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Emad

Electronic Stethoscope 2

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There is no distortion but just a sound like air or shshshshshshsh. the components around U1 are not the same like the original nor the corrected circuit . I had a diffirent circuit for U1 that Ill post it after I drow it . many many thanx .

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Hi Doctor Emad,
Why did you change so many parts of the preamp? Now it is nothing but a mid and high audio frequencies noise generator, as you reported.
1) The KIA358 (LM358 sub) opamp has a high noise level and crossover distortion. A low-noise opamp was recommended.
2) Its gain is too high so it will amplify its own high noise level and background sound too much.
3) Like the original circuit, the value of its C1 is much too low so it will attenuate low-frequency heartbeat sounds too much.
4) Its R1 and pin 3 are not decoupled from the supply so it may "motorboat", especially when the battery gets low.
5) C2 blocks the DC voltage reference that is required for the following U2 stage.
6) You probably didn't use the U2 filter circuit, whose function is to reduce noise and background sound.

Why did you do this?

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Hi Audaguru. I sent for the TL072 but I dont know it will be present or not. I told u before that I connected the output of the LM358 to the input of the LM386 using just two ICs and I have a satesfactory result. I removed some noise by adding 0.1uf between the variable R of U2 and the negative volt. I ask if I can put anoise removing circuit between the two ICS????? If I can, what is the suggested circuit????? THANX

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Hi Doctor Emad,
A TL072 or a KIA358 has two opamps inside. Your preamp circuit used only one of them. My latest schematic shows how to connect its pins for use as U1 and U2.

Before you said that your circuit just made a noise sound like shshshshshshsh. Now you say it was a satisfactory result. ???
Now you want to remove that noise. The value of your C1 was much too low so you used more gain which increased the noise. ???
The function of U2 (that you didn't use) is to remove noise!

While waiting for the TL072 low noise dual opamp to arrive, why not use an 8-pin IC socket for a dual opamp like my latest schematic and try the circuit with your KIA358? Then simply unplug the KIA358 and plugin the TL072 later.

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this is the preamplifier I used-----R1 R3 R4=10K----R2+1K--R5=100K to 1M-----C1=0.1uf ---C24.7uf


For me, I will divide R1 into two 5K ohm resistor in series and with a 22uF capacitor connected between these two resistors joint to gorund.

That will reduce much noise from the power source to the microphone.

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Hi Trigger,
The power source (battery) doesn't produce any noise. The noise comes from inside a noisy opamp, U1, and is amplified by its high gain.

If you divide R1 (10K) into two 5K resistors with their junction decoupled to ground, you will decrease the output of the microphone, because its high output impedance (current source) FET also needs a high impedance load (but too high will create resistor thermal noise).
You quoted Doctor Emad's own mic preamp circuit, with its very low value of R2 (to get more gain from U1) which loads-down the mic's output even more. Are you also going to use his very low value for C1 which attenuates the low frequency heartbeat signal? And the adjustable gain for R5 which makes the gain up to 21 times too high?
For me, it sounds like you are also building a noise generator like Doctor Emad did.

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audioguru,

Noope I am not going to build this circuit. Just quote a pass experience in handling condensed microphone circuit in manufacturing products (say for example, an microphone preamp stage inside an analog cmos camera connected to your VCR/TV).
The suggestion is to minimize the noise/ripples come from the path of the supply, which is generated by other parts of the circuit.
You may noticed "humming" sound at the later amplify stage if that modification hasn't included.

Remember, you got to eliminate the possible noise before tha amplify stage else no matter what opamp you are using, you are going to amplify both the required signal and noise..... and gradually make the circuit fails.

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Audaguru. I wonna ask about the function of U4 which is connected to the bipolar diod in this circuit . Can I make the circuit without it???????I think it produces flickering sound as light???

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hi Audioguru. I consrtucted the circuit with the TLO72 on a test board but there is just noise and nothing from the mic goes to the earphones. What is your opanion about this circuit in the site I put here , and can I use TLO72 insteed of NE5532 ?????

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Hi Doctor Emad,
Which circuit did you constuct? (We have many, now). I don't know why it doesn't work, when it worked before but was noisy. It should work fine and with less noise with the TL072.

The "auto" stethoscope that you posted is very similar to our corrected one, but uses a 3-wire electret mic and has much more gain (and therefore more noise). Its simple "tone control" noise filter is not nearly as good as our corrected U2 precision filter.
The NE5532 has lower noise than a TL072 but still has noise. I have used both for mic preamps and they sound the same (perfect).
Since the "auto" stethoscope circuit doesn't use a power amp, it must be used with high-impedance headphones.

Now that you have constructed our corrected circuit (I hope that you built that one), don't you want to troubleshoot it? There is probably something very simple that is wrong.

If you speak into the microphone, about 20cm away, do you hear your voice clearly in your earphones? The U2 filter will reduce high frequencies but your voice should still be heard.
When you turn its volume control down, does the noise reduce to nearly nothing?
What does the noise sound like?

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Hi Adioguru. I constructed your corrected circuit. It is more than excellant .No distortion , minimal noise and the sound is the same like a real stethoscpe. I can hear my heart beats and respiratory sounds laod and clear from above clothes . I dont know how can I thank you fore helping me . thaaaaaaaaaaaaaaaaaaaaank youuuuuuuuuuuuuuuuuuuuuuuuuuu

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Hi Doctor Emad,
I am happy to hear that you got your corrected project working properly and am glad to have helped you.
Thank-you for helping me prove that my ideas, corrections and simplifications work very well.
I will post my corrected circuit as a new project. ;D ;D ;D 8)

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Audioguru. Thank u again . I wonna tell u that I built the circuit without the IC which is connected to the bipolar diode.I wanted to decrease the size of the board and also the mistakes.

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Hi Audioguru

Congratulations! You have made a good stuff again!

By the way, I undersdtand that we have a university here conducting an R&D project on electronic stethoscope. It is said that the new device have many good features and would evetually replace the traditional mechanical ones ....

This is an intersting as well as useful project ! :)

Oli

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Hi Audioguru
I built the circuit named auto stethoscope. It works but your circuit is mutch better .
I want to ask about C7 and C8. You said 1000uf for C7 and 100uf f
or C8
I noticed that one battery became very week. What is the cuase???
Also could some components be changed to make the tone of the sound less huge?? Many Thanks.

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Hi Doctor Emad,
My new detailed "Electronic Stethoscope-2" project is here:
http://www.electronics-lab.com/projects/science/019/index.html
Please start a new topic about it.

I have decided that 470uF is good for C8, but 100uF will be fine.
The LM386 is powered from only one battery, so that one will wear out first. Alkaline batteries last much longer than ordinary or "heavy-duty" types.

I am not sure what you mean when you say, "huge sound". Maybe the sound has too much bass and is muffled. Maybe your headphones have a boomy sound. You can change C3 and C4 to 4.7nF (0.0047uF) to hear respiratory sounds better, but background noise will be increased.

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Hi Audioguru
You knoe I built what is called the corrected circuit using 2 TLO71 and LM386. It works well. Is the E.Stethoscope 2 the same. ?? Have you built it?? Which battery is connected directly to the LM386??That one connected to 470 uf or that connected to 1000uf. I wonna tell u that the circuit named E.auto stethoscope is bad,I asked u about it before.

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Hi Doctor Emad,
1) Your circuit with two TL071's operates the same as my new project with its TL072.
2) No, I haven't built it, I just wanted to help the many people that complained about the original faulty circuit here, and on the original author's website. I tried an electret microphone connected to an LM386 to confirm that the gain of my project is adequate and that a jar lid can be used as a stethoscope head. I have also studied and helped people with ECG circuits.
3) The "positive" supply battery is connected to the LM386 and to the 1000uF capacitor C7.
4) The "Automobile Stethoscope" schematic that you posted earlier operates poorly to reproduce heartbeat sounds because it uses a simple tone control instead of our circuit's precision low-pass-filter and does not use a power amp IC like our circuit to drive low-impedance headphones properly.

Did you try reducing the value of C3 and C4 to improve the tone of your "huge sound"?

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I tried 20 nf for the two and then100 but the heart sound became very fiant in the both attempts. The circuit is good with its original capacitors. I had put a link to your project in an arabic electronic forum. I dont know shold I have been first taken a permission from you or not ???/ If so I can delete it . her is the site:
http://www.arabteam2000-forum.com/index.php?showtopic=49466

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