Electronic Stethoscope, Electret condenser mic distortion issues

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
Hi, I'm new to this forum. I'm looking to resolve an issue in my project related to electronic stethoscope. I'm using AOM-5024L-HD-R electret condenser mic and tl071 opamp based preamp circuit as show in the link below
https://dxarts.washington.edu/wiki/electret-preamp-circuit

circuit works fine. But i hear lot of distortion when i put my chestpiece on my chest. Is it related to the mic issue? also I hear clear voice when i speak from a distance. but when i speak closer to the mic i hear distortion. Is it fixable? I need good gain to hear heart murmurs, I can't compromise on that. using a mems based analog mic would fix this issue? Please help me. Thanks in advance.
 

Alec_t

Joined Sep 17, 2013
10,424
Welcome to AAC!
What sort of distortion? Overload (clipping)? Hum? Hiss? Loss of treble? Loss of bass? Rustling sounds?
What does the amp output drive?
 

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
Hello Alec, Thank you very much for your reply

I'm not sure about what kinda distortion it is, but I've uploaded a recording for reference.
https://vocaroo.com/i/s0wHaufE4XlR
First part of the sound clip is recorded by speaking very closer to the mic capsule and later is recorded little away from it. I've connected bose soundsport earphones to the output. Please let me know if you would like to know more details.
 

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
Welcome to AAC!
What sort of distortion? Overload (clipping)? Hum? Hiss? Loss of treble? Loss of bass? Rustling sounds?
What does the amp output drive?
Hello Alec, Thank you very much for your reply

I'm not sure about what kinda distortion it is, but I've uploaded a recording for reference.
https://vocaroo.com/i/s0wHaufE4XlR
First part of the sound clip is recorded by speaking very closer to the mic capsule and later is recorded little away from it. I've connected bose soundsport earphones to the output. Please let me know if you would like to know more details.

also would you think its actually op-amp saturation? increasing supply voltage to the op-amp fixes that?
 

AnalogKid

Joined Aug 1, 2013
8,132
Sounds like the gain is way too high. The schematic you posted is adjustable from 0 dB to 33 dB of gain. The electret element you are using is much more sensitive than most.

Turn the gain all the way down.
Increase it until you just hear a heartbeat, and mark that rotation.
Continue to increase the gain until you detect the faint beginnings of distortion, and mark that rotation.

Between those two points is your operating gain range.

Consider increasing the size of output capacitor C4 for better low frequency response.

Excellent job posting a recording. You saved about 30 postings.

ak
 

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
Sounds like the gain is way too high. The schematic you posted is adjustable from 0 dB to 33 dB of gain. The electret element you are using is much more sensitive than most.

Turn the gain all the way down.
Increase it until you just hear a heartbeat, and mark that rotation.
Continue to increase the gain until you detect the faint beginnings of distortion, and mark that rotation.

Between those two points is your operating gain range.

Consider increasing the size of output capacitor C4 for better low frequency response.

Excellent job posting a recording. You saved about 30 postings.

ak
hello AK, thanks for the reply.
I'm not too much into circuits. But I would need some significant gain for my purpose, meaning i need to hear loud heart sounds. decreasing the gain makes it feeble. is there fix for my problem? like changing opamp or changing mic or circuitry? Someone suggested that increasing the supply voltage to the op-amp would fix the problem(since opamp max rating is 0v-30v or +15v--15v dual). What do you have to say?
 

AnalogKid

Joined Aug 1, 2013
8,132
Yes, increasing the supply voltage would allow a larger peak-to-peak output signal. Since the circuit already is built, the easiest thing to try would be to use two 9 V batteries in series. Check the voltage rating of C1, C3, and C5. If they are 25 V or more, fine.

The TL071 datasheet does not give operating parameters for +9 V operation. Working with the +/-15 V tables and the charts, it looks like the max output voltage is going to be about 6 V peak-to-peak. That's a lot.

What is the output signal driving? If it is a small speaker or some other low impedance load, that is a whole 'nuther problem.

ak
 

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
Yes, increasing the supply voltage would allow a larger peak-to-peak output signal. Since the circuit already is built, the easiest thing to try would be to use two 9 V batteries in series. Check the voltage rating of C1, C3, and C5. If they are 25 V or more, fine.

The TL071 datasheet does not give operating parameters for +9 V operation. Working with the +/-15 V tables and the charts, it looks like the max output voltage is going to be about 6 V peak-to-peak. That's a lot.

What is the output signal driving? If it is a small speaker or some other low impedance load, that is a whole 'nuther problem.

ak
you mean to say make it 18v by adding two 9v in series? I can try that. But i'm little surprised how can a chest piece diaphragm movement causes so loud spl? I can undrstand that it happens when i speak closer to the mic. but with a chespiece ,that too my mic is connected to the chestpiece with a rubber tube of some length in between how is it producing loud spl ??

my output is connected to bose soundsport earphones.
 

jjw

Joined Dec 24, 2013
503
What is the impedance of the headphones?
If they have low impedance, then C4 is way too small and the low frequencies are attenuated.
Try C4 about 10uF
 

Sensacell

Joined Jun 19, 2012
2,449
Your recording seems to indicate that your circuit has too much gain and is clipping.

The problem is dynamic range, you might need an AGC system (automatic gain control) that adjusts the gain to maintain an un-clipped signal level.
 

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
Your recording seems to indicate that your circuit has too much gain and is clipping.

The problem is dynamic range, you might need an AGC system (automatic gain control) that adjusts the gain to maintain an un-clipped signal level.
hi thanks for your reply.
But I need lot of gain, do you recommend any workaround or a diifferent opamp?
 

Sensacell

Joined Jun 19, 2012
2,449
hi thanks for your reply.
But I need lot of gain, do you recommend any workaround or a diifferent opamp?
All that gain is not helping if it causes the signal to get clipped.
You need the "right" amount, this varies tremendously, depending on the circumstances.
 

Thread Starter

Dr.VPot

Joined Jan 24, 2018
11
All that gain is not helping if it causes the signal to get clipped.
You need the "right" amount, this varies tremendously, depending on the circumstances.
i totally get it but is there any other opamp that serves my purpose ? following are good and bad heart recordings. note:- ive use lm386 circuit next to the current circuit .
good recording:
https://vocaroo.com/i/s0JhRtwlWcGA

bad recording (distortion):
https://vocaroo.com/i/s0dmTSaBg0n1

the gain in the good recording is not enough for me. need more
 

dendad

Joined Feb 20, 2016
3,066
Try adding gain to your speaker amp and leave the mic amp alone. As has been said, too much gain will cause distortion/clipping so increasing the mic gain further will not help. If the lower gain gives you a clean signal, keep with that.
 
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