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Emad

Electronic Stethoscope 2

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Hi Steven,
I am sorry to say the circuit won't work the way you changed it:
1) You have the volume control connected to pin 7 of the TL072 which is the output of its 2nd opamp, but that opamp's input at pin 5 is not connected to anything. For your "big ear", you don't need the low-pass filter which consists of U1b (you could use a TL071 single opamp for U1a if you change all of its pin numbers to match), R5, R6, R7, R8, C3 and C4. All those parts can be replaced by a piece of wire from pin 1 of the TL072 to terminal "a" of the volume control.
Disable U1b by connecting its pin 5 to ground and connect pin 6 to pin 7.
2) You don't have a negative 9V battery (Batt 2) connected to pin 4 of the TL072. It certainly won't work without it.
3) You don't have the battery bypass capacitors C7 and C8. The circuit will probably "motorboat" without them when the batteries run down a bit.

Then you will have a very simple circuit with U1a used as a low-noise microphone preamp, and U5 (the LM386) used as a headphones driver.

You know what? My son has a spare 61cm diameter satellite dish. I'll try my electret mic mounted in its focal point. Its diameter is much larger than the horn from your PA speaker so it should have very good pickup of distant sounds.
The trouble with it is that it is made of steel and is kinda heavy to carry around.

Since we have changed the topic, please open a new post in our forum "Electronic Projects Design/Ideas" and call it "Big Ear Microphone circuit" or something.

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:)did i make a few erors when i redrew your circuit audio guru i posted a big ear circuit recently but it suddenly stoped working again, it used to work good but i had to use a radio to receave it but id rarther something that can do it without the transmitter

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hi audioguru. i need to ask that the output of the circuit is sufficient to apply to a adc circuit. could i transfer the output to a pc in digital format. i mean i want to monitor the output signal of this circuit on pc. and second question; can i add a power amp circuit to output of this circuit to amplify better (like lm1875). is it possible? thanks alot already. ihope i can explain my questions well
sorry for the bad english ;)

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hi again. audioguru u wrote the cutoff frequency of LPF is 103hz but i read that the heartsound frequency range is 20 to 200 hz. and for medical care the cutoff should be 600hz(for listening lung etc.) what do u think?
i'd like to say smth about the lm358 circuit i built it up and i get a weak sound (from a 4ohm hop.)but no noise then i connected its output to a amplifier lm1875 but i couldnt get an output what could the reason is? could anyone send a amplifier circuit for lm358 pre amp circuit? and last one thing what kind of ecm is used i use a electret mic has very low value.
thanks much

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Hi Hakyman,
Welcome to our forum.

1) Your ADC probably has an input voltage range of 0V to +5V. The output of the Stethoscope-2 project swings about +3.5V to -3.5V. If you use the output pin of the LM386, the voltage will swing about from +1V to +8V, no longer negative but still too high for your ADC. Therefore if you attenuate it 8:5 with a 2-resistor voltage divider it will be fine.
You may want to use an active full-wave-rectifier circuit with a peak detector filter so that the signal to the ADC is a heartbeat pulse instead of the very low frequency vibration that is heard with headphones.

2) The output of the project is such a low frequency of from 16Hz to about 30Hz that you probably won't find speakers good enough and big enough to reproduce it. If you did use very good speakers then they probably must be located in another building to avoid acoustical feedback. The isolation of the mic is poor at very low frequencies.
Fairly good quality headphones sound fine. I used the "on the ears" headphones that came with my Kenwood portable CD player. Similar Sony headphones also sound fine. "Covering the ears" headphones would be best.

3) The cutoff frequency of the LPF is chosen to allow adequate heartbeat sound and adequate attenuation of background sounds such as people's voices. In the project, substitute LPF capacitors are discussed to increase the cutoff frequency to 1KHz to hear respiratory (lung) sounds. You could use a switch if the 1KHz cutoff frequency picks-up too much background sound when listening to heartbeats.

4) I have never recommended an LM358 circuit. It is too noisy for amplifying a microphone. I recommend using the low-noise TL072 dual opamp. Maybe you tried the LM358 circuit posted by Doctor Emad that has problems. Maybe your LM1875 didn't amplify it because of those problems or because capacitive coupling to your amp wasn't used.

5) I made a quick test with an electret mic from a cell phone, it worked well. Nearly any 2-wire electret mic will work in this project. They cost very little. My Newarkinone (Farnell) catalog lists about 20 having different sizes, with similar spec's and prices.

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Thank you so much audioguru for your kind and quick reply. i get the components. i'll share my experiments with you when finished and then ill add the circuits to apply an ADC circuit and try the output convert in digital. Thank You so much again...

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hi again i built the circuit but it seemed not so good. the filter part seems ok when i give a sound like sssssss it not sense to this but i gave a bass sound like boooo yeah it give an o/p. but when i heard a sound its have pıt pıt pııttpı i cant understand what it is and it get louder when i gave a louder sound(give my breath it has no sense to all standart voices in speaking). this result same in both a 4ohm 5W hop. and with a sony headphone(ear-in i havent get a earcover one yet). the reason is the load(hop or headphone)? and i only had a o/p stage as -/+ 1.5 V max. when no input(no sound) the o/p swings between +/- 20mV at this time my pot adjusted to 1.9K. i used a 5K pot no form as 2,5K pot in my land and i dont think it'll effect but if yes i can change it with a 2K pot . i took all leads as short as possible(the mic lead is about to 10 cm). any change or correction on circuit? pls help me who work it with no problem . i remember doctor emad and audioguru succesed.thanks for your helps again ...

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i' really sorry i forgot smth. i connect the mic to the stethescope's one ear. i couldnt cut the tube cause it is littmann and so expensive ;D whatever i got the heart sounds so clear but it has pıt pıt in the heart sounds i dont know how can i explain. it sounds like more sharp. i wish you get me audioguru what i mean. and its harmfull to my ears when i touch wrong place of steth. i get all the diaphram sounds and its really annoying, my ears ache cause of that. and last thing ;do i need offset the TL072 it has a null pin but some differences with 741 and i dont know sould i or how i do.
thats all i know i ask so many questions but this is important for me i struggle to do the best. im sorry so many many thanks again wait for answers...

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Hi Hakyman,
Happy New Year.
1) Did you build the Electronic Stethoscope-2 project or the original project?
2) What is a " pıt" sound?
3) What is a " 4ohm 5W hop"? The LM386 is not designed to drive a load impedance less than 8 ohms, it may be damaged by excessive output current or overheat.
4) The volume control can be any audio-taper pot between 2K and about 50K.
5) You shouldn't hear a loud sound when you touch the stethoscope head. Maybe you are hearing mains hum caused by the electret mic connected backwards so that its ground terminal is connected to the circuit's input.
6) The circuit doesn't need offset voltage nulling since the circuit's U1a plus U1B opamps have a DC voltage gain of only about 1.6. The LM386 also has a low DC voltage gain. The TL072 dual opamp doesn't have offset-null pins, just 3 pins for each opamp plus positive and negative power pins. The TL071 single opamp has the same pins as a 741.

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Hi Audioguru Happy new year too. i also checked the circuit(elec. steth.-2 project) and it really works good but i have problems with earphone i use a sony ear-in headphone and one of its ear burst. i still got some pit pats(pıt:)(seemed like this because my alphabet is not suit:) i checked the datasheet of TL072 and they named the o/p of first opamp in TL072(pin 1) as offset-null pin. i cant understand what it is(datasheet from ST microelectronics the producer site)i send the pin connection jpg. and i'd like to ask what is the purpose of peak detector using on converting the o/p of electronic stethescope to digital?
Thank You so muuuuuuuuuuuuuuuuuuch audioguru u helped me to get over this if you send me a good earphone it ll be perfect ;D thank u so much again ...

post-6280-14279142039649_thumb.jpg

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Hi Hakyman,
I am glad to hear that your Electronic Stethoscope-2 project really works good.
1) It is too bad that your broken Sony earphone makes a "pit-pat" sound that your language alphabet or fonts can't reproduce.
2) Pin #1 on a TL072 is "the output of its opamp #1".
3) I recommended rectifying the 30Hz output, peak detecting and filtering it so that each heartbeat will look like a DC pulse on a computer monitor's screen. I don't know why AC 30Hz is produced by the circuit when the heart makes a single DC pulse for each heartbeat. Maybe the skin is vibrating or the air under the stethoscope head is resonating.

I am sorry that I can't send you a good earphone. Just beg, borrow or steal one! ;D

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hi Audioguru
i see u r online and i'd like to ask that what kind of earphone u recommend?
thank for helps i dont know how thanks to you
i'm trying a new stethescope project with lf353 i ll send the circuit when i finished modyfying thank again ...

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Hi Again.
I've got some questions about audio application and the electronic stethescope-2 project components. i need the reasons of selection of the components.
1)why u choose theC7=1000uF and C8=470uF and what is the purpose of these components?
2)there are so many electret mic powering circuits in different values of components why you choose C1=470uF R1=10K and R16=1K what is the purpose?
3)U1a is a buffer and why we choose the gain about Kv=22 what if when we adjust the gain in lower or higher value?and why all buffer and booster circuits have negative feedback? why non-inversing input not select?To avoid the offset or the noise?
4)U1b is a low pass filter with the cutoff frequency 103Hz but why you choose this frequency why not 200Hz -- the max frequency of heart sounds?and if we choose the values like; C3=C4=100nF and R5=R6=1.5K(it gives the nearest cutoff value about 106Hz)i mean why you choose these values any reason for that?if i change them with the values i gave above, will be any differences?
5)what is the purpose of R7 and R8 why you need the feedback and how you define the values of R7 and R8?
6)why you dont use the bypass pin of lm386(pin 7)? what means bypass?
7)what is the reason to choose the rectifier circuit(C6 and R15) values why 1000uF why not another value why 15K.
8)what is the purpose of using C5=0.1uF and R14=3.9 as little value as this?what if we remove it or change?
9)has the filter part any effects on gain?
10)anybody calculate the matematical models of the circuit?
11)don't we need a High-Pass Filter.isn't be better with this ?
12)what if we change the LM386 with a different amplifier as 20W or 30W ?is the pre-amp part o/p can drive the power amplifier has a power of 20w or more ?
Additional Basic Questions;
1) what is a pre-amp and what is main reasons for using. which part it has got to have? buffer,filter,booster,...?
2)what is a booster what is its purpose?
3)what is buffer what its purpose ?
...
it goes like this ;D i want to know them totally i know some but i havent got a real background on audio electronics so i cant set up the relations efecting each other. Thanks to all who try to help me and answer my Questions thanks....
here is the schematic again if there is anyone not see before still

post-6280-14279142041233_thumb.jpg

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Hi Again.
I've got some questions about audio application and the electronic stethescope-2 project components. i need the reasons of selection of the components.
1)why u choose theC7=1000uF and C8=470uF and what is the purpose of these components?
2)there are so many electret mic powering circuits in different values of components why you choose C1=470uF R1=10K and R16=1K what is the purpose?
3)U1a is a buffer and why we choose the gain about Kv=22 what if when we adjust the gain in lower or higher value?and why all buffer and booster circuits have negative feedback? why non-inversing input not select?To avoid the offset or the noise?
4)U1b is a low pass filter with the cutoff frequency 103Hz but why you choose this frequency why not 200Hz -- the max frequency of heart sounds?and if we choose the values like; C3=C4=100nF and R5=R6=1.5K(it gives the nearest cutoff value about 106Hz)i mean why you choose these values any reason for that?if i change them with the values i gave above, will be any differences?
5)what is the purpose of R7 and R8 why you need the feedback and how you define the values of R7 and R8?
6)why you dont use the bypass pin of lm386(pin 7)? what means bypass?
7)what is the reason to choose the rectifier circuit(C6 and R15) values why 1000uF why not another value why 15K.
what is the purpose of using C5=0.1uF and R14=3.9 as little value as this?what if we remove it or change?
9)has the filter part any effects on gain?
10)anybody calculate the matematical models of the circuit?
11)don't we need a High-Pass Filter.isn't be better with this ?
12)what if we change the LM386 with a different amplifier as 20W or 30W ?is the pre-amp part o/p can drive the power amplifier has a power of 20w or more ?
Additional Basic Questions;
1) what is a pre-amp and what is main reasons for using. which part it has got to have? buffer,filter,booster,...?
2)what is a booster what is its purpose?
3)what is buffer what its purpose ?
...
it goes like this i want to know them totally i know some but i havent got a real background on audio electronics so i cant set up the relations efecting each other. Thanks to all who try to help me and answer my Questions thanks....
here is the schematic again if there is anyone not see before still

post-6280-14279142041434_thumb.jpg

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Hi Hakyman,
You have many questions, let's see if I can answer them.
1) C7 supplies positive supply current when the positive battery runs down and its internal resistance rises. The LM386 power amp draws a fairly high current during heartbeats and you don't want the supply voltage to fluctuate.
C8 does the same for the negative supply, except its current is less.
2) R1 is the drain load for the Field Effect Transistor inside the electret mic. R16 and C1 is a supply filter circuit to keep supply voltage fluctuations away from the circuit's sensitive input. Without this filter, the circuit might "motorboat" when the battery runs down.
3) U1a is a preamp with gain. A buffer usually doesn't have gain. The gain is not 22, but is actually only about 3.9 since the drain of the FET inside the electret mic is a high-impedance current source, therefore the opamp's input resistor is R1 plus R2. The preamp's gain of 3.9 plus the LPF's gain of 1.6 matches the output level of the electret mic to the fixed gain of 20 for the LM386. You can change the gain of the preamp by changing the value of R4.
Negative feedback in an opamp circuit reduces its output impedance and distortion, and makes the gain predictable. I think that the higher input impedance of a non-inverting preamp would work better in this project.
The low-noise opamp and fairly low-value resistors reduce noise in the preamp circuit.
The input coupling cap prevents the preamp from amplifying its small offset voltage.
4) The LPF works well with a cutoff frequency of 103Hz. You can change the frequency by changing the equal values of R5 ands R6, or C3 and C4. R5 and R6 should not be less than the 2K value that is the minimum load resistance for the preamp.
5) R7 and R8 can have any total resistance from 2K to about 1M. Their ratio must remain as shown to make the gain 1.6, which is what is required for an "equal-value resistors and capacitors, Sallen and Key LPF". The negative feedback in the LPF sharpens the cutoff so that it has a flat Butterworth response.
6) A cap at the bypass pin of the LM386 would reduce mains hum if a cheap power supply with ripple was used. It filters the input bias voltage of the LM386. Bypass means filter.
7) C6 and R15 are not a rectifier circuit. C6 couples the audio but blocks the half-supply DC voltage at the output of the LM386 from going to the earphones. R15 stops the earphones from making a very loud "pop" if they are plugged-in with the power already turned on (the earphones would quickly charge C6). The 1K value for R15 fully-charges C6 in 1 second.
The value For C6 is chosen to be a low reactance at 20Hz. Two paralleled 16 ohm earphones will cause a 3dB loss at 20Hz.
8) C5 and R14 prevent the output stage of the LM386 from oscillating at a very high frequency. The value of R14 could be as high as 10 ohms.
9) The LPF has a gain of 1.6 for the equal-value filter to work properly.
10) The circuits are very common and simple.
11) A HPF would attenuate the desired low frequency heartbeat sound. The very small value for C2 in the original defective project was a HPF.
12) The LM386 was chosen to drive low-impedance earphones to an adequate volume, with fairly low supply current and still operate well when the 9V battery voltage drops to 6V.
You wouldn't need more power to drive earphones, and speakers would cause acousical feedback.
The preamp can drive a load of 2K ohms or more. Therefore the input resistance of a power amplifier must be 2K or more, which has nothing to do with its output power.

Additional answers:
1) A preamp amplifies a very low-level signal to a line level without adding noise or distortion. Phonograph pickups also use a preamp.
2) You can boost (amplify) voltage or current. If both are boosted then power is increased.
3) A buffer has a high input impedance and a low output impedance. It prevents a low resistance load from loading down a high resistance source.

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Hey Audioguru You did again
thank you so much for your kind and quick reply again
i dont really know how can i thank to you thank you so much i'am gonna out of city for a week time. so i ll send the LF353(jfet) setehescope with a LM386 amplifier after i came back. see you
thanks for all

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hi audioguru. how r u
do you remember u recommended using a peak detector and a rectifer circuit to me to fix the output for an ADC. i tried so many times the peak detector circuits and i cant get the reason for using it. i use a adc0804. and the peak circuit just do the sampling duty like the ADC circuit can make it own. if i change the capacitor values of peak detector(change time duration) the circuit only help to find the max. peak of a cycle. and may i ask which point is the best for taking output to ADC.is the output point of amplifier(LM386) or output of the filter part. which one should i use. my amplifier(lm386) output is about 4.2V without a sound in the mic(change with the volume control pot.changes and it can up to 4.5V). and it swings with a heart sound around 4.2V(about 3.7-4.8) could i use the ADC directly and last one thing i use the 5.th pin of LM386 for output taking before 1000uF which its anot goes to headphones jack.
this is all :) i am very dogged to get the knowledge. if u can send me a circuit or just a block diagram i'll be so much happy and thanksfull. i search so much but i cant understand the death points of the circuits that help me find the circuits how to work. may be my english is not sufficient or may be their english is worse than me :)
whatever thanks already.

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Hi Hakyman,
I recommended a peak detector rectifier circuit to provide only a positive single pulse to your ADC during each heartbeat. The earphones output swings positive and negative and the negative voltage swings will damage your ADC. The direct DC output of the LM386 could swing up to 8V which could also damage your ADC unless you reduce it with a 8:5 voltage divider.

A simple positive peak detector circuit is attached. Connect its input to the earphones output of the Electronic Stethoscope-2 project. The ADC's input resistance must be at least 330K. R2 in the detector circuit can be reduced to reduce the length of its output pulse.

post-1706-1427914205912_thumb.gif

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hi,audioguru,
thanks for reply.i got the ic TL072.But i dont no the difference between dual opamp and odinary opamp.

i 've linear volume controller.you had suggested me to convert it to logarithmic by joining 3.3k resistance from slider to ground please give detailed idea about it.

i have an adapter which can supply 9 v in our circuit.you have used 2 9 v batteries if i want to use my adapter then what should i do?

what is varoboard?can i make this circuit on pcb?
waiting for your reply,
loveguru.

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Hi Loveguru,
1) I'm glad you got a TL072 which has two low-noise, low-distortion and wideband opamps inside, instead of the TL071 which has only one low-noise, low-distortion and wideband opamp inside. They cost about the same and also about the same or less than a noisy, distorted and narrowband old 741 opamp.
The TL072 is used for opamps U1a and U1b in the Electronic Stethoscope-2 schematic.
2) To make your 10K linear pot (used for R11 on the schematic) act similar to a logarithmic (audio taper) volume control to match your hearing, connect a 3.3K resistor from pin 3 of U5 to ground.
3) You need two 9V power supplies for this project. One for the positive 9V and the other for the negative 9V.
4) Veroboard (also called stripboard) has strips of printed copper running in one direction spaced 0.1 inches like the pins on ICs. The board is also perforated all over in a 0.1 inch grid. I use it for prototypes and for small quantities of circuits.
You can make the circuit on a PCB if you want. I am sorry that there isn't a PCB artwork and layout.

post-1706-14279142067419_thumb.jpg

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Hi Loveguru,
They are the same. Your TL072CP is fine.
TI used to call the plasic package "N", but now call it "P" like everyone else. Some of their parts in plastic packages still end with "N" like the TL074CN. It looks like all their 14 pins plastic packages end with "N".

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