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Re: How to amplify a the current throught a 8 ohm speaker

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Kemmy

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hi all,
I am doing a project about audio mixer. The final output from the mixer is around 0.2V and after than the signals get distorted and this voltage is not enough to drive my 8ohm speaker. So, i am trying to design a common collector BJT with 2N3904 so that it wont amplify the voltage but the it does amplify the current throught the speaker and i hope that the current throught the speaker is about 0.25A. It si possible? If yes can give me any tips
 
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Yeah, I'm confused too! How about using a high current DC amplifier to those speaker terminals? That ought to take care of your speaker issues.;)
 
Yes, now you've given us some information, you need a power amplifier - as suggested the LM386 is a popular choice for a simple low power one.

Why is your mixer going distorted above 200mV?, are you trying to drive the speaker directly? - you can't do that, but the mixer should be able to output a few volts into a suitable impedance load.
 
You should look at the datasheets for parts that you use. The absolute max current for a 2N3904 transistor is 200mA and its current gain at such a high current is very low. The datasheet shows that it will reach it max allowed internal temperature with a power of slightly more than only half a Watt.

This is about the simplest low-power amplifier that you can make that works:
 

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Kemmy said:
hi all,
I am doing a project about audio mixer. The final output from the mixer is around 0.2V and after than the signals get distorted and this voltage is not enough to drive my 8ohm speaker. So, i am trying to design a common collector BJT with 2N3904 so that it wont amplify the voltage but the it does amplify the current throught the speaker and i hope that the current throught the speaker is about 0.25A. It si possible? If yes can give me any tips

The output from the mixer is called (or used to be) line output. It is high impedance and is only designed to drive high impedance head phones or the input (high impedance) to an audio amplifier. If you have seen any audio amplifiers you see that they use dual power supplies and husky transistors with large heat for the out put stage. You are better off buying one IMHO. The proper wattage is determined by the efficiency of your speaker, the volume you need and the size of the room. How many people occupies the room also comes into the equation.
 
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