Electronic Stethoscope

[audioguru]

Cdak,
MP's expander circuit is a good idea. But your scope picture seems to show a continuous low-frequency.  Are you using shielded cable from your mic to avoid hum pick-up, and insulating the mic and its cable from the patient to avoid a ground loop?

[MP]

what are you talking about with 2 of the op amps running open loop? I am looking at this circuit to see what problems you have noted.

MP

[audioguru]

MP,
In the schematic, U2 and U5 are shown without negative feedback. Therefore, DC-wise, their outputs would be idling against a power rail since their enormous gain would amplify their offset voltage. AC-wise, any tiny signal (or noise) that is greater than their offset voltage (only a few millivolts) would therefore be rectified (causing severe distortion) and also be over-amplified.
U2 is fixed by adding a dot on the schematic at pin6 so that it connects to C3 and R7. Then it will have feedback and will be a classic "Sallen and Key equal-capacitor (requiring its gain of 1.6) Butterworth 2nd-order low pass filter" with a cutoff frequency of about 103Hz, as others have noted and as described in the project's text.
U5 is fixed by changing its part number in the parts list to an LM386 with its built-in feedback and biasing, and correcting the pin numbers on the schematic for its output and +9V. If U5 had feedback and its pin4 connected to -9V so that it could function as a 741 opamp, then it certainly would not be able to drive an 8 ohm earphone like the LM386 is designed to do.  

[MP]

audioguru:
I would have built this dofferently myself. However, I would have built it, tested it, and posted it at the time.
I see what you are talking about with the feedback in the schematic. I am not the author of this project and will not speculate what he meant to do. However, it should be noted that there is not always a feedback resistor used on an op amp and these are not open inputs. Sometimes op amps have a different purpose than amplification of voltage. Also, U3 is a buffer. If you get rid of this, the pot right after this will not work as intended. Especially if you replace R9 with wire. If you use a 386 power amp chip, you might as well get rid of all the op amps, because they would not be needed. The 386 can power a head phone without preamplification.
The C2/R2 combination determines the Low frequency cut off. The author has chosen .047uf and 2.2k, which dictates a 1.6KHZ low cut off. If you change the value of this capacitor to 4.7uf as you have suggested, the low frequency cut off will be changed to approximately 16 Hz as you have indicated. This will pass more low frequencies in the circuit, but I am not so sure that the heartbeat pickup is intended to duplicate the Pink Floyd imitation of a heartbeat. Medical equipment usually uses a higher frequency in the middle or lower end of the speech spectrum. Not in the lower end of the full hearing range. The low frequencies also bring in more hum and interference from other devices used in the room.

Lastly, let me say this: When someone finds a circuit that is not correct, I think that it is welcome by all to hear about it. But the changes that are welcomed by all come from someone building a working circuit and posting the fix for the circuit.

MP

[audioguru]

Hi, MP, thanks for your anaysis of the original circuit and your comments on my recommended fixes. But I am sorry to have additional corrections for you:
1) U2 can easily drive the 2.5K volume control. The 741 is spec'd with a minimum load of 2K. The 2.5K load on U2 will not affect its gain nor filtering function since it has an output impedance of 75 ohms divided by its feedback of about 30,000 (at 16Hz) = 2.5 milli-ohms. Therefore the effect of a 2.5K load on its output is very, very small. Since U2 can drive the volume control then U3 is not needed.
2) What is the function of R9? Maybe it will limit the loading on U2 from the input of U4. But the input resistance of U4 is 2M ohms, and its input current is almost nothing (nano-amps)! Therefore since R9 does not limit anything, it can be replaced by wire.
3) If the entire circuit was replaced by an LM386 (with its gain-adjust pins connected for a gain of 200) then it will be noisy, without having a sharp-cutoff low pass filter. The LED driver might also be missed but I doubt that it will work as intended (thump is red, thomp is green) since that would require DC coupling at the input. The LED probably just flickers with each heartbeat.
4) With the original low-value for C2 then the output would sound like a "tic" instead of a "thump" or "thomp". Low frequency response is needed to reproduce a true replica of a heartbeat sound. Even a mechanical stethoscope makes a sound like "boom-boom, boom-boom" (have you tried one? I have). Look again at CDAK's professional-equipment's output which shows a very low-frequency "thump".
I am not going to build a working circuit and apparently neither are you, so maybe the defective original one should be replaced in the Projects Section with CDAK's working one.          

[MP]

There are problems with that analogy. You are missing the point and becoming argumentive. I am not going to waste my time arguing with you about this simple amplification circuit. As with my other post, if you have something to offer here, then build it. If all you have is theory, put it in the theory category. Because so far, all you have offered is theory. If you want to offer a different design, draw it out and post it.
It is simple.

MP

[CDAK]

Sorry once again for the late reply. Thanks a lot MP and audioguru for the healthy discussion between u. Here I give my reply to some of the questions asked  by u during ur discussion.
           The e-steth I made works fine with the circuit modified by me. I don't know whether LM387 is commercially available or not, but it was available in bulk in my Institute lab. So I made use of it and got the  desired output.
            As I told earler, for the analysis work ( my work is not just an e-scope but the analysis is also there), I took the pre-amp out. I used sheilded cable from mic to amp input.
       I haven't done the body-grounding, but the low frequency interference is reduced with a 7th order digital highpass filter (cut-off at 60Hz) after recording the sound from the e-steth in .wav format.
        I've given a set of figures showing the sounds recorded and then the HPFed version.The first set shows a normal heart beat on the left hand side and its HPFed version on the right hand side. It's clear that the filtered version is almost same as  that from a standard e-steth(second fig). The second figure is given only for the comparison purpose and here RHS figure is a single beat of the one on the LHS.

   

[CDAK]

The first figure

[CDAK]

The second figure (normal heart beat from a standard e-steth)

[CDAK]

      MP, till now, I was extremely busy with my thesis work and was in a struggle to get proper result. Finally,my work ended with a  success note. The credit goes to this forum too.
    I'll try ur expander circuit idea as I'm going to do more specialised work on the design of the stethscope part, in order to improve my diagnosing system.
     I'll inform the timely improvement of the system.
                                                           CDAK

[MP]

CDAK,
Thank you for the feedback on this project.

Also, if you would like to forward your modifications to the circuit, we will have them posted for the next person who is considering this project.

From your description, it seems that the circuit worked fine with removal of the preamp and a 60 HZ high pass filter added. Was there some amplification in the high pass filter that compensated for the removal of the pre-amp or was it simply not needed?

MP

[audioguru]

Cdak,
I am glad that you got your LM387 (obsolete) pre-amp working so well with an external 7th-order HPF used to reduce 50Hz hum pickup. Have you considered a simple notch-filter?
Are you still using the 9th order LPF that you mentioned earlier?
What frequency does the LPF cut-off at?
Do you think that an available TL071 opamp will work as a preamp here?

[alhilaly]

Hi bros.
I need a description about this circuit and how it works?
becouse ihave this cicuit as apart of graduation project as apre amp by using 741 op- amp to transmitte the voice signal and hert sound then receive it at(88-108Mhz)  can you help me?

[audioguru]

Alhilaly,
As you can see the many replies about this project, the schematic and parts list have errors. Until the errors are corrected it is awkward to give you a detailed circuit description.