Electronics > Beginners

TL071 distortion - bad amplifier design?

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t1d:

--- Quote from: AngraMelo on December 10, 2018, 03:39:12 am ---I actually bought the boards. I got them on our local "ebay" kind of website.

--- End quote ---
Well, that may be a good portion of the problem. Even with the extremely knowledgeable and gracious experts here on the forum sorting out the design, the components are probably not up to specs and there may be no chasing down the problem, due to that fact. Don't get me wrong, I buy Ebay stuff, when appropriate. So, for learning the circuit, maybe what you have is a good tool. But, for your discerning ear, maybe not. Again, I am not faulting you, by any means.

spec:
Hi AngraMelo

That is a pretty standard audio power amplifier architecture and has the potential for high quality reproduction, but there are two areas that you need to know about.

[1] The drive from the TL071 to the voltage amplification stage is troublesome.

[2] The amplifier will generate severe cross-over distortion.

To fix [1] above change the two 4.7nF capacitors for 47uF. The amplifier should then work.

I am quite busy at the moment but, maybe in about ten hours time. I will post a revised circuit to fix both issues properly.

perieanuo:
And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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Zero999:

--- Quote from: perieanuo on December 10, 2018, 10:55:06 am ---And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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--- End quote ---
You mean the final transistors, not resistors, i.e. the 2SC52EE and 2SA1943?

If so, how can they saturate? They're emitter followers.

perieanuo:

--- Quote from: Hero999 on December 10, 2018, 11:51:24 am ---
--- Quote from: perieanuo on December 10, 2018, 10:55:06 am ---And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF


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--- End quote ---
You mean the final transistors, not resistors, i.e. the 2SC52EE and 2SA1943?

If so, how can they saturate? They're emitter followers.

--- End quote ---
They can.for example, from thermal considerations or if driver goes short.that's why between emiter driver and final transistor base it's good idea to put resistor.see the second schematic that does have.

Look here and try to understand in practice it can arrive this situation"The bias resistor must connect to a voltage that is at least 0.7V higher than the supply voltage for the upper emitter-follower transistor to saturate." (eg if you have voltage loss at high power on your wires inside the amplificator, yes, the wires between the final transistor and the +Vdd, I seen this in a clone philips schematic, in practice) :
https://www.electro-tech-online.com/threads/current-limiter-using-transistor-how-to-saturate-an-emitter-follower-bjt.118323/

What you are saying should never arrive, but in practice can arrive, that's the trick.
I agree final transistor will die in short-circuit load scenario and 0.22 resistor should prevent thermal derive.
For me this schematic is not the most stable choice possible, but maybe you're right ....
Anyway there are better ways to prevent signal to be overamplified, thermal and oscillation tendances.
It's too complicated to talk about all the aspects with so simple amplifier.
Pierre


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