Bias Class B vs AB When Using HG2879

[SuperLid]

I have noticed that some amps I have seen lately are class B, rather than AB. I am wondering why and what others have seen, when biasing class AB in a HG2879 box. Rumor has it, that when running AB in a HG2879 box, there is a high probability of self oscillation. I am wondering what truth there is to this.

I personally have seen this occur in Texas Star amps, that use the HG devices in class AB. I don't partake in a lot of these amp repairs, but tend to help out the locals when possible. Looking for experience from others on this subject.

Thanks,

SuperLid

 

[9C1Driver]

No reply's on this yet. I was messing around and did find this. Not sure how the HG finals behave when they get warm.

Here is what happens in a class AB amplifier when it gets too hot:

• Thermal runaway: The core of the issue is that transistors become more conductive as they get hotter. In a class AB amplifier, a small amount of "bias" voltage is applied to keep the output transistors from fully shutting off during the signal's zero-crossing point. This prevents "crossover distortion," a type of distortion present in class B amps. If the amp's cooling is insufficient, the transistors can become too hot, which causes them to draw even more current. This, in turn, causes them to get even hotter, leading to a vicious cycle of increasing heat and current draw.
• Bias instability: The bias circuit is designed to provide a small, stable current, but it can be affected by temperature changes. If the bias current increases too much due to heat, it can push the amplifier's operation closer to class A, which is even less efficient and generates more heat. Conversely, if the bias were to drop, it could move the amp closer to class B, causing crossover distortion, but this is less common with overheating.

 

[SuperLid]

No reply's on this yet. I was messing around and did find this. Not sure how the HG finals behave when they get warm.

Here is what happens in a class AB amplifier when it gets too hot:

• Thermal runaway: The core of the issue is that transistors become more conductive as they get hotter. In a class AB amplifier, a small amount of "bias" voltage is applied to keep the output transistors from fully shutting off during the signal's zero-crossing point. This prevents "crossover distortion," a type of distortion present in class B amps. If the amp's cooling is insufficient, the transistors can become too hot, which causes them to draw even more current. This, in turn, causes them to get even hotter, leading to a vicious cycle of increasing heat and current draw.
• Bias instability: The bias circuit is designed to provide a small, stable current, but it can be affected by temperature changes. If the bias current increases too much due to heat, it can push the amplifier's operation closer to class A, which is even less efficient and generates more heat. Conversely, if the bias were to drop, it could move the amp closer to class B, causing crossover distortion, but this is less common with overheating.

Thank you for that, but the post was specific to HG2879.

73,

SL