Today, I have tried to build the circuit using resistors and capacitors i have while waiting for ordered parts to arrive. I was building preamp circuit with the gain of 21 based on below schematic. R1+R2+R3 are used to drop 5.6 V across them since my electret seems to draw only 0.33 mA rather than 0.5 mA. Keep in mind that the output jack is trrs for connecting to phone.
View attachment 132727
What I found was that I couldn't hear any heartbeat, recorded by voice recorder program on my phone, using the circuit above. However, if the electret mic directly connected to the trrs jack, the sound of heartbeat could be heard from the recording. Results seem to point the preamp circuit being the culprit. I am not sure why the preamp circuit cuts off all the heart sound.
It gives me concerns because the amplification is similar to the one of first stage of audioguru's circuit. If anyone knows, please enlighten me. I could also hear heartbeat using lavalier mic and phone. Below are sound recorded during my experiment.
this is a sound recorded using electret mic connecting directly to trrs jack
recorded using preamp circuit
bought lavalier mic with phone
I have compared with electret mic and lavalier mic and found them to be relatively similar in size. I am also expecting the similar performance. I have attached pictures on what my diy stethscope looks like. I have not yet tried adding diaphragm.