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Audio amplifier using a preamp and an lm386 audio ampl

ARNAB MONDAL

Jun 28, 2016
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I had tried to build an audio amplifier using a preamp build out of a transistor and thereafter using lm386 for the audio amplifier. I had received a suitable response using an earphone. But when I use an 8 ohm 0.5 speaker the audio output gets distorted and also the sensitivity reduces. Also I am unable to eliminate the mains noise which distorts the output of the speaker. Along with it when I increase the volume control or the gain control too high i hear a sharp beep sound of 2.7 kHz. I would love to hear what improvments i could make in the circuit to make it sensitive as well as clear
 

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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Posting the same question three times will not garner you friends. Please don't do it again.
 

BobK

Jan 5, 2010
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Where did you get that circuit? Just use the one on the LM386 datasheet.

Bob
 

davenn

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ARNAB MONDAL

Jun 28, 2016
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While there is a gain of 50 the lm386 circuit works fine. When I use a gain of 200 there is a strange noise coming out when the volume control in the 3rd pin comes to zero. Also i am hearing a hum noise from all the ac supplies near the circuit. How do I eliminate it?
 

davenn

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When I use a gain of 200 there is a strange noise coming out when the volume control in the 3rd pin comes to zero. Also i am hearing a hum noise from all the ac supplies near the circuit. How do I eliminate it?

probably poor routing of wiring, lack of proper ground connections / use of screened cable etc

show us some sharp and well lit pics of your construction so we can see if there are some obvious problems or not
 

ARNAB MONDAL

Jun 28, 2016
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These are some of the modifications that I did in the circuit . But i am still getting acoustic howling. What can I do to prevent it dynamically. Also I want to increase the sensitivity to the extent that when I rub my fingers in front of the electret mic the speaker should be able to play it .

 

BobK

Jan 5, 2010
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Are you talking about feedback from the speaker to the mic? If you remove the mic, does the "howling" stop?

I would expect acoustic feedback with the mic that close the speaker if you have enough gain.

Bob
 

ARNAB MONDAL

Jun 28, 2016
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Yes this is happening . Even when I cover the speaker the howling stops . But the sensitivity is reduced and also I think that to eliminate the mains noise I need to eliminate the ground loops . How do I eliminate ground loops.
Also Can i use a simple op amp like 741 c to eliminate the noise where the inputs are a scaled version of the output and the preamp input
 

(*steve*)

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if your amplifier is picking up mains hum and it is battery powered, you are best off looking at options to shield the circuit. The wires from the microphone to the input are probably the most likely place to pick up hum, so using shielded cable here would be my first action.
 

BobK

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Why do you want a microphone within inches of the speaker?

The way to eliminate acoustic feedback is to use a directional mike and point it away from the speaker. Putting the mic in a housing can make it more directional.

Bob
 
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