The BD139/BD140 little power transistors have a max allowed current of 1.5A. That is the peak current of a sinewave with a power of 9W into 8 ohms.

An amplifier has a voltage loss at its output of about 2.5V. Then the peak-to-peak voltage swing with a 12V supply is 9.5V. Then an amplifier can make 3.5V RMS into 8 ohms which is a power of only 1.5W. The average power supply current is about 250mA.

The little power transistors have a low current gain like all power transistors. Then the transistor feeding them has a high output current which reduces its voltage gain and reduces the amplifier's negative feedback. Then the distortion is high.

Amplifiers usually use driver transistors to drive the power transistors so that the voltage amplifier transistor can have a high voltage gain, and a high amount of negative feedback can be used in the amplifier resulting in low distortion.

__________________
Uncle $crooge

 

hmmm does it matter? i don't want it to hear music!
i have made many builds to enjoy myself.... DACs, LM3875 Gainclone amp, TDA1553Q amp etc... but this is the first time i'll make an amplifier which is not Chipamp... so it it wrong to start with something small that i can understand?

 

There are many simple good circuits. Make one that works well.

__________________
Uncle $crooge

 

I think a great learning amp that works as a headphone amp is here:

http://sound.westhost.com/project113.htm

Real common way of doing things from what I understand. Should drive any headphone on the planet. I've got parts for a (modified) version I am working on now.

 

looks easy and very simular to what i'm studing here!!! but it uses
IC it's not totaly from discrete components and also i think it needs a PCB..
i don't think i can make it on a breadboard probably it will behave far
worst even than the amp that Audioguru advices me not to build.

 

You will very likely have oscillation problems on the breadboard from reading the article. You will have large capacitive coupling between the + and - inputs due to the structure of the breadboard. I did not know you couldn't use an op amp, sorry.

 

Heres the simplest amp i could find.

http://scitoys.com/scitoys/scitoys/c...amplifier.html

 

So did you read the web page before or after starting the other thread?

 

It is a very lousy amplifier. Read what I said about it in the other thread.

__________________
Uncle $crooge

 

audioguru,
You little amplifier needs emitter resistors to prevent the transistors from going in to thermal runaway.

__________________
I also post at the following sites:
http://www.stop-microsoft.org http://www.heated-debates.com
Screen name: Aloone_Jonez
And http://www.silicontronics.com, same screen name as here.

 

Probably not?, as it's under-biased - particularly if the bias diodes are thermally coupled to the output transistors. But really it's a just a piddling little amp, based on the minimum component count.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk