltage levels you suggested in reply #135 and they are good except for the 3rd check. Pin 5 of U5 is at 4.3 v when pot is turned down. But as I turn it up it gradually increases as it should until it jumps way high, then bounces all over - oscillating. The circuit is breadboarded and I checked the ground paths as you suggested to keep them separate for the in and out. I suspect I wired my pot wrong. I'm very rusty and unsure of it. The middle contact (wiper) I connected to pin 3 of U5. One of the other pot contacts I connected to pin 7 of U1b and the 3rd pot contact I connected to pin 3 of the 741. My headphones are over-the-ears with 32 ohms. I'm using a stethoscope head. Put in a Knowles NR-3160 electret mic into the steth head. I think I'm close to getting this working correctly but not quite there. Will you offer some more assistance? Or are you getting tired of this thread? :)