help bulding Class ab amp

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alisancak1

New Member

• Jan 7, 2012

• #1

hello everyone
am trying to design a class ab amp ...
32W @ 8 ohm
i choose tip 31 and tip 32 Am I on the right track?

 

Sceadwian

Banned

• Jan 7, 2012

• #2

For the drive transistors, sure...

 

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alisancak1

New Member

• Jan 8, 2012

• #3

i basically design my first class ab amp. like this. What should i change to gain 32W ?

 

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Last edited: Jan 8, 2012

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 8, 2012

• #4

Pretty well most of it, but for a start where's the negative feedback?.

You're also missing an input transistor if you're going with that kind of old design.

 

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aerozan

New Member

• Jan 8, 2012

• #5

Q11 is input bjt, isnt it?

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 8, 2012

• #6

aerozan said:

Q11 is input bjt, isnt it?

Click to expand...

No, there should be a PNP in front of it - with negative feedback to it's emitter.

I also think TIP31/32 are a bit too small for 30W, use a 41/42 instead.

 

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alisancak1

New Member

• Jan 8, 2012

• #7

i add another bjt but i still can not get 32W from r8 . i shoul miss something ? . i get almost 1mW @ r8

 

alec_t

Well-Known Member

Most Helpful Member

• Jan 8, 2012

• #8

There is no DC current path to Q1 emitter / Q11 base.

 

Nigel Goodwin

Super Moderator

Most Helpful Member

• Jan 8, 2012

• #9

alisancak1 said:

i add another bjt but i still can not get 32W from r8 . i shoul miss something ? . i get almost 1mW @ r8

View attachment 59981

Click to expand...

You can't just randomly throw an extra transistor in there - it needs connecting properly - why not just look at existing circuits like that?.

There's still no proper feedback - which MUST be DC, with lower AC feedback to give AC gain. If you're hoping for 30W your emitter resistors are also too high.

Here's an example of an excellent design, the Leak Stereo 70 - which provided a GOOD 35W in to 8 ohms, at high quality and low distortion - a classic quality design of it's day.

 

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audioguru

Well-Known Member

Most Helpful Member

• Jan 8, 2012

• #10

32W into 8 ohms is 45.3V peak-to-peak. Add maybe 6V for losses after you fix your serious losses with the high value emitter resistors and tiny output capacitor. Then the supply must be 51.3V.

But your supply is only 12V so you are lucky to get only 1W into 8 ohms.