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Audio Amplifier. Which class should I use?

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Shayaan_Mustafa

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Hello experts!

I am designing a circuit for audio amplifier.

I know there are classes for amplifiers. Class A(low efficiency and mid Q point), class B(better efficiency than class A and some distorted), Class AB(slightly distortion in compare to class B and more distortion than class A) and class C(highly distorted but efficient amplifier).

So which class should I use? Should I consider their efficiency or their operating points?

If I designed class A amplifier then it has low efficiency but mid Q-point then what will be audio output in this case? Will it be low sound due to low efficiency?

In short, what effect does the efficiency or distortion makes on output sound? What sound I will hear in both cases?

Thanks in advance.
 

Class A and class AB are both good for audio. Comparing the two, class A makes it easier to get low distortion and good sound with a very simple circuit. The disadvantage is the low efficiency which means they are much more expensive than class AB amps, due to the need for bigger heatsinks and power supplies.

Properly designed class AB amps can have very low distortion and excellent sound too. Most commercial amplifiers are class AB.

There is not much point to class B amplifiers (with no idling current at all in the output devices). The efficiency is only slightly better than class AB, but the distortion is much worse.

Class C is only used for radio transmitters. It is no good for audio due to very high distortion. There are other classes of amplifiers as well, but they are all more complicated.

The quickest, easiest and cheapest way to make a nice amplifier is to use a chip like the LM3886 but that's so simple there's not much challenge or fun in the design work.

p.s. I don't understand what you mean by "mid Q-point".
 

For "mid Q-point", I think the OP means that the quiescent point of the transistor is in the middle of the potential range.

You can also look at Class D, if you want to add a lot of complexity but improve the efficiency dramatically.
http://www.irf.com/technical-info/appnotes/an-1071.pdf

As for the impact of efficiency, it should have no impact on the audio output, but it will impact the power consumption and how you handle heatsinking your active devices. If you want 100W of audio output from a Class A amp, you need to put in at least 400W of power form the outlet (Class A theoretical max eff = 25%, 100W / 25% = 400W, minimum). That means that you will have to dissipated 300W of heat into the air... that's like cooling 5 light bulbs, simultaneously.

I'd lean toward AB and get some efficiency improvement, and suffer a little distortion (most people can't hear less than 0.1% THD). AB will get you closer to 50% in a well-tuned audio amplifier... so 100W output needs 200W input and 100W dumped as heat.

Read the section on efficiency to get some good numbers on real world amp applications (audio amps are noted and discussed).
http://en.wikipedia.org/wiki/Amplifier#Efficiency

Class D can get you up to 90% efficiency. So 100W would need 111W input and 11W left as dissipated heat.
 
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If you want to drift of into nerd land and try to achieve .005% distortion or some thing similar, there is the Quad designed Amp which used a class AB for the power and used a balancing technique with a low power class A amp to feed "anti distortion" products to the load. This is based on the fact that for a 50W amplifier, the total power of ALL the distortion products are less then 100mW. That should soak up a few evenings!
Frank
 

enjunear said:
For "mid Q-point", I think the OP means that the quiescent point of the transistor is in the middle of the potential range.
Click to expand...

Yes, this I meant from mid Q-point.

enjunear said:
You can also look at Class D, if you want to add a lot of complexity but improve the efficiency dramatically.
http://www.irf.com/technical-info/appnotes/an-1071.pdf
Click to expand...

In university I have studied until only three classes, class A, ,class AB, class B and class C. Not other class. So, I have to experiments with only these.

Now I need clearifications on two.

1) If amplifier is low efficient it means it would dissipate more heat. If amplifier is more efficient then it would dissipate less heat.
2) If amplifier Q-point (quiescent point or operating point) is near to cutoff like class C, this means it will be more noisy, it will generate noisy sound at output. Similarly if amplifier Q-point is in the middle of the potential range like class A then it will not create noise at the sound at output?

Am I right about above two?

Please explain in detail but in easiest way.

Sorry for my bad English as English is not my native language.

Thank you.
 

Shayaan_Mustafa said:
1) If amplifier is low efficient it means it would dissipate more heat. If amplifier is more efficient then it would dissipate less heat.
Click to expand...
Yes.

2) If amplifier Q-point (quiescent point or operating point) is near to cutoff like class C, this means it will be more noisy, it will generate noisy sound at output. Similarly if amplifier Q-point is in the middle of the potential range like class A then it will not create noise at the sound at output?
Click to expand...
The problem with class B is crossover distortion. See here: https://en.wikipedia.org/wiki/Crossover_distortion
 

About point 2 I am just asking that what effect does distortion do at output i.e. what type of sound I would hear due to distortion? will it contain noise?

and also can an amplifier catch noise from environment?
 

Shayaan_Mustafa said:
About point 2 I am just asking that what effect does distortion do at output i.e. what type of sound I would hear due to distortion? will it contain noise?

and also can an amplifier catch noise from environment?
Click to expand...

To be technically correct, distortion and noise are two different physical effects. Distortion is a corruption of the input signal, such that the output is no long just a linearly scaled version of the original signal. Since the output signal is not an exact copy of the input signal, it has been changed, corrupted, distorted. The resulting sound output from a speaker would sound warped/garbled, but not "noisy" (which would be more like a hissing sound).

Noise can come from many places, but it's generally considered to be random amplitude and phase over an infinite bandwidth. "White noise" has a power spectrum that is flat vs. frequency... thermal noise (noise generated by the random motion of electrons within devices above absolute zero) appears to be very close to the "white noise" frequency spectrum.

Other noise sources are shot noise, 1/f or flicker noise, avalanche noise, etc. These contributions of noise will manifest themselves as the "hiss" you hear coming out of your speaker when the power is on, but there is no input signal. Thermal noise exists at all frequencies, so your amplifier will pick it up and amplify it, just like the desired signal.

Wikipedia - Noise (electronics)
 

Class A type is desirable in pre-amp stages. Tone control, freq band splitting, audio effects, etc.

The advantage is that class A can use a single ended supply, and at low power levels. You can have linear performance (hence low distortion) with an amplitude of 15, 20, 30 V.

Then where power is needed, such as to the speakers, it is typical to use class AB.
 

To hear what very bad crossover distortion sounds like, you can build this circuit. All you need is two back-to-back diodes in series with the loudspeaker.

 

@enjunear Thanks for detailed explanation.

@BradtheRad Class A has low efficiency. Therefore more power will loose through heat sink. Therefore I chose class AB.

@godfreyl I have got your point.

Now I want to ask something more.

As in discussion godfreyl suggested me to use class AB. Now my question is how much maximum power can a class AB amplifier deliver to its load? and how much power will it dissipate through heat sink into the air?
 

Dear Shayaan
Hi
At a class AB amp , the maximum dissipation across the each transistor is about 0.2poacmax . and it's efficiency can't exceed from 78.5 percent but in practice you can't get this efficiency .
BTW : to select a good class it depends on how much power do you need .
Best Wishes
Goldsmith
 

It was answered in the second response I think..
There is quite a bit of knowledge involved in designing a good amplifier; might be worth reading a book on it, e.g. books by D.Self, or check the diyaudio site, they have some circuits.
 

@sky_123

I have knowledge about electronics by the mercy of Almighty ***. But I never come into practical examinations with electronics. I am student of electronics engineering. In our university, in our department of Applied Physics, it is very bad point that they don't give any project to make to the students. Therefore, we students have knowledge about individual stages of circuits. But, as I am trying to make audio amplifier, in practical life then we have difficulties to integrate our knowledge for one complete project. I hope you can understand.

@goldsmith

As you said it will me 0.2poacmax. Kindly tell me what is meant by "po"

And how much output power is should be deliver to load in order to gain good sound quality?

Sorry for my bad English.

Thanks for everyone.
 

Po , means out put power . don't forget that in class AB amplifier , the transistor will work as a high power , variable resistor .
Thus when your out put power is in maximum mode , your efficiency will be at highest value , and it is a great disadvantage of class AB amplifier .
It think you are trying to make a project but you are confused by some definitions , and then loosed the way . right ?
If you tell me , that what you want to do ( how much power do you need ) exactly , thus i can help you as well .
Best Luck
Goldsmith
 

@goldsmith

Yes you are probably right.

Approximately 400W I need.

I hope this would give the somehow best sound quality. Can you help me now in designing of this amplifier?

Should I start a new thread?
 

Of course it will be possible to achieve high quality voice with that . but before helping you to design , is that your first design in audio range ? if yes , i suggest you to start with lower wattages , such as 50 watts , and then if you could do it as well , you can start to design a higher wattage , with the experience that you have earned from previous design .
 

@goldsmith

OK sir. Thank you very much.

I am following your suggestion. As it is my first design so I will make 50W audio amplifier.

Will you help me now??
 

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