Class B amplifier with SINAD of 120, how is that possible?

[DanielT]

I thought that class B based amplifiers advantage were a lot of power in combination with the disadvantage of high distortion. To solve the negative aspect class A design was integrated, creating the classic combination: Class AB amplifier.
BUT why bother with this AB combination if it can be solved in another way? Just add plenty of feedback and suddenly a class B amplifier, (Topping B100), with incredibly low distortion appears.
(note that what I mentioned above is as I understood it all. Correct me if I'm wrong).

How is it technically possible to create a class B amplifier with extremely low distortion? I warmly leave it to you more technically savvy to explain this. I think there are more people than me who are interested in this.

The class B amplifier I am referring to is Topping B100. Read about the test results here:

Topping B100 Amplifier Review

Not saying it’s what you mean, but to me that reads a bit like “can we only talk about the good things” Nah :) i just thought the power argument was done by now ...

audiosciencereview.com

Here is a Wikipedia link for those who want to read about different amplifier classes:

Power amplifier classes - Wikipedia

en.wikipedia.org

 

[Old_School_Brad]

I'll just quote PMA in this thread as I think it's more appropriate than in the review thread.

I heard an issue with the crossover distortion, back in the eighties, DIY power amplifier with adjustable idle current. Quite audible at low volume level, and disappeared when idle current set to at least several mA.
It depends on loop gain and real level of spectrum residuals. You will definitely hear them if there is a comb of equivalent height of high order harmonics of some -40dBr.

My point was that I've never seen, or heard rather, it as an issue though. The lowest quiescent current I've seen was 1.25 mA over 0.22 ohm emitter resistor with the set 0.275 mV per the manual. A Yamaha p5000s pro amp with smps and classic sanken BJT. And no distortion was audible. Essentially this would be a class B amplifier while technically it's an AB.

 

[solderdude]

I thought that class B based amplifiers advantage were a lot of power in combination with the disadvantage of high distortion. To solve the negative aspect class A design was integrated, creating the classic combination: Class AB amplifier.
BUT why bother with this AB combination if it can be solved in another way? Just add plenty of feedback and suddenly a class B amplifier, (Topping B100), with incredibly low distortion appears.
(note that what I mentioned above is as I understood it all. Correct me if I'm wrong).

How is it technically possible to create a class B amplifier with extremely low distortion? I warmly leave it to you more technically savvy to explain this. I think there are more people than me who are interested in this.

The class B amplifier I am referring to is Topping B100. Read about the test results here:

Topping B100 Amplifier Review

Not saying it’s what you mean, but to me that reads a bit like “can we only talk about the good things” Nah :) i just thought the power argument was done by now ...

audiosciencereview.com

Here is a Wikipedia link for those who want to read about different amplifier classes:

Power amplifier classes - Wikipedia

en.wikipedia.org

The biggest issue with crossover distortion in class-B is the transistors Vbe diving down to 0V or negative which makes them slower reacting when they have to start conducting again.
Using better reacting output devices, smart biassing of the output devices (not switching completely off), feedback trickery and lots of open loop gain and high BW could all be part of the solution.
In any case it won't be a class AB amp with just a lowered idle current.

 

[pma]

@DanielT : guys in the local DIY community here also designed and built class B amplifiers with THD as low as -120dB. It is possible to do it. Never forget that THD+N here (Topping) is achieved with a very low and impractical gain. Then, with such a low gain, it is about PCB design, wiring and feedback transfer function optimization. It is possible to do it and it is less difficult than with class D. You should rather ask how do they do it in class D.

 

[SIY]

BUT why bother with this AB combination if it can be solved in another way? Just add plenty of feedback and suddenly a class B amplifier, (Topping B100), with incredibly low distortion appears.

Just pouring on feedback will not fix inherent crossover distortion from Class B. The key issue is that during that crossover, all of the output devices are switched OFF and the feedback can't have any effect because the output is off (simplified explanation). Also, just increasing feedback will generally cause stability issues.

I don't know what Topping did and would love to know more about it, but there must be a very clever workaround. Speculation hat on, some feedforward? A sliding bias?

 

[pma]

The biggest issue with crossover distortion in class-B is the transistors Vbe diving down to 0V or negative which makes them slower reacting when they have to start conducting again.
Using better reacting output devices, smart biassing of the output devices (not switching completely off), feedback trickery and lots of open loop gain and high BW could all be part of the solution.
In any case it won't be a class AB amp with just a lowered idle current.

You can switch them off however you must not use Re additional resistors. This is a fact and amplifiers like this were built.

 

[SIY]

Never forget that THD+N here (Topping) is achieved with a very low and impractical gain.

The data show similar distortion performance at higher gains as well.

 

[pma]

Just pouring on feedback will not fix inherent crossover distortion from Class B. The key issue is that during that crossover, all of the output devices are switched OFF and the feedback can't have any effect because the output is off (simplified explanation). Also, just increasing feedback will generally cause stability issues.

I don't know what Topping did and would love to know more about it, but there must be a very clever workaround. Speculation hat on, some feedforward? A sliding bias?

Feedback optimization and no additional Re resistors. I am speaking about real, built and measured amplifiers. Not my design, however. A colleague here.

 

[wwenze]

Is Topping B100 class B?

 

[SIY]

Feedback optimization and no additional Re resistors. I am speaking about real, built and measured amplifiers. Not my design, however. A colleague here.

With no Re, how did he achieve thermal stability?

 

[DanielT]

The biggest issue with crossover distortion in class-B is the transistors Vbe diving down to 0V or negative which makes them slower reacting when they have to start conducting again.
Using better reacting output devices, smart biassing of the output devices (not switching completely off), feedback trickery and lots of open loop gain and high BW could all be part of the solution.
In any case it won't be a class AB amp with just a lowered idle current.

Compare a good class B based amplifier that is designed with what you mention: Using better reacting output devices, smart biassing of the output devices (not switching completely off), feedback trickery and lots of open loop gain and high BW could all be part of the solution.
....against a good class AB based amplifier. In such a case, what does a class AB based amplifier add that a class B does not have?

I'm just trying to get the hang of Topping B100.
Only Topping themselves can answer that but if you think about it: Why do you think Topping chose class B instead of class AB?

In general, why even choose to design a class AB amplifier when it seems possible to get superb performance with a class B based amplifier?

I think that for this thread we should exclude the pros and cons of class D amplifiers. There are already so many threads bringing it up, class D vs ....X class that is.

 

[pma]

The data show similar distortion performance at higher gains as well.

117dB with 20dB gain. This is as expected with the best class B design.

 

[Blumlein 88]

With no Re, how did he achieve thermal stability?

They did extensive auditioning. Then changed components until it sounded thermally stable? /sarc

 

[pma]

With no Re, how did he achieve thermal stability?

No Re and almost no Vbias. You want a link to Czech forum? The guy who designed it was my fellow student at the Czech Technical University.

 

[pma]

You guys are NOT amplifier circuit designers.

 

[pma]

One more controversial post of mine - it is not necessary to give all the know how to chinese guys for free.

 

[wwenze]

One more controversial post of mine - it is not necessary to give all the know how to chinese guys for free.

But can we suck it from Topping? Honest question

 

[Old_School_Brad]

One more controversial post of mine - it is not necessary to give all the know how to chinese guys for free.

Too late.

 

[pma]

I'll just quote PMA in this thread as I think it's more appropriate than in the review thread.


My point was that I've never seen, or heard rather, it as an issue though. The lowest quiescent current I've seen was 1.25 mA over 0.22 ohm emitter resistor with the set 0.275 mV per the manual. A Yamaha p5000s pro amp with smps and classic sanken BJT. And no distortion was audible. Essentially this would be a class B amplifier while technically it's an AB.

In case you guys are interested, I will prepare a listening test with various level of crossover distortion, based on output stage bias of the old class AB amplifier topology. Measurements will go together with music samples.

 

[SIY]

No Re and almost no Vbias. You want a link to Czech forum? The guy who designed it was my fellow student at the Czech Technical University.

If it details his design, sure. I'd love to see how he pulled it off.

You guys are NOT amplifier circuit designers.

I'll just stare at the ground and drool.

What compels you to repeatedly post condescending crap like that?