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I 'm designing a sound system consisting of an audio source (say MP3) as an input to an amplifying circuit which will provide the output signal with a certain gain. This output signal will go through 3 speakers.

This is the amplifier circuit enter image description here

What I need to know is : can those three speaker be connected to only one amplifier circuit or do I need a circuit for each one ?

What are the elements in the circuit do I need to manipulate in order to control the output signal in terms of increasing and decreasing the volume of one speaker and without affecting the other 2 speakers ?

note : speakers are 8 ohms and 0.25 watt each just for testing ... larger speakers will be used eventually.

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  • \$\begingroup\$ What is the power rating and impedance of the speakers? If you want independent volume control of one speaker you will need at least 2 amplifiers \$\endgroup\$ – Icy Nov 6 '15 at 8:27
  • \$\begingroup\$ I have 8 ohm and 0.25 watt small flat speakers for testing \$\endgroup\$ – Ahmed Samy Nov 6 '15 at 8:46
  • \$\begingroup\$ You could wire the Volume Control part of FakeMoustache's answer with the Wiper connected to your "Input Signal". Driving each speaker from one of these would let you control them independently. \$\endgroup\$ – Brian Drummond Nov 6 '15 at 11:14
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This amplifier will not be able to drive large 8 ohm speakers very loud. I conclude this from the two 5 ohm resistors present at the emitters of the output transistors. The amplifier will be sufficient to drive a small speaker like the ones most people use with their PC.

It will be able to drive three 8 ohm speakers in parallel but it will not be able to drive these very loud. Most power will be lost through the two 5 ohm resistors !

But why build your own amplifier from discrete components ? If you want the learning expercience then EXCELLENT :-) go ahead and build. If you want something a bit easier, there are ready-to-use modules on ebay like this class D type or this TDA2030 based one. These can only drive one speaker per amplifier but they are cheap. Also, these don't require the +/- 15 V but only a single 5 or 12 V supply.

Oh, and volume control is done like this:

audio volume control

This uses an LM386 audio amplifier, you can just replace it with the amplifier circuit from your question. Use a LOGARITHMIC potmeter (marked in red) to get nice volume control. For fun temporarily replace it with a standard LINEAIR potmeter and notice how the volume control is "crazy" ! :-) Why is that ? Because our ears are also sort of "logarthmic".

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  • \$\begingroup\$ It's for academic purposes ... So do I conclude from your answer that the two 5 ohm resistors will control the output signal power which will enable me to manipulate the volume of each speaker ? \$\endgroup\$ – Ahmed Samy Nov 6 '15 at 9:17
  • \$\begingroup\$ +1 because of "for academic purposes" :-) No, the 5 ohm resistors are just a design choice for the particular amplifier. They determine the DC current through the end-stage. Volume adjustment is done with a logarthmic potmeter at the input. I advise you to google on some audio amplifier projects and see how people do this. Also many textbooks have been written about audio amplifier design. It's not very hard but very rewarding in an academic learning way :-) \$\endgroup\$ – Bimpelrekkie Nov 6 '15 at 9:32
  • \$\begingroup\$ Well ... it always comes down to google, doesn't it :) ... Thanks for caring \$\endgroup\$ – Ahmed Samy Nov 6 '15 at 9:58
  • \$\begingroup\$ It does, when I was a boy all I had was magazines and the odd book. Google would be something you could only dream about. \$\endgroup\$ – Bimpelrekkie Nov 6 '15 at 10:17
  • \$\begingroup\$ @AhmedSamy google is more useful here for the learning process because it has way-more resources about this subject than could ever be given in the limited space we get for answers here. So it's right that google is recommended. If you went to the doctor you wouldn't expect him to provide an overnight bed for treatment! \$\endgroup\$ – Andy aka Nov 6 '15 at 10:20

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