ATI Battle Class AB vs Class D Amplifier Shootout

[Verdinut]

If you have a positive standing bias voltage on both transistors in a push-pull configuration, there must be a power level at which the configuration operates in Class A, at which both transistors can satisfy the waveform with the bias voltage. It may be a very low power level, but it has to exist.

In class A, where there is only one transistor in the output stage, there is no need for any bias as the transistor works on the full positive and negative portions of the wave. The question of crossover distortion doesn't apply.
In my opinion, if you have an amplifier with both transistors each operating half-wave in a push-pull configuration, it cannot be a class A amp since you have that crossover point at 0 volt. It's a class B amp. Then you apply some bias voltage to eliminate that distortion at that critical crossover point and it becomes a class AB amp.

 

[Irvrobinson]

In class A, where there is only one transistor in the output stage, there is no need for any bias as the transistor works on the full positive and negative portions of the wave. The question of crossover distortion doesn't apply.
In my opinion, if you have an amplifier with both transistors each operating half-wave in a push-pull configuration, it cannot be a class A amp since you have that crossover point at 0 volt. It's a class B amp. Then you apply some bias voltage to eliminate that distortion at that critical crossover point and it becomes a class AB amp.

Not correct. You've confused a single-ended output stage with Class A. Differential (sometimes called push-pull) output stages can also operate in Class A.

 

[Irvrobinson]

Interesting he writes for that blog and all the phoolishness it represents.

For his two current companies, Pass Labs and First Watt, Positive Feedback readers are an important segment of his target market. Also, Nelson believes that amplifier design can significantly impact audible performance, even down to the type of transistor he uses, so he seems to embrace the audiophile market more than operate outside of it.

I think PENG owns a First Watt amp... true?

 

[Verdinut]

A McGill University text more clearly explains amplifier classes operation:

https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/e/Electronic_amplifier.htm

"A practical circuit using Class B elements is the complementary pair or "push-pull" arrangement. Here, complementary devices are used to each amplify the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small glitch at the "joins" between the two halves of the signal. This is called crossover distortion. A solution to this is to bias the devices just on, rather than off altogether when they are not in use. This is called Class AB operation. Each device is operated in a non-linear region which is only linear over half the waveform, but still conducts a small amount on the other half. Such a circuit behaves as a class A amplifier in the region where both devices are in the linear region, however the circuit cannot strictly be called class A if the signal passes outside this region, since beyond that point only one device will remain in its linear region and the transients typical of class B operation will occur. The result is that when the two halves are combined, the crossover is greatly minimised or eliminated altogether.
However, it is important to note that while the efficiency of Class AB is greater than Class A, it is less than Class B.


Class B Push-Pull Amplifier

Class B or AB push-pull circuits are the most common form of design found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is reproduced with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well."

 

[Irvrobinson]

My best advice for people posting on internet forums, which I try to follow myself, is when you find yourself in a hole, stop digging.

 

[Trell]

My best advice for people posting on internet forums, which I try to follow myself, is when you find yourself in a hole, stop digging.

Is that the reason for your choice of an avatar

 

[PENG]

What AH wrote is consistent with what Nelson wrote. The bias current essentially "lifts" the waveforms conduction duration above 180 degrees for those waveforms with a peak power requirement greater than the bias current. So that means that another way of describing the effect is that the time that both transistors are conducting for a given waveform at a given power level will effectively increased as the bias current increases.

As I mentioned, the only part in NP's write up that bothers me a little, is the part I highlighted, that is "Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias. ". That actually contradicts with what he wrote in the sentence prior, "Class B has no bias current, Class AB has a moderate bias current............................" That part was perfect, until the second sentence in the very same paragraph that muddies the water.

By his own definition, how does a class AB amp become class B amp. One can argue he added "determined by the bias"...but that would be for argument sake only as it then becomes circular logic, because by definition, to be considered class AB, there has to be a minimum amount of bias current to keep the transistors conduct more than 180 degrees. I hope I am clear this time.

Back to your original point, do you agree, at least by definition that even at 0.01 W, a class AB amp could have left class A? As NP said,..... determined by the bias right? PS: I do think you are right from practical stand point, though one never knows for sure..if something can happen in theory, it just might happen in practice too.

 

[PENG]

For his two current companies, Pass Labs and First Watt, Positive Feedback readers are an important segment of his target market. Also, Nelson believes that amplifier design can significantly impact audible performance, even down to the type of transistor he uses, so he seems to embrace the audiophile market more than operate outside of it.

I think PENG owns a First Watt amp... true?

I own the kit, has not started building yet because I want to finish building two pairs of speaker first. I still have to shop for the power supply components for the F5.

 

[Irvrobinson]

As I mentioned, the only part in NP's write up that bothers me a little, is the part I highlighted, that is "Class AB run Class A at low power levels, and become Class B amplifiers at output currents determined by the bias. ". That actually contradicts with what he wrote in the sentence prior, "Class B has no bias current, Class AB has a moderate bias current............................" That part was perfect, until the second sentence in the very same paragraph that muddies the water.

By his own definition, how does a class AB amp become class B amp. One can argue he added "determined by the bias"...but that would be for argument sake only as it then becomes circular logic, because by definition, to be considered class AB, there has to be a minimum amount of bias current to keep the transistors conduct more than 180 degrees. I hope I am clear this time.

Back to your original point, do you agree, at least by definition that even at 0.01 W, a class AB amp could have left class A? As NP said,..... determined by the bias right? PS: I do think you are right from practical stand point, though one never knows for sure..if something can happen in theory, it just might happen in practice too.

I do agree, it is definitely possible (though hardly advisable) to build a Class AB amp that would leave Class A operation before 0.01W. In fact, it wouldn't shock me to find that some old Class AB automotive amplifiers use very low standing bias levels, but Benchmark was comparing themselves to mains-powered consumer amplifiers.

Regarding Nelson's wording, you are a stickler for precision, and I'm not going to complain about it; I like that trait.

 

[PENG]

Not correct. You've confused a single-ended output stage with Class A. Differential (sometimes called push-pull) output stages can also operate in Class A.

Agreed, for some reason he forgot there are class A push pull amps, but I guess his point is also that not all class AB amps operate in class A even at very low output level. On that point, he could be right because he has been told (or read from a website) already by QSC, that his DCA class AB amp operates in class AB full time, no transition from A to AB as such. Or we can all be right, if the DCA still in theory runs in class A up to only 0.01W or even less, then it is practically/almost class AB full time.

Case in point, Parasound, and I am sure there are others (probably Emotiva too),call their Halo amps:

• High bias Class A/AB operation

implying their non Halo, the classic series are just class AB, no high bias.

The idling power consumption spec generally should give a good indication on whether the class AB amp has enough output in class A (imo, at least 1 W) to be considered "high bias A/AB".

 

[RichB]

Agreed, for some reason he forgot there are class A push pull amps, but I guess his point is also that not all class AB amps operate in class A even at very low output level. On that point, he could be right because he has been told (or read from a website) already by QSC, that his DCA class AB amp operates in class AB full time, no transition from A to AB as such. Or we can all be right, if the DCA still in theory runs in class A up to only 0.01W or even less, then it is practically/almost class AB full time.

Case in point, Parasound, and I am sure there are others (probably Emotiva too),call their Halo amps:

• High bias Class A/AB operation

implying their non Halo, the classic series are just class AB, no high bias.

The idling power consumption spec generally should give a good indication on whether the class AB amp has enough output in class A (imo, at least 1 W) to be considered "high bias A/AB".

From posts by Parasound folks on AVS, I think it's fair to say that the non-Halo series (class A/B) have less bias (less high-bias if that's a thing).

- Rich

 

[PENG]

From posts by Parasound folks on AVS, I think it's fair to say that the non-Halo series (class A/B) have less bias (less high-bias if that's a thing).

- Rich

I think that is correct regarding their current non Halo series, but some of their older classic amps were high bias too, they just didn't call any (not even Halo's) of them A/AB in those days. Again, I welcome the A/AB "class", as it helps clarify the myth (sort of) that all class AB amps run in class A at low level. Just checked Emotiva site, as I alluded to earlier, they do call their monoblock class A/AB now.

 

[Irvrobinson]

...he has been told (or read from a website) already by QSC, that his DCA class AB amp operates in class AB full time, no transition from A to AB as such.

This is an absolutely correct explanation. There is no such thing in conventional Class AB amps as an "A to AB transition", implying a change in amplifier class. It is simply a power level in a Class AB amplifier at which the bias voltage is no longer sufficient to keep both transistors in the differential pair powered for all 360 degrees of the sine wave.

 

[RichB]

There are amplifiers using advanced DSP's for monitoring, bias, and protection that are outside the signal path. Benchmark (Spartan 6 FPGA), Anthem, and Shitt are some of them.

Since I am a bit wonk, I love this post by John Siau:

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-benchmark-ahb2-amp.7628/page-18#post-180791

No this is not a problem at all. Capacitive or inductive speaker loads will not cause stability issues. The feed-forward system is inherently stable. The only limitation is the over-current protection circuit. The amplifier will shut down if repetitive peak currents exceed 29 Amps or if the RMS current exceeds 20.5 Amps for more than a few seconds.

The TEMP lights will flash when peak currents exceed 29 Amps. This is a warning that the protection will kick in if the condition persists. The CLIP lights will flash whenever the amplifier is driven into voltage clipping causing the THD to exceed about 0.5%. If the CLIP and TEMP lights flash together, it indicates that you have simultaneously reached the voltage and current limits of the AHB2. Each channel has its own set of lights and has its own protection monitoring. If protection is triggered, both channel will mute. To protect your tweeters, the distortion monitoring will not allow sustained operation if the amplifier is driven into voltage clipping resulting in a sustained THD exceeding 1%.

The feed-forward correction keeps the AHB2 distortion free when driving very low impedances and difficult phase angles. The protection system in the AHB2 monitors output current, output voltage, distortion, temperature and other critical parameters. The AHB2 can cleanly drive a 1.4-Ohm resistive load to full output voltage on both channels for several seconds without triggering the protection circuits. This is much longer than any musical peaks. Load impedances below 1.4 Ohm (stereo mode) or 2.8 Ohms (mono mode) may flash the TEMP lights indicating that peak currents exceed 29 Amps. If the overload condition is severe enough, and persists for a long enough time, the over-current protection may trip. The amplifier remains distortion free until the protection system shuts it down.

Unlike most amplifiers, the protection circuits are not in the audio path. For this reason, the protection circuits have no impact on the audio quality until they activate and mute the amplifier to protect the amplifier and speakers from damage. The speed at which the protection circuits react is a function of the severity of the overload. You can place a short circuit across the output of the amplifier while it is delivering full output. This will immediately activate the protection and the amplifier will be fully protected from this short-circuit event. In contrast, we allow peak currents to exceed 29 Amps without immediately shutting down. The time interval is determined by the severity of the overloads. This is all controlled by digital signal processing (DSP) in a Xilinx FPGA.

From the amplifiers perspective, -45 degrees at 4 Ohms is equivalent to driving a 2.8-Ohm resistive load. This can be driven to full output voltage in stereo mode or in mono mode.

-50 degrees at 4.1 Ohms is equivalent to driving a 2.6-Ohm resistive load. This can be driven to full output voltage in stereo mode and can be driven to within 0.1 dB of full output in mono mode without flashing the TEMP lights. Even in mono mode, it would be virtually impossible to trip the protection with music although it could be done with a 100 Hz sinusoidal test tone if it was played at, or slightly above, full power for a few seconds.

One unique characteristic of the AHB2 is that the power supply rails are tightly regulated. In contrast, virtually all other amplifiers use unregulated power supplies. This means that the DC rails sag in a conventional amplifier when the amplifier is driving a difficult load. With the AHB2 the rails maintain their voltage and the amplifier continues to deliver nearly the same rail-to-rail voltage swing. This can be seen from the fact that the AHB2 delivers almost exactly twice the power into 4 Ohms as into 8 Ohms. It is also why we get a near 4:1 increase in power when running in bridged mono.

I've asked questions of other manufactures about clip indicators but never received this level of detail.
Bi-amping the Salon2's I have seen the Clip indictor for the woofer amp illuminate playing Lorde's Royals but not the Temp, indicating voltage clipping only. That's interesting.

- Rich

 

[Irvrobinson]

I think that is correct regarding their current non Halo series, but some of their older classic amps were high bias too, they just didn't call any (not even Halo's) of them A/AB in those days. Again, I welcome the A/AB "class", as it helps clarify the myth (sort of) that all class AB amps run in class A at low level. Just checked Emotiva site, as I alluded to earlier, they do call their monoblock class A/AB now.

I admit that I get a charge out of these high-end companies that use the invented term "Class A/AB". I suppose you could stretch the impact of the variable bias designs to say that they sometimes act like Class A amps, but all of this is really just marketing talk.

 

[Verdinut]

The idling power consumption spec generally should give a good indication on whether the class AB amp has enough output in class A (imo, at least 1 W) to be considered "high bias A/AB".

For example, the QSC DCA 1222 consumes 0.8 A at idle while the 1622 which @everettT owns consumes 0.9 A. This would confirm the fact that they are both heavily biased and operate constantly in class AB.
I confirm being told by QSC that the DCA 1222 constantly operates in class AB.

 

[Verdinut]

For example, the QSC DCA 1222 consumes 0.8 A at idle while the 1622 which @everettT owns consumes 0.9 A. This would confirm the fact that they are both heavily biased and operate constantly in class AB.
I confirm being told by QSC that the DCA 1222 constantly operates in class AB.

They consume about 100 watts at idle, despite the fact that they both have a switching power supply.

 

[PLE]

@Irvrobinson @Verdinut
I believe you both have some correct understanding, and the actual real meanings will vary somehow depending on who you ask, but, at least to my understanding, you also both bring up points that are not factually correct. First There is no rules about class A that there should only be one transistor and that it should be biased at 0 V. The only rules is that each transistor operate for the full 360 degree phase, meaning that each transistors do not only push or only pull, they all do the full cycle. That is why a class AB amp can have a region where they operate in class A. If the signal is small enough both the pushing and pulling can do the full cycle and it's class A. Class AB only mean that they both do more than half the phase, one start pushing while the other one is still pulling but if the bias is high enough yes there is a zone where they could be class A. Now, Differential and push pull are certainly not the same concept. It doesn't refer to the class of operation, differential only means two symmetrical signals, opposed to single ended which means you have one signal only and a ground reference. It is not correct to say Differential also called push pull one have nothing to do with the other.

 

[PENG]

In class A, where there is only one transistor in the output stage, there is no need for any bias as the transistor works on the full positive and negative portions of the wave. The question of crossover distortion doesn't apply.

Between class A, AB and B, only class B can get away with no bias.

 

[Irvrobinson]

If the signal is small enough both the pushing and pulling can do the full cycle and it's class A. Class AB only mean that they both do more than half the phase, one start pushing while the other one is still pulling but if the bias is high enough yes there is a zone where they could be class A. Now, Differential and push pull are certainly not the same concept. It doesn't refer to the class of operation, differential only means two symmetrical signals, opposed to single ended which means you have one signal only and a ground reference. It is not correct to say Differential also called push pull one have nothing to do with the other.

This post is so dumb I should ignore it, but I can't resist asking you to explain these silly statements.

Exactly what do you think, electrically, happens in a push-pull output transistor pair? What does "pushing" and "pulling" electrically consist of? Extra credit question - does a push-pull amplifier have a phase splitter prior to the gain stage?