hello guys​

attached is a preamplifier i'm using in my project!i want to feed the output of this circuit into PIC for ADC and then further programming! i have few problems with the circuit​

this preamplifier bias the output voltage to avarage half of the supply voltage, let's assume in my case to about 4.2V! the problem is that since i use mic to detect sound, this reference voltage of 4.2 varies with the sound detected by the mic( in other word when i detect sound of different frequencies and intensities this voltage varies up and down but when there's no sound on the mic it becomes stable) it varies in range about -,+ 0.1-0.9 added to the reference voltage.
is there any way to stabilize the reference voltage while detecting the sound? (using it to detect heart sound)
or does it affect on the process of ADC if it's unstable?​

another things, do i need to add buffer to the output of the circuit or no need since the active LPF gives me low impedance?​

how is the way to calculate the HPF filter that produced by the capacitors in the circuit? i need it to be about 30 at -3db and i want to know how to calculate it!​

last question, is my schematic ready to be fed into PIC or i have missing parts?​

the amplifier will be replaced by another type that accept 5V supply voltage so that i can feed it into PIC​

(the original preampilfier schematic has been givin to me by the member Audioguru)​

thank you​

 

this preamplifier bias the output voltage to avarage half of the supply voltage, let's assume in my case to about 4.2V! the problem is that since i use mic to detect sound, this reference voltage of 4.2 varies with the sound detected by the mic( in other word when i detect sound of different frequencies and intensities this voltage varies up and down but when there's no sound on the mic it becomes stable) it varies in range about -,+ 0.1-0.9 added to the reference voltage.
is there any way to stabilize the reference voltage while detecting the sound? (using it to detect heart sound)
or does it affect on the process of ADC if it's unstable?

Audio is an AC voltage. It causes the output to swing above and below the DC reference voltage .

do i need to add buffer to the output of the circuit or no need since the active LPF gives me low impedance?

The buffer is useless. Remove R7, R8, C3, C4 and U2A.
Your circuit does not have a lowpass filter. Make a 2nd-order Butterworth Sallen and Key lowpass filter that is fed from the output of the preamp opamp.

how is the way to calculate the HPF filter that produced by the capacitors in the circuit?

The formula for an RC filter is one divided by pi x R x C.
The mic is about 3.3k and it is in parallel with the 10k resistor so they are 2.5k ohms. The opamp has FET inputs so its input resistance is infinite. The two 100k divider resistors are in parallel for the signal so they are 50k ohms. The total resistance of 52.5k ohms and the 0.22uF coupling capacitor produces a highpass filter that is -3dB at 13.9Hz.

The 1k resistor is in series with the 10uf capacitor so it produces another highpass filter that is -3db at 10Hz.

The total of the two filters reduces the output to nearly -6dB at about 12Hz.

The gain is 1+ (20k/1k)= 21 from about 100Hz to about 200kHz.

is my schematic ready to be fed into PIC or i have missing parts?

Depending on which opamp you use, the output with a 5V supply will be about 1V to 4V and will be AC with a 2.5VDC bias.

 

Audio is an AC voltage. It causes the output to swing above and below the DC reference voltage .

so that means it doesn't affect the reading of ADC of the pic when it swings up and down! right?

The buffer is useless. Remove R7, R8, C3, C4 and U2A.
Your circuit does not have a lowpass filter. Make a 2nd-order Butterworth Sallen and Key lowpass filter that is fed from the output of the preamp opamp.

oh sorry, i think i made a mistake in drawing the schematic, C3 actually should be connected to the output of U2A as feedback to form 2nd-order Butterworth Sallen and Key lowpass filter! in this case should i have buffer or the active LPF gives me low impedance so that i don't need buffer?

The formula for an RC filter is one divided by pi x R x C.
The mic is about 3.3k and it is in parallel with the 10k resistor so they are 2.5k ohms. The opamp has FET inputs so its input resistance is infinite. The two 100k divider resistors are in parallel for the signal so they are 50k ohms. The total resistance of 52.5k ohms and the 0.22uF coupling capacitor produces a highpass filter that is -3dB at 13.9Hz.

The 1k resistor is in series with the 10uf capacitor so it produces another highpass filter that is -3db at 10Hz.

The total of the two filters reduces the output to nearly -6dB at about 12Hz.

The gain is 1+ (20k/1k)= 21 from about 100Hz to about 200kHz.

you advised me before to change input capacitor value into 0.22uF and also to change the feedback capacitor to ground into 10uF so that i can have total cutoff frequencies is about 25Hz at -3dB (last reply in this thread). http://forum.allaboutcircuits.com/showthread.php?t=34563
my need is to have a total of HPF about 30Hz out from the preamplifier as well as LPF

Depending on which opamp you use, the output with a 5V supply will be about 1V to 4V and will be AC with a 2.5VDC bias.

i'll use OPA2134 or OPA4134

 

Or..
You could just re-orientate R1 (the 1.0M ohm resistor that is referencing Vcc) so that it references common (ground).
..
I believe this resistor orientation is the root cause of many of your issues..
..
Dataman19

no because i want the output to be in a range of 0-5V so that the ADC of the pic can read the voltage.

thank you

 

so that means it doesn't affect the reading of ADC of the pic when it swings up and down! right

I don't know what your ADC will do.

oh sorry, i think i made a mistake in drawing the schematic, C3 actually should be connected to the output of U2A as feedback to form 2nd-order Butterworth Sallen and Key lowpass filter! in this case should i have buffer or the active LPF gives me low impedance so that i don't need buffer

Then the corrected lowpass filter is a droopy Bessel not a sharp Butterworth.
An opamp has an extremely low output impedance so a buffer is not needed.

you advised me before to change input capacitor value into 0.22uF and also to change the feedback capacitor to ground into 10uF so that i can have total cutoff frequencies is about 25Hz at -3dB (last reply in this thread).

But why didn't you continue in your last thread? I don't know which sounds you have and why you are blocking high audio frequencies.

my need is to have a total of HPF about 30Hz out from the preamplifier as well as
LPF

Then do it.

 

i think mine is butterworth filter!! i'll double check it again!

the sound is heart sound n i want to have HPF with fc=30Hz,
i want to know, is it possible to have about 30hz at -3db in this preamplifier without affecting the biased output reference voltage? if yes show me the way and i'll calculate it myself.

thank you

 

You are not following what I am saying...
Sure I get the 5VDC Reff for the ADC...
,..
But the 1.0M Resistor is sourcing the 5VDC VCC to the Microphone.
..
Are you doing this to provide power to the Microphone?
..
If So - you are letting the Microphone Vary the VCC at the audio rate.
..
This will cause all kinds of problems..
..
R1 the 1.0M Resistor has no bearing on the ADC input - that is determined by the VCC and the Output from Pin 1 of U2. But the 1.0M resistor is alowwing the microphone to vary the VCC - therefore indirectly influencing what the ADC sees in the output of this preamp.
..
Just what is your intended function of R1 (the 1.0M resistor)?
..
If it is to set the load for the mic - then you are not doing this. Referencing the Mic to the VCC is not setting the load - it is causing VCC stability problems.
..
If you want to reference load the Mic - then R1 should go to common/ground.
..
This is what I am saying. Your preamp would make for an interesting SFX preamp. Incidentally, SFX is Special Effects.
..
dataman19

oh i'm confused here, R1 is not 1M, it's 10K ohm!!
do u mean that i shouldn't connect the mic to the resistor R1? and instead of that connect R1 directly to ground?? this thing might affect the mic output right?

 

Dataman,
What are you talking about?
R1 is not 1M, it is 10k. It powers the Jfet transistor inside the electret microphone.
R1 is decoupled from the supply by R4 and C5 and the signal from the mic is very low level anyway (10mV).

Alarassi,
Heartbeats are around 16Hz so a highpass filter at 30Hz is too high. Use 10Hz or less for less tilt in the waveform.
A 2nd-order Butterworth lowpass filter has its feedback capacitor double the value of thew capacitor to ground but your capacitor are the same so it is a droopy Bessel filter. A Butterworth filter can use the same R and C values if the opamp has a gain of 1.6.

The output is AC audio so the output voltage goes above and below the output DC reference voltage.

 

Heartbeats are around 16Hz so a highpass filter at 30Hz is too high. Use 10Hz or less for less tilt in the waveform.

i really appreciate your help but i really need to know how to calculate them to have the total value at -3db!
is it by followint the normal formula of RC passive filter ? or it has another way!

 

One RC filter has an output at its cutoff frequency that is down -3dB which is 0.707 times the voltage or half the power.
Two RC filters that are buffered so they do not affect each other have a cutoff frequency that is down -6dB which is half the voltage or 1/4 the power and the response is droopy.

At the cutoff frequency the phase of the output is changed which causes the display to be tilted.

 

Wouldn't just eliminating C1 do the same thing?
Dataman19

Nobody knows the exact current drawn by an electret mic because they are all different so without a coupling capacitor the opamp might be cutoff or saturated.

If you use larger coupling capacitors then they will pass the low frequencies you need. You need a highpass filter if you have many earthquakes or have crazy people jumping around.

 

hello guys

i used the circuit given in the first page and it works really nice but something i couldn't understand! how the voltages get biased to half of the supply voltage!?
i know it's through the three resistors given there but what i mean is, what is the technique used? and how its calculations?
i tried to find on the net and understand but i really couldn't !!
could any one explain for me or at least guide me to any sources that i can learn from!

another thing, i didn't find the internal connections or structure for the mentioned amplifier on its datasheet!! i don't mean the connection of pis but the FET connections! how can i get it ?

thank u

 

ok guys, i searched and got the answer! simply by using resistive divider

another question! what are the charactiristics that make single OPA1234 or dual as good amplifier for audio application!? i know it's low noise and distortion and others but what i mean here is why it's low noise? what features they have to make them low noise op-amp?

thank you

 

I never designed an opamp. I just look at their spec's and select the opamp that works well in my application.

The old opamps like the 741 and LM324 were made for amplifying DC and low frequencies. They were noisy (hiss) and cut high audio frequencies that didn't matter. The LM324 and LM358 were designed for low power so their output transistors did not have enough current which produces crossover distortion.
Better opamps like the TL07x and OPAx134 were designed for good audio.

 

Heartbeats are around 16Hz...

Anyone with a heartbeat of sixteen times a second isn't going to be around long, 1.6Hz would be more reasonable.

 

Something vibrates at about 16Hz during each heartbeat. Maybe it is the skin trapped under the stethoscope head. The Dark Side of The Moon album begins with a 16Hz heatbeat. My 3" computer speakers just played it and had a difficult time producing 16Hz. My 8" speakers produce it fine. The album is 37 years old.

 

The song you reference has what sounds like a typical heartbeat, thump-thump, thump-thump of about 1.5Hz (count the seconds between), it's definitely not a 16Hz "heartbeat" which would definitely be ventricular tachycardia or ventricular fibrillation.

The 16Hz could be some resonance or artifact of the blood flowing into the atria and ventricals or along the arterial wall.

 

Since I had two blocked heart arteries fixed last year with angioplasties, my heart beats at about 58 beats per minute when I am not excercising. My home made electronic stethoscope plays the 16Hz vibrations of each heartbeat loud and clear.

 

So, as you know, 58 beats/minute is just under 1 Hz. Your perceived 16Hz vibrations are not heartbeats, but most likely blood movement heard as a vibration.

 

audioguru

i have made the circuit and connected it to PIC and i had the correct output! thank you

i'm thinking to have an extra feature on my project by adding a headset to listen to the heart sounds!
the one i constructed feeds the signal values into PIC but here i want to add a headset circuit so that i can listen to and watch the signals.

can i do that without affecting the current constructed circuit's values?
if yes, how can i do it?

thank you