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TL071 distortion - bad amplifier design?

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Mechatrommer:
i thought its class AB near 0V (with the existence of 2 biasing diodes) and B when not.
https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html

why class C came into picture? its not suppose to be in audio anyway?
http://www.circuitstoday.com/class-c-power-amplifier

this thread seems to follow the fate of the others... as usual..

Kleinstein:
There is no need to use a large, expensive polypropylene capacitor at the input. Just a more normal polyester capacitor (PEN or PET) is well good enough.
For the 220 µF a more normal electrolytic cap is likely OK too.

If DC coupling is used, it might be a good idea to adjust the offset - at least with an TL071. It may work with other lower offset OPs.
Even with the capacitor at the input one might consider adjusting the offset in a way to get the correct polarity for the 220 µF capacitor.  Over a long time electrolytic caps may not like reversed polarity, even if only a few mV.

The distinction between class B and AB audio amplifiers is normally from the bias / standing current: class B would be without a standing current and class AB would be with some bias current at low output level, but less than the normal load current. Ideally there would be a small bias, so that in the cross over region the slow onset of operation from both halves add up - so no gm-doubling. There is still the problem of staying stable at that point of operation with changing temperature - so the coupling of the VBE multiplier to the transistors can be important.

The output stage uses a darlington configuration. So it would take about 4 diodes to get class AB.

Mark Hennessy:

--- Quote from: Mechatrommer on December 11, 2018, 01:29:17 pm ---why class C came into picture? its not suppose to be in audio anyway?
http://www.circuitstoday.com/class-c-power-amplifier

--- End quote ---

As your link explains, the definition of a class C power is one where the output devices are conducting for less than 180 degrees.

If you take a class B audio amplifier and turn down the quiescent current, you will get crossover distortion. This is the effect of the output devices conducting for less than 180 degrees. Hence, the amplifier has moved from class B to class C. As you rightly say, this is not good.

Note that the quiescent current does not tell you what class of amplifier you have. This is a common myth - I'm not really sure where it comes from. In short, transistors are not binary devices, so require a particular standing current to reach class B. Slightly less bias moves into class C, and slightly more will move into class AB. Lots more standing current will move the amplifier into class A (in practice you'd choose a value that fits with the power output and minimum load impedance). All that without changing the circuit; simply adjusting a variable resistor.

It's all in the book I referred to, which is a very readable yet thorough analysis.



--- Quote from: Kleinstein on December 11, 2018, 01:32:13 pm ---The distinction between class B and AB audio amplifiers is normally from the bias / standing current: class B would be without a standing current
--- End quote ---

Not so; class B requires some bias to achieve 180 degree operation. No standing current means class C  :-+



--- Quote from: Kleinstein on December 11, 2018, 01:32:13 pm ---There is no need to use a large, expensive polypropylene capacitor at the input. Just a more normal polyester capacitor (PEN or PET) is well good enough.

--- End quote ---

Agree completely. Coupling capacitors lead a simple life - obviously it's good to avoid electrolytics when forming a high-pass filter as they are known to cause distortion in that role, but an ordinary polyester is more than good enough. The VAS capacitor is much more critical, but easily overlooked.


Bassman59:

--- Quote from: mikerj on December 10, 2018, 09:50:01 am ---
--- Quote from: Bassman59 on December 10, 2018, 03:25:27 am ---
--- Quote from: oPossum on December 10, 2018, 03:15:26 am ---The stage following the op-amp is inverting, so the feedback to the + input of the op-amp is correct.
--- End quote ---

no.

--- End quote ---

Yes.  The drive stage after the op-amp is inverting, so the feedback connection to the non-inverting op-amp input is correct.

--- End quote ---

You guys are all correct -- I missed how the drive stage was connected. I even simulated the thing.

Ya learn something new every day, if you're willing to do so.

Thanks.

spec:

--- Quote from: Kleinstein on December 11, 2018, 01:32:13 pm ---There is no need to use a large, expensive polypropylene capacitor at the input. Just a more normal polyester capacitor (PEN or PET) is well good enough.
For the 220 µF a more normal electrolytic cap is likely OK too.
--- End quote ---
What does 'there is no need' and what does, '(PEN or PEN) is well good enough' mean?
Capacitors have a big impact on the sound of an amplifier- I had a good reason for specifying those components.
A suitable polypropylene capacitor costs 70p UK and is not excessively big.
As I have said to you before, let us see your design for an amplifier, rather than criticizing/contradicting other people's work.

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