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Stereo Audio power amplifier 2 x 40W - 80W

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I found a commercial audio power amplfier and i have just finished it's PCB mapping . although some parts need a little image enhancement. because some connection were broken when i mapped the PCB. When i am finished i will place the corrected PCB , and details, also the preamplfier module. so here is a little not so god , under improvement , reverse engineered PCB layout


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It looks like an old design, not very good performance and uses mostly Oriental transistors.
The input transistors look like they are mounted backwards.
Draw a schematic, cut its pcb in half and make it if you want, but there are many newer and better amplifiers designs on the internet.

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So what is the final verdict , which one should i make for my project , the commercial amplfier that i showed you first which has easy to find components , or the other which has high power , but have just found it on the internet . One more thing , will the power increase , if i replace the 2N3055 with MJ15023 or MJ15022 in my first amplfier .

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Make the 1st amplifier if its power output and distortion are acceptable to you.
Changing its output transistors won't increase its power output.

The supply voltages for the 1st amplifier from a dual 24VAC transformer are about plus and minus 32V while they are plus and minus 36V for the 2nd amplifier. Therefore the power output from each amplifier are nearly the same.
Changing the output transistors won't affect the max power output of an amplifier when the transistors are in the same package and have good cooling.

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Even if you used high voltage output transistors with a dual 60V supply, the peak current into an 8 ohm speaker is only about 7A which all power transistors can pass.
But transistors will melt due to the high power in them, so need to be paralleled.
Here is a pretty good 150W amplifier (into 8 ohms) that uses high voltage transistors with a dual 56V supply. The transistors are paralleled.
The same site has a 1.5kW amplifier with 18 output transistors.


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The power supply suggests a toriodal transformer of 2x38V = 76V total . and a power 500VA , but i have a transformer of 2 x 28V with a current of 8A . now calculating the current required , Watt = power factor x VA = .5(typical) x 500 , so i get 300W and divide by 76V , i get 3.94A , which is the current it needs , right  , and there is also a fuse of 4A . and a bridge of 35A / 250V ????  ??? , one more thing will m 2 x 28V transformer rated at 8A , work with the M250 power supply , does it fullfill the needs .


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Your transformer has a sine-wave RMS value of 28VAC.
Its peak voltage is 28 x 1.414= 39.6VDC.
The two rectifier diodes in series will drop it to about 37.6VDC at full output which is far less than 55VDC.
At clipping, the amplifier will make a sine-wave at its output of about 69Vp-p which is 24.4V RMS. Therefore it produces 74W RMS into 8 ohms.

The 38VAC transformer will have a peak voltage of 53.7VDC. After the rectifier diodes it will be 51.7VDC at full output.
At clipping, the amplifier will make a sine-wave at its output of about 96Vp-p which is 34V RMS. Therefore it produces about 144W into 8 ohms. Nearly double the power of the smaller transformer. 

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ok you are right but how much ampere should i have , a transformer of 2x38VDC @ how many amperes ??? A . what i think it would 500VA so if i multiply by power factor which i don't know , but assuming , it to be 0.5 , it is 300W that is 4A , so the transformer should be rated at 2x38VDC @ 4A , or else .. what do you suggest  ???

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you'll need a supply with the voltage the schematic was designed to. About it's current, the more the better (the circuit will only use the required amperes). The problem is that if your transformer has less power, the circuit will cause the transformer to dissipate more power (incresing it's temperature) and therefore... kaput  :P

Don't forget to have a rectifier bridge with support for more than 1.5x the maximum current you'll need.

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A class-AB amplifier is about 50% efficient at clipping, so a stereo amplifier with 144W output from each channel will have a supply power requirement of 576VA which is a current of 7.6A. Amplifiers don't work at full power continuously so a smaller transformer with a current of 5A will be fine.

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i have studyed just now that Class AB is about 79% efficient and Class A transformer coupled have 50% efficiency. However as far as the transformer is concerned , i have decided , a transformer of 2 x 38VAC @ 6A , with 2 x 12VDC @ 1 A for something(the other winding is seperate for from the first BUT IN THE SAME HOUSING) other then powering up a amplfier . i mean for the Detector circuits and the 10 band VU LED meter that i will insert in it .

i will also need a bridge of 35A @ 230V , well i will begin in it later , seems like a tough project and a little expensive too . i will keep it as a mega audio project and now i will make the First Audio AMplfier i posted with PCB . i am going to draw a schematic and post it sooon.

Wish me good luck in the exams , tommorow is my Solid state Devices and Basic Electrical Engineering paperz.

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