electret microphone amplifier

I tried this circuit. but it doesn't work. Doesn't matter how loud I speak to the microphone there is no signal out of the speaker.

what is wrong with this schematics.

Thanks


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The value of the 200k resistor is 20 times too high. Use 10k ohms and feed it in series with 1k ohms from the battery and a 100uF capacitor from the junction of the two resistors to ground as a filter.

The value of the 100uf input capacitor is far too high and will take all day to charge.

The value of the 10k volume control is too low and will reduce the output level from the mic.

Your LM386 has a gain of only 20. Add a capacitor from pin 1 to pin 8 to boost the gain to 200.

The LM386 needs a supply bypass capacitor so it does not oscillate. Use 100uF to 470uf.

If your mic can hear the speaker then it will produce acoustical feedback howling.

thank you Guru. BTW could you explain how to choose the most optimum value for the capacitor from mic to the amplifier.

If I want to use LM380, the mic section would be similar wouldn't it.

Thanks Again.

XT.5155 said:

BTW could you explain how to choose the most optimum value for the capacitor from mic to the amplifier.

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The input resistance of the LM386 is typically 50k ohms. If the 100k volume control is turned down a little then the slider is halfway and 50k ohms is in parallel with the 50k ohms of the IC and the other 50k of the trimpot is in series with the resulting 25k making a total resistance of 75k for the 0.22uF capacitor to feed.
The mic and the 10k resistor thay powers it are about 2.5k. Then the total resistance affecting the low frequency response of the 0.22uF coupling capacitor is 77.5k.
The low cutoff frequency is one divided by 2 x pi x R x C so the cutoff frequency is 9.4Hz and the response is flat down to about 47Hz. If you don't like deep bass then reduce the value of the capacitor.

If I want to use LM380, the mic section would be similar wouldn't it.

Click to expand...

The LM380 does not have enough gain to be a mic amplifier. Its gain is only 50 and the gain of the LM386 with the optional gain boosting capacitor is 200. Maybe its gain can be increased with some positive feedback. it has an input resistance that is 3 times higher than the LM386 so the input coupling capacitor from the mic can have a lower value for good bass response.

thank you very much Guru.

Audioguru said:

the cutoff frequency is 9.4Hz and the response is flat down to about 47Hz.

Click to expand...

Ahha, the fundamental reason of capacitor is the frequncy response of the circuit.

Thank you sooo much

XT.5155 said:

Ahha, the fundamental reason of capacitor is the frequncy response of the circuit.

Click to expand...

No.
The fundamental reason for the coupling capacitor is to block the DC from the 10k resistor and the mic from going to the grounded volume control and into the 0V input of the amplifier. The value of the capacitor affects the low frequency response.

The output capacitor blocks the DC from the amplifier from going into the speaker. Its value also affects the low frequency response.

i want to know the design procedure for amplification of electret micro phone output.to give he amplified o/p to the adc.

Added after 6 minutes:

**broken link removed**
In the above mentioned link they have used bc549c translstor instead of that can't we use sL100 y?

b_manohar said:

In the above mentioned link they have used bc549c translstor instead of that can't we use sL100 y?

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Simply look at their datasheets:
1) The BC549 is low noise for use in mic preamps. It is available all over the world.
The SL100 is very old and has a fairly high noise level that is not spec'd. It is available only in Asia.
2) The current gain of the BC549 is typically 520.
The current gain of the SL100 is typically 80.

i want to know the design procedure for amplification of electret micro phone output to give the amplified o/p to the adc.

b_manohar said:

i want to know the design procedure for amplification of electret micro phone output to give the amplified o/p to the adc.

Click to expand...

an electret mic has 48VDC built into its electret material. a Jfet transistor inside the mic is powered from a resistor to a filtered supply voltage. the Jfet matches the extremely high impedance of the "condenser' parts that move in the mic to a much lower impedance.

You must learn about how transistors work and how to properly bias them to understand how the preamp circuit works and how to design your own transistor amplifier.

b_manohar said:

i want to know the design procedure for amplification of electret micro phone output.to give he amplified o/p to the adc.

Click to expand...

perhaps you could get an electret mic capsule, use plier to open the metal casing. learn what inside it. it work for me.

Added after 4 minutes:

Audioguru said:

The mic and the 10k resistor thay powers it are about 2.5k.

Click to expand...

(1/10k) + (1/x) = 1/2.5K

would "x" constant for all audio spectrum, and would it be the same for different kind of FET or JFET. which is used in mic capsule.

thanks alot for clearing the cloud in my mind

The electret mics I have used and measured are powered with a 10k resistor from 6V to 12V and draw about 0.5mA. Their impedance is about 3.3k ohms which is 2.5k ohms including the 10k resistor.

Then the frequency response of a half-decent electret mic is a flat line from 20Hz to 20kHz like this one: