Stereo Audio power amplifier 2 x 40W - 80W

[faizanbrohi]

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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[faizanbrohi]

So here is the PCB layout and PCB silkscreen for PCB toner transfer the image is mirrored . AUdio guru please i require your help with the schematic . and one more thing , there are two channels , can you draw it for one channel seperately.

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[audioguru2]

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.

 

[faizanbrohi]

Well this amplfier is goood for starters

 

[faizanbrohi]

Can you tell me which class of amplfier is it . Class B or AB or not likely A

 

[faizanbrohi]

How is this POwer amplfier of CLass AB i found on the web

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[audioguru2]

This amplifier produces a little more power than the other amplifier you showed and both operate in class-AB.
Class-A creates too much heat and class-B has crossover distortion.

 

[faizanbrohi]

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 .

 

[audioguru2]

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.

 

[faizanbrohi]

If we increase the input voltage upto +-40V on the first amplifier i posted , will it then effect the power output. if not  is there any way to increase the power to 100W per channel or 60W per channel .  ???

 

[faizanbrohi]

Hmm if we add a darlington current amplifier at the end of the 1st amplfier , it will increase current , wouldn't it . so this can increase overall power and if we increase the voltage up to say 50V or 60V plus and minus both. What do you think   

 

[audioguru2]

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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[faizanbrohi]

But there is one problem , the transistors used cannot be found easily , can u suggest a replacement , for them like the MPSA and the MJL series transistors

 

[audioguru2]

Find an amplifier designed for parts available in your country.
In Canada, I can buy any transistor I want, except some Oriental ones are hard to get.

 

[faizanbrohi]

hmm , now let me give you another amplfier . to play with . i have a good felling that these parts can be found.  How is this amplfier

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[audioguru2]

The M250 looks like another good modern amplifier. It has enough paralleled output transistors to produce high power in a 4 ohm speaker.

 

[faizanbrohi]

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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[audioguru2]

Making a high power amplifier with high voltage transistors and with them paralleled for high power is a waste if you use your little transformer that has a low voltage. The amp will have the same low power as the other amplifier that uses this little transformer.

 

[faizanbrohi]

But i can produce a DC 55 and -55 from the transformer that i have because it has reserver power of 4A , and 8A is the total , the circuit uses 4A @ 2 x 38 , and  i have 2x28 V but 8A . so the voltage can be tackeled , because it is a multiple V=IR