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P3A Comparison table  ( long .... )
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Old 23rd May 2019, 08:07 PM   #971
johnego is offline johnego  Indonesia
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Quote:
Originally Posted by east electronics View Post
Some of the P3A i made had a hatch down under on the belly and trimmers located under the board specifically for the above reasons .
Many ways to solve it, so the bias stability is not really an issue. We should choose output R not based on the one that will give the best/easiest bias stability. Otherwise, just build with latfet EF, or the Blameless.
Based on initial prediction, i guessed that the optimal R was slightly above 0.33. But I started with 0.47 because it is the most stable option. I gradually improved the design by ears. When i tried 0.33, I didn't have the same patient to improve it, mainly because I don't have components that i needed, so it couldn't better my 0.47 one.

The hardest or most tricky thing in upgrade process is to find the correct bias value, because it can make or break the design. Everytime there is any component change, the bias should always be rechecked again by ears. Not enough just defining the standard value for base-to-base voltage of the drivers.

Last edited by johnego; 23rd May 2019 at 08:13 PM.
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Old 24th May 2019, 04:20 AM   #972
Ian Finch is offline Ian Finch  Australia
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johnego, you seem confused because you say stability is not a problem, but then you say you started with 0.47 because it's the most stable option. Also, to this point we are only concerned with bias stability. Let's not mix it up with satisfying our personal and subjective sound quality requirements.

Obviously, if you are using lower than 75 mA bias current, the temperature will be reduced and the drift slower - maybe stable enough for home use but maybe not in hot weather, so we need to know what degree of stability is necessary and how to ensure it. That's the first priority.
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Old 24th May 2019, 09:28 AM   #973
johnego is offline johnego  Indonesia
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Quote:
Originally Posted by Ian Finch View Post
johnego, you seem confused because you say stability is not a problem, but then you say you started with 0.47 because it's the most stable option. Also, to this point we are only concerned with bias stability. Let's not mix it up with satisfying our personal and subjective sound quality requirements.

Of course I understand the relationship between output R and stability. Hence, when I said bias stability shouldn't be an issue, it means that bias stability is not a problem that cannot be solved. Many people mentioned latfet is the best option because of its temp coefficient. For me it is something strange to mention when everyone knows how to work with vfet or bjt. It will be silly if I choose 0.47 simply because it is the most stable. I choose the most stable options as a starting point. Another example is the Miller cap, I always start with 100pF (this fits with BD139/140), at the end it will be the lowest possible. Some topology is harder because the compensation value must be precise (higher or lower is not stable).



Quote:
Originally Posted by Ian Finch View Post
Obviously, if you are using lower than 75 mA bias current, the temperature will be reduced and the drift slower - maybe stable enough for home use but maybe not in hot weather, so we need to know what degree of stability is necessary and how to ensure it. That's the first priority.


Take a look at P3B. Same circuit, class-A. For me it is not a first priority, may be last one.
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Old 24th May 2019, 03:30 PM   #974
Ian Finch is offline Ian Finch  Australia
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Class A current control needs are different to those of AB and can be quite simple. All that is required is sufficient bias to ensure the transistors are at or above the Q point at which they remain in the linear region of their characteristic curve at all times. Temperatures are constantly high and small changes or drift of the bias should make little or no difference to performance. Some designers go to a lot of effort to reduce the high dissipation by various tricks with tracking bias but this is another topic.

Class AB, where a lesser and in this case arbitrary amount of bias is applied, should be controlled as best we can to avoid changes in sound quality with power supply voltage and temperature because the Q point will be changing with it and so too, the harmonic spread of distortion products. So generally, the bias current controller needs to be well matched to the crossover characteristic of the output stage and also quite stable.

Many old style class AB amplifiers tend to be simple and sensitive to temperature variations so audiophiles can be fussy about their warm-up period. Some go to the extreme of leaving their equipment on 24/7 to ensure it sounds right whenever they use it. This is another reason why we need to get class AB biasing under tight control and avoid the risky practice.
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Last edited by Ian Finch; 24th May 2019 at 03:34 PM.
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Old 25th May 2019, 12:29 PM   #975
johnego is offline johnego  Indonesia
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Originally Posted by Ian Finch View Post
Some designers go to a lot of effort to reduce the high dissipation by various tricks with tracking bias but this is another topic.
Yes, and i have a few up my sleeve, so i'm not worry at all. I have only a few months experience with CFP but it is not as difficult as i thought it was. Packaged darlington was very hard. High output R was mandatory. But like anything else, if we can get access to other people previous works then we know how to set expectation.
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