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t_ang4

Electronic Stethoscope

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Hi guys,
Sorry for this late post. This project has attracted a lot of attention but has also created much frustration and confusion, due to schematic and parts list errors in the original article. Let us stick to the original circuit using standard parts and having 9V batteries. Dpak showed a nice circuit but the LM387 is obsolete (discontinued in '98) and it cannot be powered with 9V batteries easily.
Corrections to the original circuit follow:
1) Connect pin6 of U2 to the junction of C3 and R7. This applies proper negative-feedback which dramatically reduces gain (and noise) and allows U2 to function as a 2nd order low-pass filter. Thanks, T_ang4 and Staigen.
2) Use an LM386 for U5 and swap its pins: pin5 is output and pin6 is +9V. The LM386 has built-in feedback and biasing to be used with its inputs as shown, and can drive an 8 ohm earphone. Thanks, Staigen and Mozikluv.
3) Change C2 to 4.7uF (the + lead to the mic) so that it can pass a heartbeat sound. Listen to Pink Floyd's "Dark Side Of The Moon" rock song since it starts with a 16Hz heartbeat. The original circuit's gain is 'way down at 16Hz.
4) Get rid of U3 which doesn't do anything. U2 can easily drive U4 and the volume control.
5) Get rid of R3 and R9 and replace them with wire . They also don't do anything.
6) Use a TL071 (or a dual amp TL072) for U1 and U2. It is low-noise and inexpensive. Or use a quad amp TL074 for U1 to U4.
7) Disconnect the junction of R1 and C1 from +9V and add a 1K resistor from their junction to +9V. This will filter the amplifier's input from the bouncing +9V power supply. Add an additional 1000uF capacitor fom +9V to ground. This helps the battery provide power to U5.
8) Add a 1K resistor across the ouput jack. This will stop a loud "pop" when you plug-in your earphone if the stethoscope is already turned-on.
If the mic is properly mounted in a stethoscope-head (jar lid or whatever) then it should reproduce a heartbeat sound well without much background sound. If breathing sounds must be heard then change R5 and R6 to 1K resistors, but background will be louder, and keep the mic away from your earphone to avoid howling. Add a switch to hear either sound properly.
Please reply if you make these mods and let us know how it works.

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CDAK, thanks for coming back with a scope diagram. It is not really all that bad. A scope will make things look ugly when they are not perfect. Does your circuit have a little better looking signal when not amplified as much?

You might be able to clean this up a bit with an expandor circuit and then trim down the overall gain. An Expandor circuit will amplify the louder parts and quiet down the softer signals. Then you would not have as much problem with the other sounds.

One such chip is the NE570D compander. This compander chip has two sections which can be wired as an expandor or a compressor. The word compandor or companding a signal refers to using a compressor at the input of a device and then an expandor at the output of the device to return the level to normal. In the process the noise is reduced. I do not think you would need to do all of this. Perhaps only the Expandor part would be needed. I have used this a lot in music equipment that I have designed to get rid of inherent noise. More importantly, background noise. This is not a noise gate. This is completely different. A noise gate will turn your sound down until it reaches a preset threshold. An Expandor increases the dynamic range. It gives you more db distance between the background noise and the applied signal.
Here is a data sheet: http://www.semiconductors.philips.com/acrobat/datasheets/NE570_3.pdf
Hope the info is helpful. It might keep you from having to redesign.
I think there is an application diagram in the data sheet. If it is not clear and you want to go this route, let me know and I will post a diagram.

audioguru: How are all of these changes "sticking to the circuit?" ::)
Also, have you built the circuit with the changes you mentioned or is this in the theory stage? I think this is important to know.

MP

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MP,
Well, let's see: Electronics-lab posted an audio circuit which has 2 op-amps running open loop without any feedback, an input capacitor which can be calculated to severely cut the desired sound, an output stage without proper bias and many complaints that it doesn't work. The author of the circuit no longer supports it and apparently hasn't even tried it.
I am just trying to be helpful in assisting members, getting this circuit corrected, improving it with newer parts and pruning the useless parts. Otherwise, the next guy who tries it will also be frustrated.
The 8 changes that I recommend are quite minor, physically. The original circuit is basically the same, but corrected.
If I had a need for this circuit then I would build it, but for now it is experience and attention to detail that is talking.

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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?

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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.

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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

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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.

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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

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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.

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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

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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

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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?

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Guest 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?

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MP,
I haven't yet started further work in the project. It may take a few weeks to restart. (I finished my course in the current institution and may go back start working in another environment. So it may take few weeks to come back to the project again.) I shall inform every step in the enhancement work. The first thing I would try will be ur expander circuit. Then a notch filter ( audioguru also suggested it, thanks). I had this notch filter idea, but since I had a long signal processing left in that work, I left the hardware there.
Reagarding the HPF, I haven't used any amplification and even without that it worked well in MATLAB ( .wav format keeps the value between +1 and -1).
Audioguru, I don't know why it is obsolet if somebody uses an available chip ( atleast it was available to me)...
CDAK

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LM387 is still available through digikey. I have not tried other sources, but I am willing to bet that they are also available through other vendors. Also, many parts which go obsolete in the US and Canada are still widely available in other countries. Obsolescence of a part is many times dependent upon profit margin of a vendor.

I see two changes made here by CDAK:
1. HP filter added, but CDAK states that the signal was not very different from it.
2. Pre-amp was not used.

If this is so, I do not see why the circuit in it's original form would need to be replaced. Perhaps a second circuit which is different.

MP

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MP,
Digikey won't sell me an LM387 (quantity available = 0) and neither will Newark Inone. Why would anyone want to use it now, without any manufacturer second-sourcing it and with its non-standard pin-out?
There is nothing special about it in this circuit, a standard low-noise opamp is fine instead. Cdak used it only because he had some.
Answering your last post:
1) His external HP filter made a BIG difference because it attenuated hum pickup, as is shown in his graphs.
2) He didn't use a preamp? Of course he did, he used his LM387.

I mentioned replacing the original circuit because it has errors (which are easily corrected) and therefore does not work. Nobody is correcting it.

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audioguru, there are different ways to look at this. From a manufacturer's point of view, yes, you would not build a product with questionable parts availability. However, a hobbyist would have no problem with it for a one up circuit. As CDAK has pointed out, in his application, he had availability to several.
I took his previous comment that he did not use amplification to mean that he did not use the preamp ckt. As I read it again, I think he means he did not use post amplification.

Digikey will not sell the LM387N-ND but they do still offer the LM387AN-ND pin for pin compatible last time I checked.
Partminer gives me an availability of just under 400 pieces.
Dial Electronics in UK has both versions LM387N and AN with different prices. I do not know how many they have.
I am sure a serious search will bring up many possibilities.

MP

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someone builds it, fixes the part that does not work, posts the fix. This has been done with other projects such as the voltmeter project.

MP

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