Electronics > Beginners
TL071 distortion - bad amplifier design?
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AngraMelo:
Spec, the only polyproplyne 1uf cap I can find in my country are these types: https://proesi.com.br/capacitor-partida-cbb61-1uf-450vac-fio.html

They seem to be for mains filtering. As they are 450v would impact negatively on the ESR or other parameter?
About the 220uf Tantalum, I think it is better for me to get a bunch of 100uf and I can combine them in series/parallel and find the sound I like.
What is the voltage rating I need for it? A 6.3V or 2V would do?

The main idea for this amp is to be a good sounding amp. I know it sounds vague but Im a musician and I have plenty of high fidelity amps around. From studio monitors to specialized stuff. Im diving in the audio amplifier world as a way to learn electronics and to be able to put together a amp that sounds the way I like, not that is 0.00000001% distortion.

I dont have a need for high power amps, given that I listen to music in a small space, so when I heard you telling me to choose the capacitors to suit the sound I like I got really excited because that is precisely the way Im tackling this subject.

Yes, it will be great to learn how to make extremely low distortion amps and maybe I will pursue that in the future. But right now, what Im looking for is finding the amp configuration/components/class that best suits my ears and speakers.
spec:
UPDATE #1  2018_12_12

As to the class of an amplifier, there is no confusion at all:

* In class A both output elements conduct all the time. This is, by definition, the lowest distortion class of all, but at the cost of low efficiency and thus high waste heat.
* In class A/B both output elements conduct for small signals, but move over to class B for larger signals.
* In class B one output element conducts for the positive half of the waveform (180 deg) and the other element conducts for the negative half (180 deg) of the waveform. AFAIK there are no low distortion class B audio amplifiers. In fact there would be little point, because class lowA/B (conventional audio amps) are so simple to implement and have essentially the same efficiency as class B, without the high crossover distortion.Class AB  amplifiers predominate, because they give good efficiency with reasonably low distortion. Typically, a class AB amplifier will have a quiescent current of 20mA to 40mA, because this gives the lowest crossover distortion. Another approach is to have a quiescent current of 100ma to 1A. This would still be a class A/B amplifier but would operate in class A for a greater part of the signal. Guitar valve amps are nearly always class hiA/B.

Incidentally you can change the crossover point on most class A/B amplifiers, by simply adjusting the quiescent current. Of course, you need to increase the heat-sinking to get rid of the extra static heat dissipation.

You can even turn a class A/B amplifier into pure class A by increasing the quiescent current to a high value. Much more heat-sinking, would of course be required. For example, you could turn the reply #44, 72W class A/B amplifier into pure class A by increasing the quiescent current to 4A375.This wold require a change of output transistors and vastly increased heat-sinking, but that does not detract from the principle.

I have often run a standard Douglas Self amplifier in all three modes: A/B hiA/B and A.

By the way, it is dead easy to make a class B amplifier with no quiescent current: just feed the bases of a complimentary emitter follower output stage with a constant current. In fact the voltage amplification stage (VAS) of most audio power amplifiers do produce a constant current. That is one of the major objectives of most VASs.

In practical terms the absolute value of quiescent current is not that critical, so long as it does not drop below around 20mA. If it were essential to accurately fix the quiescent current exactly to a particular value, it would be done, because it is not a difficult feature to achieve.

spec:

--- Quote from: AngraMelo on December 11, 2018, 09:25:25 pm ---Spec, the only polypropylene 1uf cap I can find in my country are these types: https://proesi.com.br/capacitor-partida-cbb61-1uf-450vac-fio.html.
--- End quote ---
There are polypropylene capacitors available for small signal use. I will advise on this.


--- Quote from: AngraMelo on December 11, 2018, 09:25:25 pm ---They seem to be for mains filtering. As they are 450v would impact negatively on the ESR or other parameter?
About the 220uf Tantalum, I think it is better for me to get a bunch of 100uf and I can combine them in series/parallel and find the sound I like. What is the voltage rating I need for it? A 6.3V or 2V would do?

--- End quote ---
I will change the schematic to show a 100uF Tant, if that is what you can get. 100uf will be more than adequate.
2V will be fine, as will any higher voltage. The objective anyway, is to get rid of the capacitor altogether. The tant capacitor is one of the biggest sources of distortion/coloration because it is part of the feedback network. In general capacitors are troublesome components, so it is always a good approach to get the best you can, consistent with cost/size. Of course, the best approach is to eliminate capacitors in the signal path whenever possible.



AngraMelo:
Awesome! Thank you again!
spec:

--- Quote from: AngraMelo on December 11, 2018, 09:25:25 pm ---
The main idea for this amp is to be a good sounding amp. I know it sounds vague but Im a musician and I have plenty of high fidelity amps around. From studio monitors to specialized stuff.
--- End quote ---
nice :)


--- Quote from: AngraMelo on December 11, 2018, 09:25:25 pm ---I'm diving in the audio amplifier world as a way to learn electronics and to be able to put together a amp that sounds the way I like, not that is 0.00000001% distortion.

I don't have a need for high power amps, given that I listen to music in a small space, so when I heard you telling me to choose the capacitors to suit the sound I like I got really excited because that is precisely the way I'm tackling this subject.
--- End quote ---
OK, that gives a good picture of how perceptive you are, and the audio quality standard you would be happy with.

The amplifier of reply #44, with good +-40V supply lines will produce 72W into 8 Ohms. Which is ideal for music reproduction in a small room with modern speakers, which tend to be rather inefficient. If 72W sounds a lot, it is not because music normally consists of low level base-line signals with huge peaks every so often. So while the average power at fairly loud listening levels may be 100mW, the peaks could easily reach 72W instantaneous (theory says 1kW). Don't forget that the human ear's response to sound is logarithmic so, for example, increasing the power by 10%(+1dB) is barely perceptible. 


--- Quote from: AngraMelo on December 11, 2018, 09:25:25 pm ---Yes, it will be great to learn how to make extremely low distortion amps and maybe I will pursue that in the future. But right now, what Im looking for is finding the amp configuration/components/class that best suits my ears and speakers.

--- End quote ---
It is extremely persnickety and tiresome to make a top end audio amp. Much of it is mechanical: the thickness and kind of wire and solder. The order of the connections. etc. And there are capacitors splattered all over the place- just take a look at a picture of the innards of a high end amp.

So I would advise this as a theoretical study for the future.

To summarize then, I suggest that we get the amplifier of reply #44 working well so there is a reference and reliable test bed and after that stage, which will be no mean achievement, look into improvements, like changing the opamp. Of course, if anything simple crops up on the way we can incorporate that.
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