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Boost LM380 with 2N3055

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captain_haddock

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Hello all,

Please advise me how to increase LM380 output power using transistor 2n3055, which can withstand 3 ampere of current.

what would be the better choice of modern complementary pNp/nPn power transistor.

Thank you very much

your Captan
 

schmitt trigger

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The LM380 had sufficient stability issues with their vertical-PNP output stage, to complicate this further with external transistors.
You would lose voltage swing, which is not great to start with, causedt by the transistor's Vbe.
The transistors would be outside the feedback loop. No distortion correction available.
The DC biasing would have to be investigated to ensure that it would not cause a DC current flow from the 380.

I'm sure AudioGuru will rspond to this thread and add further insights of his own.

It actually it would be better if you used a modern audio amplifier IC to get the power level you require.
 

Audioguru

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Why add output transistors to a low power LM380 amplifier?
It is designed to use an 18V supply to produce 14V peak-to-peak. If you add emitter-follower transistors then they will produce distortion because they will not have any negative feedback and their output will be 12V p-p. 12V p-p is 6V peak that will produce a peak current of 3A into 2 ohms. Is your speaker only 2 ohms?
12V p-p into an 8 ohms speaker is 2.25W which an LM380 can do without output transistors and the distortion will be less. With the transistors the output will be about 4W into 4 ohms.
 

Vbase

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Hello all,

Please advise me how to increase LM380 output power using transistor 2n3055, which can withstand 3 ampere of current.

what would be the better choice of modern complementary pNp/nPn power transistor.

Thank you very much

your Captan

Connect the bases of complementary transistors to the output of LM380 and take the feedback to the inverting input from the emitters, this way the transistors are in the loop and you will get low distortion. A small cap on the feedback will take care of the instability.
 

Audioguru

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Connect the bases of complementary transistors to the output of LM380 and take the feedback to the inverting input from the emitters, this way the transistors are in the loop and you will get low distortion. A small cap on the feedback will take care of the instability.
Why? As I showed, the supply voltage limit does not allow higher power unless you parallel many speakers.
The LM380 already has built-in negative feedback so if you simply add output transistors without biasing them properly then the resulting crossover distortion will be horrible.

If you want more power then simply buy an amplifier IC that produces more power.
 

schmitt trigger

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Exactly, what Audioguru said.

External Output transistors on an LM380 is not a good idea.
 

KlausST

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Hi,

Nowadys I mostly use class D BTL audio amlifiers for this kind of quality.
They drive a lot of power while staying cool.

Klaus
 

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