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Pre-amplifier circuit for electret capacitor microphone

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irreversibleman

New Member
Hi,

I wanna design a circuit which collects sounds, whose frequency is constant in the range between 1-5kHz ,from the enviroment and filter and amplify this sound with audio amplifier.As you know ,the voltage of mic is in the range of micro volts ,hence what should I use to amplify this input considering noise with the coming input wave?A bjt or mosfet or directly an opamp.Is it necessary to power the electret mic.Can you suggest a circuit to make this job(pre-amplifying the mic output)?
 
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Hero999

Banned
Yes, a TL071 op-amp IC will do the job.

What output voltage do you require?
 

irreversibleman

New Member
I think for the output voltage of preamplifier milivolts is enough..I am new in this analog circuit design.Hence,I want to ask somehing.In the articles I have read, it is said that pre-amplifier usually comes before the amplifier.It is like a preparation for amplification ,I think as it is obvious from its name.My question is that: Why should I use pre-amplifiier to amplify the signal before the audio-amplifier?and do you have the TL071 circuit and can you send or give link of the circuit?
 

audioguru

Well-Known Member
Most Helpful Member
What will you do with the amplified signal?
You will have a loud howling acoustical feedback sound if the amplifier feeds a speaker. Sealed headphones might be OK.

This preamp might have enough gain but it has a frequency response from about 30Hz to about 20kHz.
 

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irreversibleman

New Member
Firstly, thank you for message.I will filter the output of the preamplifer and amplify it once more.Then, I set the same circuit once more using another electret mic and find the direction of the coming way comparing the phase difference like a radar.

@audioguru

As I understand from your circuit ,mic require no polarizing voltage, it requires power from preamplifier .And the gain of the opamp is adjusted by VR1 100k.What is the max gain of this circuit and the average load?Because I connect a bandpass filter to preamplifier and I don't want the filter load the pre-amplifier.Also,can you explain more what you mean by

"You will have a loud howling acoustical feedback sound if the amplifier feeds a speaker."

And can I obtain the input signal ,whose frequency is determined in advance, by filtering by a bandpass or two to be more precise considering the noise.I think I have asked lot questions but as I mentioned before, I am new :) and hungry to learn.
 

irreversibleman

New Member
@audioguru

After I amplify the output of the mic, I filter the this wave then again I amplify.As I mentioned before, I am new and I wanna know why I should amplify the signal after amplifying using pre-amplifier.Can I amplify the output of mic by using only audio amp or pre-amp?I set one more circuit same with this and at the end of these two circuit I find the direction of sound by comparing the phase difference.

Also,I wanna to ask whether it is necessary to power electret mic.I think in the circuit you send mic is powered.The gain of the circuit is controlled by Vr1 100k what is the max gain of this circuit?AS I mentioned before , I connect the filter after pre-amp.I don't want the per-amp to load the filter.Whta should be the load of filter?You said that how I should change the circuit to make suitable for the 1-5kHz.
 

TheVictim

Member
Electret mics contain a FET or other transistor that acts like a tiny preamp, it IS possible to feed a small power amp such as a 386 with just an electret mic.

Most of the time pre-amps and power amps are used in combination because of how transistor amplifiers work. The two commonly used types of transistor amplifiers are common emitter and common collector (also known as emitter-follower). Common emitter amplifiers produce a voltage gain, but no current gain (they also invert the signal, which is of no consequence for audio), so are very poor choices to drive a speaker or other load, but make great preamps. Common collector amplifiers have the opposite trait. They will produce a current gain, but the voltage remains the same (actually there is a tiny bit of voltage loss, but it can be ignored in most cases. Also, they won't invert the signal). In combination, a common emitter amp feeding a common collector amp produces a very large gain in power.

When considering IC's, virtually all of your Op-Amps will act as preamps, and you will need a power amp to follow it up for driving a load such as a speaker.

Edit: Yes, electret mics DO need power. Typically you'll see a resistor and cap on the positive lead of an electret mic. The resistor supplies the mic with the proper voltage and the cap blocks this supply voltage from the input of your amplifier while passing the audio signal through it.
 
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audioguru

Well-Known Member
Most Helpful Member
The preamp I showed has a max voltage gain of (100K/4.7K) + 1= 22.2. You might need a voltage gain of 500 for your application. The TL071 opamp is fairly wideband but frequencies above about 3kHz are reduced when its voltage gain is 500 or more.
Therefore I suggest using two opamp circuits in series, each with a gain of 22.2.

A bandpass filter can be made with a highpass filter followed by a lowpass filter. Sallen and Key 2nd-order or 3rd-order Butterworth filters work well.
 
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