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The real answer is "Because Class A amps don't need to have such low THD numbers. Engineers could use feedback and other techniques to get the THD number down but they don't.
The reason they don't is that the distortion products of a class A amp sound different from the distortion products of an AB or B amp. The class B distortion is more harsh to the ears and really can't be allowed to be large whereas the class A distortion is not quite so bad and more of it can be allowed to reach the speakers.
I think what I'm saying is that designers try to build amps that sound good to them and let the THD number fall where it may.
Think of "THD" number is measuring about the same thing as "Pounds" caught in a fishing net. The fisherman cares about pounds but he cares even more about the kind of fish.
The "T" in "THD" stands for Total and lumps together a lot of different kinds of artifacts. I think you can use THD only to compar very similar amps. If you use THD to compare a Class A and Class B amp it is like saying "The class A has 3 pounds of apples and the Class B has 1 pound of oranges.
No it's not useless. If you are trying to sell amps to people who read specs THD is second only to "watts" as a marketing tool. You have to remember that most people who buy amps don't know much about them.
We see the same thing with cameras. People think they know what a "megapixel" is so that is the number the marking people push.
You will have distortions by turn-off.
If polarize the transistors in class A is necessary more current (Ib) in drivers, in most cases increases the distortion in class A, using BJTs is difficult to achieve high gain, I prefer Super A instead of Class A.
I'm not turning off anything. Just raising supply voltage. How does that make for different sound, if I play the same music at the same level?
jd
Yes, if you keep a low level you will not have distortions Turn-off, but the current in output stage must be less than current Bias...
Why?
If you increase the supply voltage on my class A amp, it will never become an AB. However, at some point it may turn into a stove, and you will be able to grill a crusty smelly chicken.
It is different in a couple areas. There is greater transistor
temperature, and the voltage across the device is different,
which alters the load line.
Both may be reasonably subtle, but they are easily measured.
It is different in a couple areas. There is greater transistor
temperature, and the voltage across the device is different,
which alters the load line.
Both may be reasonably subtle, but they are easily measured.
Agreed. I wonder if the higher temp in itself would be audible. And also with a higher supply, whether the relative Vce (or Vds) change over the same cycle as with the class A case would be audible; measureable it would actually be better I think.
But I don't think you can say in general that class A is better than AB in all cases.
BTW Thanks for the beer
jd
shaan,
Edit: don`t bother with measurement data.
I see, you are not familiar with my standpoint regarding this subject.
Edit: don`t bother with measurement data.
In order to correspond with our sense of hearing, the generated harmonics must decline evenly with increasing order. Unfortunately, not so many devices are willing to produce harmonics in that pattern even in a single-ended class A configuration. The single-ended class A, however, implies no harmonic cancellation, which is a condition of healthy tonal balance. (Harmonic cancellation is a measure of linearity). Low THD figures indicate an impoverishment of harmonic content.
Agreed. I wonder if the higher temp in itself would be audible. And also with a higher supply, whether the relative Vce (or Vds) change over the same cycle as with the class A case would be audible; measureable it would actually be better I think.
But I don't think you can say in general that class A is better than AB in all cases.
BTW Thanks for the beer
jd
My experience listing to the same amp/system (nothing changed but temperature) was that there was very little, if any, difference in sound when the temperature of the entire amp was changed in 10 degree increments (Fahrenheit). The amp was in a constant temperature oven and the oven temperature was set at 80F initially. The amp was stabilized for one hour while powered up. Listening begin at this time. What I found to be clearly audible was the period when the amp was going from one temperature to another, either up or down. Once the temperature stabilized, I could hear no difference at 90F, 100F or 110F.
As for increasing the supply voltage, you might hear an improvement if the amp were playing very dynamic music. Would you rather listen to switching or clipping?
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