Does 2 mic malfunctioning ?

MD MUBDIUL HASAN · Jan 4, 2018

Hello there,

I am going to develop a circuit for audio power amplifier using, https://www.ti.com/lit/ds/symlink/lm48511.pdf
Looking at the application circuit , I am using SD_AMP(pin5) input for Mic input at pin 16 IN+. Did -IN pin grounded.

Trying to suppress noise at the circuit in attachment 2 in 300Hz-3kHz audio amplifier, using https://www.ti.com/lit/ds/snas371c/snas371c.pdf audio amplifier.

Kindly, let me know whats wrong I am doing here in attachment 2.
Do you think,

1. Low frequency noise can be cancel properly?
2. Does the 2 mic malfunctioning ?
3. Discuss about its L and R signal from MIC.

audioguru · Jan 5, 2018

1) A mic preamp needs a gain of at least 100 but yours has a gain of only 4.4 times and the 0.01uF capacitor in series with the 4.7k resistor will feed only high frequencies at an extremely low level to the ear plug that is probably 32 ohms.
2) The 0.01uF capacitor from mic 1 to the 10k input resistor of its opamp cuts frequencies below 1600Hz. The 0.01uF capacitor from mic 2 to the 10k resistor cuts frequencies below 1600Hz.
3) Why are you cancelling most audio signals picked up by the mics?*
4) The Sallen-key lowpass filter cuts all audio frequencies. I changed it to cut frequencies above 2.9kHz, then speech will be difficult to understand and music will sound very muffled.
5) SD is Shut Down (mute).
6) The Mosfet will cause a very loud POP when it is turned on if you can find one that works with a gate input of only 3.3V. Control the SD voltage instead.
EDIT: Sorry, I wrongly thought the mic preamps had opposite phase.

MD MUBDIUL HASAN · Jan 7, 2018

Dear Sir Audioguru,
Thank you again for such nice solution.
I know you are the one who can understand this circuit well.
I saw it this sunday, but due to weekend I had no way to respond it.

Here we go,

A mic preamp needs a gain of at least 100 but yours has a gain of only 4.4 times and the 0.01uF capacitor in series with the 4.7k resistor will feed only high frequencies at an extremely low level to the ear plug that is probably 32 ohms.

Which part you have calculate the gain? At mic 1, 10k and 22k? If 0.01uF and 4.7k will not fit well, then what do you want me to feed ? This is SMT type 3.5mm phone plug .

2) The 0.01uF capacitor from mic 1 to the 10k input resistor of its opamp cuts frequencies below 1600Hz. The 0.01uF capacitor from mic 2 to the 10k resistor cuts frequencies below 1600Hz.

This information proves that MIC inputs are not more then 1600Hz? My allowable range is 300Hz to 3kHz, need to change the values?

3) Why are you cancelling most audio signals picked up by the mics?*

Could you kindly put some comment how the signals can be cancelled here?

4) The Sallen-key lowpass filter cuts all audio frequencies. I changed it to cut frequencies above 2.9kHz, then speech will be difficult to understand and music will sound very muffled.

I wish to make it for Human voice, like waki-tokey!

5) SD is Shut Down (mute).

Then you dont want to use SD_AMP pin ?

6) The Mosfet will cause a very loud POP when it is turned on if you can find one that works with a gate input of only 3.3V. Control the SD voltage instead.

How about to make a Tx signal by non inverting Schmitt trigger 74AHC1G14; take a look,

EDIT: Sorry, I wrongly thought the mic preamps had opposite phase.[/QUOTE]

audioguru · Jan 7, 2018

MD MUBDIUL HASAN said:
Which part you have calculate the gain? At mic 1, 10k and 22k? If 0.01uF and 4.7k will not fit well, then what do you want me to feed ? This is SMT type 3.5mm phone plug .

The gain of the first opamps is 22k/10k= 2.2 times and the gain of the mixer is 2 so the total gain is only 4.4 times.
Please learn the formula of a coupling capacitor. 0.01uF into 10k ohms cuts frequencies below 1600Hz.
0.01uF into the 4.7k ohms at the output cuts frequencies below 3400Hz. Also the 4.7k ohms at the output feeding a 32 ohms earplug attenuates the signal to 1/148th.

This information proves that MIC inputs are not more then 1600Hz? My allowable range is 300Hz to 3kHz, need to change the values?

Yes, you must calculate, not guess the capacitor values.

Could you kindly put some comment how the signals can be cancelled here?

I wrongly changed my previous post, the mixer does cause the mic signals to be anti-phase and cancel.
If you speak closer to one mic then your voice will not be cancelled much. Background noises will be cancelled.

I wish to make it for Human voice, like walkie-talkie!

Human speech produces consonants letters p, t, k, h, f, s, m and n that produce frequencies up to 14kHz. Also speech produces digraph sounds sh, ch, th and zh that also reach 14kHz. "Th" and other sounds are done different ways. The word "this" and the word "thin" have the "th" digraph sounded differently. When the high frequency consonant sounds are not produced then speech is difficult to understand.

Then you dont want to use SD_AMP pin ?

you can use Standby if you want.

How about to make a Tx signal by non inverting Schmitt trigger 74AHC1G14; take a look

I do not know why you need a tx signal.

MD MUBDIUL HASAN · Jan 7, 2018

The gain of the first opamps is 22k/10k= 2.2 times and the gain of the mixer is 2 so the total gain is only 4.4 times.

If its not enough, do you want to make it 100times?

Please learn the formula of a coupling capacitor. 0.01uF into 10k ohms cuts frequencies below 1600Hz.

Dont you mean this formula ?

0.01uF into the 4.7k ohms at the output cuts frequencies below 3400Hz. Also the 4.7k ohms at the output feeding a 32 ohms earplug attenuates the signal to 1/148th.

Then what do you think for good matching ?

If you speak closer to one mic then your voice will not be cancelled much. Background noises will be cancelled.

To make it better, do you have any idea?

Human speech produces consonants letters p, t, k, h, f, s, m and n that produce frequencies up to 14kHz. Also speech produces digraph sounds sh, ch, th and zh that also reach 14kHz. "Th" and other sounds are done different ways. The word "this" and the word "thin" have the "th" digraph sounded differently. When the high frequency consonant sounds are not produced then speech is difficult to understand.

Need to study more, but dont you mean 14kHz component should be included ?

you can use Standby if you want.

Actually, in this design I have connected both L and R signal together in 3.5mm phone plug as their behavior is almost same if you consider superposition, just use 10k with one of them.

I do not know why you need a tx signal.

This application will allows to speaker in some time to speak through, other wise it will go to main circuit. What do you suggest for de-bouncing in this circuit, diode 1n4148 is OK?, how about to use a RC filter after push button?

audioguru · Jan 8, 2018

MD MUBDIUL HASAN said:
If its not enough, do you want to make it 100times?

The gain needed should be calculated.
1) A 32 ohm earphone is very loud with 100mW. Then its AC voltage will be 1.8V. When you talk loudly 10cm away from an electret mic its AC level is 20mV. Then the gain needed is 1.8V/20mV= 90 times.

Dont you mean this formula ?

Click to expand...

Yes.

Then what do you think for good matching ?

Click to expand...

If one earphone is 32 ohms then two is 16 ohms. I calculated that 1.8VAC is loud and its peak voltage will be 2.55V so the amplifier's peak output current will be 2.55V/16 ohms= 159mA which is too high for an opamp, a little power amplifier like an LM386 must be used. A resistor in series with the earphones would waste too much voltage so it should not be used. A highpass cutoff frequency of 100Hz will sound good with voices so the coupling capacitor should be 100uF.

To make it better, do you have any idea?

Click to expand...

Yes, use an expensive noise cancelling mic.

Need to study more, but dont you mean 14kHz component should be included ?

Click to expand...

Yes. Normal clear speech includes all the consonants and digraph sounds that I listed that go to 14kHz. If you cutoff frequencies above 3kHz like an old fashioned telephone then speech is difficult to understand and the conversation will be full of "what?" or "say it again" or "spell it". Without the high frequency sounds then the words "sailing" and "failing" sound the same. If you cut high frequency noise then you also cut high frequency speech sounds.

Actually, in this design I have connected both L and R signal together in 3.5mm phone plug as their behavior is almost same if you consider superposition, just use 10k with one of them.

Click to expand...

Then you hear the same in both earphones. The resistor is not needed.

This application will allows to speaker in some time to speak through, other wise it will go to main circuit. What do you suggest for de-bouncing in this circuit, diode 1n4148 is OK?, how about to use a RC filter after push button?

Click to expand...

Your debounce circuit is fine without the diode. Aren't you going to use the LM48511 power amplifier with SD?

Click to expand...

MD MUBDIUL HASAN · Jan 8, 2018

The gain needed should be calculated.
1) A 32 ohm earphone is very loud with 100mW. Then its AC voltage will be 1.8V. When you talk loudly 10cm away from an electret mic its AC level is 20mV. Then the gain needed is 1.8V/20mV= 90 times.

If earphone facilities are built in the smart phone, then you will also consider the AC voltage level and gain? Those belongs to phone circuit.

If one earphone is 32 ohms then two is 16 ohms. I calculated that 1.8VAC is loud and its peak voltage will be 2.55V so the amplifier's peak output current will be 2.55V/16 ohms= 159mA which is too high for an opamp, a little power amplifier like an LM386 must be used. A resistor in series with the earphones would waste too much voltage so it should not be used. A highpass cutoff frequency of 100Hz will sound good with voices so the coupling capacitor should be 100uF.

Your explanation indicates my design would be poor, I am confused also in this sense that, LME49721 output current at RL = 250Ω, VS = 5.0V, about 9.3-9.7mA. May be you are ignoring LM48511.
What I have did is, I have tied those L and R signal together, connect them with 16th +IN pin of LM48511 with RC filer.
Other IN- pin is grounded.
Now all those voltage and current sensing will handle by LM48511, results will be show on +LS and -LS.

Yes. Normal clear speech includes all the consonants and digraph sounds that I listed that go to 14kHz. If you cutoff frequencies above 3kHz like an old fashioned telephone then speech is difficult to understand and the conversation will be full of "what?" or "say it again" or "spell it". Without the high frequency sounds then the words "sailing" and "failing" sound the same. If you cut high frequency noise then you also cut high frequency speech sounds.

I am still keep in mind.

Then you hear the same in both earphones. The resistor is not needed.

Good suggestion.

Your debounce circuit is fine without the diode

Dont you consider the recovery time of diode after getting an bouncing from switch?

audioguru · Jan 9, 2018

What are you making? You have two mics in a noisy room and stereo headphones connected for mono.
You hear your own voice until you press the TX button then you do not hear your voice when transmitting?

The debounce circuit can be made with one resistor, one capacitor and the Schmitt trigger. The capacitor immediately charges fully on the first contact and discharges only a small amount if there are contact bounces, then discharges completely when the button is released.
When the Mosfet turns on it severely changes the DC bias on the opamp following it causing a very loud POP sound. Another loud POP sound will be made when the Mosfet turns off. Add a coupling capacitor from the drain of the Mosfet to the audio you want to mute to prevent the POPs.

MD MUBDIUL HASAN · Jan 9, 2018

You have two mics in a noisy room and stereo headphones connected for mono

Headphone connection will slide in smartphone. Let me know where you can find noise here beside 2 mics, these are PDA type mic.

The capacitor immediately charges fully on the first contact and discharges only a small amount if there are contact bounces, then discharges completely when the button is released.

Nice description, dont you suggest the matched RC values.

When the Mosfet turns on it severely changes the DC bias on the opamp following it causing a very loud POP sound. Another loud POP sound will be made when the Mosfet turns off. Add a coupling capacitor from the drain of the Mosfet to the audio you want to mute to prevent the POPs.

I am also curious about POP sound you mentioned.
Then this MOSFET should be one which has a protection diode on source to drain!
Not sure why you are considering OPAMP dc bias with it. I have chosen 2N7002LT1G. Take a look, https://www.onsemi.com/pub/Collateral/2N7002L-D.PDF

audioguru · Jan 9, 2018

MD MUBDIUL HASAN said:
Headphone connection will slide in smartphone. Let me know where you can find noise here beside 2 mics, these are PDA type mic.

I do not know what "connection will slide in smartphone" means.
I do not know what is a "PDA".

Nice description, dont you suggest the matched RC values.

The switch quickly charges the capacitor the moment it makes contact then the capacitor stays charged during contacts bouncing. Then when the switch is turned off the resistor parallel with the capacitor slowly discharges the capacitor. One resistor and one capacitor, plus the Schmitt trigger so nothing is matched.

I am also curious about POP sound you mentioned.
Then this MOSFET should be one which has a protection diode on source to drain!

no, a protection diode will not help.
Please understand that ALL audio opamps must have their input biased at half the supply voltage so that the input can swing equally up and down. Then if the Mosfet shorts the input to ground and the opamp has a 5V supply, the input has a 2.5V drop causing a loud POP sound. When the Mosfet turns off the input of the opamp suddenly rises 2.5V and creates another loud POP sound. If you use coupling capacitors then the Mosfet does not change the DC but instead shorts only the AC then there are no POPs.

Not sure why you are considering OPAMP dc bias with it.

When the Mosfet turns on it shorts the bias voltage at the input of the opamp to ground.

MD MUBDIUL HASAN · Jan 9, 2018

I do not know what "connection will slide in smartphone" means.
I do not know what is a "PDA".

I am using a connector like, broken link removed
Take a look the MIC specification,broken link removed

The switch quickly charges the capacitor the moment it makes contact then the capacitor stays charged during contacts bouncing. Then when the switch is turned off the resistor parallel with the capacitor slowly discharges the capacitor. One resistor and one capacitor, plus the Schmitt trigger so nothing is matched.

Yes, correct. Capacitor discharge need a resistor path. I think we can change the push switch button in this way, need a resistor afetr 3.3v to switch,

The diode is an optional part needed only when the math goes haywire. It's possible, with the wrong sort of gate where the hysteresis voltages assume other values, for the formulas to pop out a value for R1 + R2 which is less than that of R2. In this case the diode forms a short cut that removes R2 from the charging circuit. All of the charge flows through R1. The equation still applies

, except we have to account for drop across the diode. Change Vfinal to 2.6 volts (3.3 minus the 0.7 diode drop), turn the crank and R1 pops out.

no, a protection diode will not help.
Please understand that ALL audio opamps must have their input biased at half the supply voltage so that the input can swing equally up and down. Then if the Mosfet shorts the input to ground and the opamp has a 5V supply, the input has a 2.5V drop causing a loud POP sound. When the Mosfet turns off the input of the opamp suddenly rises 2.5V and creates another loud POP sound. If you use coupling capacitors then the Mosfet does not change the DC but instead shorts only the AC then there are no POPs.

Auido amplifier LME49721 and the MOSFET works here at 3.3v,
I am using a LDO XC6222D331MR-G, https://www.mouser.com/ds/2/760/XC6222-837497.pdf

AnalogKid · Jan 9, 2018

Please explain carefully: What is it you are trying to do with this circuit?

Why do you have two microphones?

ak

MD MUBDIUL HASAN · Jan 9, 2018

AnalogKid said:
Please explain carefully: What is it you are trying to do with this circuit?

Why do you have two microphones?

ak

If you can take a deep look in the conversation, you will understand may be.
Circuit diagram and discussion might not useless what you are asking for.
Best way is to respond to the point.

AnalogKid · Jan 9, 2018

I read all of the posts in this thread and I don't have a clue.

To The Point:

Why do you have two microphones????????????????????????????????????????????????

ak

MD MUBDIUL HASAN · Jan 9, 2018

Why do you have two microphones????????????????????????????????????????????????

One is inside, other is outside.

AnalogKid · Jan 10, 2018

Inside and outside *what*?
Why are you subtracting the signals?

MD MUBDIUL HASAN · Jan 10, 2018

AnalogKid said:
Inside and outside *what*?
Why are you subtracting the signals?

Its a mixer circuit, take a look the idea ,

audioguru · Jan 10, 2018

If you subtract audio signals then sounds common to both microphones like background noise are cancelled.
But it is unlikely that inside and outside noises are the same.

MD MUBDIUL HASAN · Jan 10, 2018

audioguru said:
If you subtract audio signals then sounds common to both microphones like background noise are cancelled.
But it is unlikely that inside and outside noises are the same.

Yes, understand.
By looking at my last post reply did you find any problem related to MIC pop out and mosfet biasing?
All ICs are set to be with 3.3v.

Still now the circuit wont work properly?

audioguru · Jan 10, 2018

Why don't you understand that the opamp inputs must be biased at half the supply voltage so that their outputs can swing equally up and down?? But when your your Mosfet turns on it slams the input of the last opamp to ground causing a huge negative going POP sound, then another positive going POP sound when the Mosfet turns off and when the input of the opamp returns to half the supply voltage.
You do not want the Mosfet to disturb the DC, you want it to mute the AC audio so add a capacitor between its drain to the biased audio at the input of the last opamp.

3.3V is a very low supply voltage for an audio circuit because any loud sound will cause extreme clipping distortion.

Does 2 mic malfunctioning ?

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