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Help needed

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I have successfully tried public address system using a microphone and a loudspeaker using LM386 so that whatever I speak in front of mic is heard clearly on speaker. Now I want to do speech processing on Matlab. For that the main problem I am facing is how to give the output of amplifier which is analogue to the computer through its port for microphone? Should I use 3.5mm universal jack? I also attach circuit diagram of the amplifier I am using.
 

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Grossel

Well-Known Member
Yes, a 3.5 mm jack is the standard way to connect audio to a PC. Remember to use the "Input" on the sound card (not same as mic input).

Aside from that, the problem is Matlab specific. How to use Matlab to read from audio input.
 
You mean to say that in the circuit diagram I have posted I should not directly connect output pin(i.e pin no 5) to the jack?
What is this sound card? Is it the one that is already there in computer? Like my computer already has two ports (one is green for output to headphones and other is pink for microphone input). Or something which needs to be connected externally besides this?
Talking about Matlab, I have heard that the sound can be saved as .wav through recorder(there are recorder software available) and then the file can be accessed through Matlab for processing.
 
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audioguru

Well-Known Member
Most Helpful Member
With the amplifier using a new 9V battery, its pin 5 is at 4.5VDC when there is no sound. With no load its pin 5 swings up to +8V and down to +1V. The +8V will destroy your computer because it is too high.
Make an attenuator with two series resistors from pin 5 to ground to limit the voltage to +5V or whatever is the max allowed input of your computer.
 
After the output capacitor(470uF) the DC level is still reduced I think it's around 2.5V.
Ok I will take care of that and using a simple voltage divider I will attenuate the output.But the main problem is how to feed the sound to the jack? Like the jack I have purchased has 3 terminals. So should I give this attenuated output(with peak of max 5V) of amplifier directly to this jack?
 
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audioguru

Well-Known Member
Most Helpful Member
The output pin 5 of the amplifier feeds the speaker through the 470uf output capacitor.
Pin 5 can also feed two resistors that make an attenuator so that the max voltage is +5.0V. The attenuated signal will be +2.5VDC when there are no sounds.

You said your computer has a mic input. Then its DC bias for an electret mic must be removed with a coupling capacitor and the attenuator must cut the signal down to mic level which is about 10mV peak.
 
The mic I am using is condenser mic.You mean to say that I should plug the JACK into the mic input of my sound card which my PC already has.Then to this JACK I should apply directly the output of my condenser mic without using LM386 amplifier? Or should I use LM386 amplifier and make an attenuator circuit at the output to reduce the amplitude of the output to 10mV peak?
 
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