Electronic Stethoscope, Electret condenser mic distortion issues

[Dr.VPot]

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
http://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.

 

[dknguyen]

The circuit page says it doesn't handle an SPL (sound pressure level) that is too high. What you describe where things that are quiet come through clear and things that are loud do not matches this. You are feeding sounds to the mic that are too loud for the amplifier and the amplifier is saturating. Turn the gain down.

 

[Dr.VPot]

hello dknguyen, thanks for the reply. But the problem is we need a stethoscope with significant gain, decreasing gain wouldn't help our project. is there any workarounds for this? also do you think chestpiece diaphragm movement would produce that much sound pressure level?
here is the recorded sound for reference(its not heart sound though)
https://vocaroo.com/i/s0wHaufE4XlR

 

[dknguyen]

The datasheet says the op-amp can handle a power supply of up to 30V across across it's power pins. So you can change 9V to 30V, or you can change both AGND and 9V to -15V and +15V, respectively.

That will allow the op-amp to produce a higher voltage to correspond to the higher output volume without saturating. What was happening before was the
Input signal voltage x Gain > Maximum Output Voltage Possible

 

[Dr.VPot]

The datasheet says the op-amp can handle a power supply of up to 30V across across it's power pins. So you can change 9V to 30V, or you can change both AGND and 9V to -15V and +15V, respectively.

That will allow the op-amp to produce a higher voltage to correspond to the higher output volume without saturating.

I see. but my microphone has 10v as max rating, having a 15v would be a problem right? but i can try with +9v and -9v. so you think that distortion is due to opamp saturation? not related to mic itself?

 

[dknguyen]

Th

I see. but my microphone has 10v as max rating, having a 15v would be a problem right? but i can try with +9v and -9v. so you think that distortion is due to opamp saturation? not related to mic itself?

Then power your microphone off 10V and power the amplifier off 30V. Just make sure they share a common. Separate the voltage supplies between the amplifier and the microphone. Probably disconnect the top of R1 and C3 from everything so you can connect it to another power supply.

This op-amp doesn't have a common connection for a bipolar supply so it might be easier to just go 0V and 30V so that it can share the ground with the microphone. Or else you will have to use a couple of capacitors to produce an artificial common for the microphone if you use a bipolar supply on the op-amp.

To tell if it's the op-amp saturating and not the mic, measure the output of the amplifier on an oscilloscope and put a sound through it. If the output voltage is clipping at the op-amp's maximum output voltage swing then the distortion is probably from the op-amp. It's about Vcc - 1.5V to 3V according to your op-amp's datasheet.

If it's clipping at some other voltage below that, then it's probably the microphone.

BTW, if you try +9V and -9V in the circuit as drawn, you will blow your micropone since you will be putting 18V across the microphone. Maybe you should brush up on your electrical knowledge a little bit. If you use a bipolar supply, connect the microphone between 0V and +V, not between -V and +V if the difference is greater than 10V (unless your microphone is actually rated +/-10V, then it could take 20V across the terminals)

 

[Dr.VPot]

Th

Then power your microphone off 10V and power the amplifier off 30V. Just make sure they share a common. Separate the voltage supplies between the amplifier and the microphone. Probably disconnect the top of R2 from everything so you can connect it to another power supply.

This op-amp doesn't have a common connection for a bipolar supply so it might be easier to just go 0V and 30V so that it can share the ground with the microphone. Or else you will have to use a couple of capacitors to produce an artificial common for the microphone if you use a bipolar supply on the op-amp.

I would definitely try that, thanks. Were you able to hear the sound clip and you think its saturation?
there is a famous circuit by audioguru https://www.aaroncake.net/circuits/steth.asp , they used tl072 with +9v and -9v (it also has low pass filter). do you think that's better or would you recommend any other opamp or a different electret condenser mic instead, that serves my purpose best? also what's your take on MEMS mic? i was thinking of implementing this https://www.instructables.com/id/Electronic-Stethoscope-With-MEMS-Microphone-and-Re/
I apologize for lot of questions. take your time

 

[dknguyen]

I would definitely try that, thanks. Were you able to hear the sound clip and you think its saturation?
there is a famous circuit by audioguru https://www.aaroncake.net/circuits/steth.asp , they used tl072 with +9v and -9v (it also has low pass filter). do you think that's better or would you recommend any other opamp or a different electret condenser mic instead, that serves my purpose best? also what's your take on MEMS mic? i was thinking of implementing this https://www.instructables.com/id/Electronic-Stethoscope-With-MEMS-Microphone-and-Re/
I apologize for lot of questions. take your time

I'm not an audio guy so I don't really know. Your question and the circuit just seemed like a straightforward one to answer so I did. If you increase the input volume once distortion starts, the output volume doesn't get any louder right? That's certainly what it sounded like to me.

 

[audioguru]

The mic circuit I posted at Aaron Cake's site uses a dual opamp. One opamp is the preamp and the second opamp is a lowpass filter. The "stethoscope head" shields the mic from background sounds.
Your recording sounds like the first speech has the opamp clipping like crazy, increasing the supply voltage will make only a small improvement. Turn down the gain when the sounds at the mic are loud.
If you need high gain for weak sound levels then you need an automatic volume control (AVC) like is used in a sinewave generator circuit that uses a Jfet to adjust the gain. Adafruit has a mic with preamp and auto-gain control.

EDIT: The circuit might be clipping on loud sounds because it is overloaded. An opamp cannot properly drive a speaker or headphones, a power amplifier must be used like the LM386 in my mic stethoscope project at Aaron Cake's site.

 

[dknguyen]

The mic circuit I posted at Aaron Cake's site uses a dual opamp. One opamp is the preamp and the second opamp is a lowpass filter. The "stethoscope head" shields the mic from background sounds.
Your recording sounds like the first speech has the opamp clipping like crazy, increasing the supply voltage will make only a small improvement. Turn down the gain when the sounds at the mic are loud.
If you need high gain for weak sound levels then you need an automatic volume control (AVC) like is used in a sinewave generator circuit that uses a Jfet to adjust the gain. Adafruit has a mic with preamp and auto-gain control.

Can you elaborate why the clipping isn't resolved by increasing the supply voltage? Isn't it just because the op-amp wants to output more voltage than it has available?

EDIT: Ah, I see your edit regarding the power-amp.

 

[Dr.VPot]

The mic circuit I posted at Aaron Cake's site uses a dual opamp. One opamp is the preamp and the second opamp is a lowpass filter. The "stethoscope head" shields the mic from background sounds.
Your recording sounds like the first speech has the opamp clipping like crazy, increasing the supply voltage will make only a small improvement. Turn down the gain when the sounds at the mic are loud.
If you need high gain for weak sound levels then you need an automatic volume control (AVC) like is used in a sinewave generator circuit that uses a Jfet to adjust the gain. Adafruit has a mic with preamp and auto-gain control.

EDIT: The circuit might be clipping on loud sounds because it is overloaded. An opamp cannot properly drive a speaker or headphones, a power amplifier must be used like the LM386 in my mic stethoscope project at Aaron Cake's site.

Hello audio guru, i appreciate your reply.
I understand your explanation but my only surprise is why a chestpiece diaphragm movement is causing so much loud spl to clip opamp? isn't it very feeble?
Also my goal is to amplify heartsounds significantly loud and i'll be using only earphones. so will your popular circuit can serve my purpose? yes i would be interested in AVC, but are there any chips or circuits i could use for my prototyping? thanks in advance

 

[dknguyen]

The mic circuit I posted at Aaron Cake's site uses a dual opamp. One opamp is the preamp and the second opamp is a lowpass filter. The "stethoscope head" shields the mic from background sounds.
Your recording sounds like the first speech has the opamp clipping like crazy, increasing the supply voltage will make only a small improvement. Turn down the gain when the sounds at the mic are loud.
If you need high gain for weak sound levels then you need an automatic volume control (AVC) like is used in a sinewave generator circuit that uses a Jfet to adjust the gain. Adafruit has a mic with preamp and auto-gain control.

EDIT: The circuit might be clipping on loud sounds because it is overloaded. An opamp cannot properly drive a speaker or headphones, a power amplifier must be used like the LM386 in my mic stethoscope project at Aaron Cake's site.

Hello audio guru, i appreciate your reply.
I understand your explanation but my only surprise is why a chestpiece diaphragm movement is causing so much loud spl to clip opamp? isn't it very feeble?
Also my goal is to amplify heartsounds significantly loud and i'll be using only earphones. so will your popular circuit can serve my purpose? yes i would be interested in AVC, but are there any chips or circuits i could use for my prototyping? thanks in advance

I think Audioguru is saying that it's not a matter of the signal being too loud. It's that the op-amp doesn't provide enough current to drive the speakers or headphones.

In the chatroom you said you already tried my method of increasing the supply voltage and it still clips so it's not that the amp can't produce enough voltage for the volume, it's that it does not produce enough current to drive your speakers.

 

[audioguru]

Most opamps are spec'd driving a load no less than 2000 ohms. What is the impedance of your headphones? 32 ohms for each ear then in mono (both ears in parallel) the load is 16 ohms which is much too low for an opamp to drive.

 

[Dr.VPot]

Most opamps are spec'd driving a load no less than 2000 ohms. What is the impedance of your headphones? 32 ohms for each ear then in mono (both ears in parallel) the load is 16 ohms which is much too low for an opamp to drive.

hey AG,
I'm not sure exactly what's the impedence of bose soundsport inear headphones. but my project should work irrespective of headphones. a few updates i added lm386 to the circuit after my current circuit. it still distorts. I have a good recording and bad recording of heart sounds for you. both recordings were made with lm386 at hardly 5% i only varied tl071 circuit pot. the good recording gain is not enough, i need more gain but it distorts.
good recording:
https://vocaroo.com/i/s0JhRtwlWcGA

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

would you suggest any other opamp that gives me the gain i'm looking for without distortion? how about your stethoscope circuit? would that give me significant gain without distortion or clipping?

note:-i use lm386 circuit from here

 

[Colin]

2k2 is far too low for 9v supply. It should be 22k to 47k. I make electronic stethoscopes for safe cracking and you can also use a piezo diaphragm

 

[audioguru]

Your distorted recording has the amplifier clipping like crazy. All amplifiers clip when they are trying to produce more output level than is possible. You need an auto level control or compressor circuit but some circuits react too slowly then the distortion comes before they cut the level down. Maybe you have the mic touching and moving on the chest.
When I tested my electronic stethoscope I mounted the mic on a peanut butter jar plastic lid then the lid touched the chest and blocked background noise and the mic was not touching the chest. I was able to use an LM386 power amplifier and loudspeaker without having any acoustical feedback howling. My heart sounds were loud and the amplifier was not clipping.

Bose provides no spec's on their expensive headphones like frequency response, maximum power and impedance. They just say important things like the PRICE, color and weight.

 

[AnalogKid]

2k2 is far too low for 9v supply.

This 2.2K over here, or that one over there?

 

[Dr.VPot]

2k2 is far too low for 9v supply. It should be 22k to 47k. I make electronic stethoscopes for safe cracking and you can also use a piezo diaphragm

hello colin, thanks for replying

were you talking about the 2.2k in the tl071 circuit? if i increase the resistor value from 2.2k to 22k the gain drops drastically right? correct me if i'm wrong

when you said piezo diaphram, were you talking about piezo based microphone or sensor?

 

[Dr.VPot]

Your distorted recording has the amplifier clipping like crazy. All amplifiers clip when they are trying to produce more output level than is possible. You need an auto level control or compressor circuit but some circuits react too slowly then the distortion comes before they cut the level down. Maybe you have the mic touching and moving on the chest.
When I tested my electronic stethoscope I mounted the mic on a peanut butter jar plastic lid then the lid touched the chest and blocked background noise and the mic was not touching the chest. I was able to use an LM386 power amplifier and loudspeaker without having any acoustical feedback howling. My heart sounds were loud and the amplifier was not clipping.

Bose provides no spec's on their expensive headphones like frequency response, maximum power and impedance. They just say important things like the PRICE, color and weight.

Hello AG,
My mic is not touching chest at all, chest piece and mic has a rubber tube in between like any conventional stethoscope, sound waves travel through the tube from chest piece to the mic on the other end. I'm planning on to implement your circuit today. so even your lm386 didn't clip when you turn the knob to much? in my case even lm386 circuit alone without any preamplifier circuit is clipping after turning the knob 20-30% . My objective of the project is too use earphones at the end of the day, no speakers.

I've attached the chest piece setting of mine. It would be great if you could share your chest piece setting you talked about for reference. thanks

 

[audioguru]

were you talking about the 2.2k in the tl071 circuit? if i increase the resistor value from 2.2k to 22k the gain drops drastically right? correct me if i'm wrong
when you said piezo diaphram, were you talking about piezo based microphone or sensor?

The TL071 circuit has a 100k gain control labeled VOL. With the 100k trimpot set to 100k then the gain is 1+ (100k/2.2k)= 46.5 times which is good when you talk in a normal conversation level about 5cm away from the mic. With the trimpot set at half rotation then the gain is 23.7 times and you would need to talk a little louder or hold the mic closer to your mouth. With the trimpot turned down all the way then the gain is only 1 and if you scream very close to the mic then you might not be heard. Then why change the 2.2k to 22k??

ANY amplifier will clip if its input signal level is too much causes the output to try to go higher than is possible. An LM386 has a gain of 20 times but it can be 200 times if a capacitor is added between pin 1 and pin 8.
Your recordings had some very loud sounds like you were rubbing the mic in your hand or scraping the mic on the floor. When I hold my mic mounted in a lid on my chest then it does not move and does not produce those noises.

A piezo transducer picks up vibrations like from a guitar, piano or car engine. It makes a poor microphone.