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Electret mic and phone for heart and lung sound for telemedicine

rjenkinsgb

Well-Known Member
Most Helpful Member
Couldn't work out which cap you meant?

One of (2) labelled 1/16 or the one labelled 100/16. or...
Both stay in. Only the 1uF is near Q9.

NF1-Noise-Gate_mod.png
 

audioguru

Well-Known Member
Most Helpful Member
A high quality low noise OPA2134 dual opamp should not make the high level wideband noise in your recording when its gain is only 3.13 x 1.6= 5.0 times.
Maybe the noise is produced by your phone? Maybe the electret mic was 1 or 2 meters away from your chest? Maybe a big jet flew past?:)

Check its polarity then remove C10 from the noisy preamp and make the recording again:
 

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Ruidep

New Member
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.
ex Doctor my 9 v battery is to heavy to carry around your neck has any body has a 3v amp heartbeat, recorded thanks
 

doldett

Member
Sorry guys for a lack of update.

Been trying all week to mold silicone for plastic cap (poorly executed I know :facepalm: ).

IMG_20210903_140517.jpg

After putting these on my friend and I thought, sound during the video call was better.
Here is the breathing sound recording.

Check its polarity then remove C10 from the noisy preamp and make the recording again:
Sorry AG. Have not done what you are suggesting yet. Only check polarity of C9 and C10, which is correct.

ex Doctor my 9 v battery is to heavy to carry around your neck has any body has a 3v amp heartbeat, recorded thanks
I do not plan for patient or doctor to carry this around. It could be put on the table. Sorry cannot help you with 3V preamp circuit as I am not circuit design expert. If you manage to find one, please share it too ;).
 

Buk

Active Member
ex Doctor my 9 v battery is to heavy to carry around your neck has any body has a 3v amp heartbeat, recorded thanks

Pretty much every electret module pre-pre-amps the signal using a j-fet.

When passive headsets/microphones are connected to a smartphone -- apple or android -- that j-fet provides the only amplification prior to the signal reaching the phones input stage; and that ppa is powered from (at most) 2.9V.

The issues of distortion due to over-amplification don't only affect this thread, but also this one and this one and many more; and the cause is always the same (person).
 

doldett

Member
A high quality low noise OPA2134 dual opamp should not make the high level wideband noise in your recording when its gain is only 3.13 x 1.6= 5.0 times.
Maybe the noise is produced by your phone? Maybe the electret mic was 1 or 2 meters away from your chest? Maybe a big jet flew past?:)

Check its polarity then remove C10 from the noisy preamp and make the recording again:
AG. C10 has been bypassed and the voltage drop across R13 is 1.25 V. Mobile devices need to detect whether the mic is connected, I don't think that voltage could be at 0V as you had suggested.

Sorry for lack of updates recently, been occupied almost completely by other works. Still determine to finish this project :).
 

audioguru

Well-Known Member
Most Helpful Member
The mic preamp circuit I designed years ago used an 0.1uF input capacitor value that passed music frequencies, not heartbeat very low frequencies.
The preamp opamp can have a load no less than the input of an amplifier, not a speaker or headphones. The output capacitor was only 0.1uF to pass music frequencies to the 50k input of an LM386 power amplifier. Then the power amplifier drove the speaker or headphones.
 

audioguru

Well-Known Member
Most Helpful Member
Cancelled a double post.
Why can't this website remind me that I already posted it before I ate my lunch??
 

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