I love class-AB amplifiers. You get most of the midrange magic of the space heaters that are pure class-A amps, while also getting meaningful amounts of power. They’re not too big, not too expensive, and -- crossover distortion aside -- have no major limitations in sound quality. And a well-engineered class-AB amp should last for years, even decades.
But we’ve all known all that for a while now. Despite what some advertising campaigns might have you believe, there haven’t been any breakthroughs in class-AB circuit design in recent times. Material qualities, tolerances, and reliability have greatly improved over the last decade, but I think it’s fair to say that most competently designed, modern class-AB amps sound broadly the same. Manufacturers’ differing design philosophies may result in subtle differences in sound here and there, but the overall sound should have similar characteristics: wide bandwidth; linear sound from 20Hz to 20kHz; and low levels of noise and harmonic distortion.
That’s not to say that each new generation of class-AB amps we review doesn’t reveal incremental improvements. Massive toroidal transformers have long been the standard for top-flight class-AB amplifiers: the bigger the transformer, the greater the current capacity and power output. However, one limitation of using a linear power supply is that a circuit designer can push the signal/noise ratio only so far, due to the AC-line hum and buzz caused by magnetic interference from a power transformer. This has led several manufacturers, such as Germany’s T+A Elektroakustik, to begin marrying their class-AB circuits to high-quality, high-voltage switching power supplies. When well executed, such designs can offer substantially more current than their linear counterparts, and exert greater control over speakers that present a challenging electrical load. Such an approach can also dramatically decrease the number of magnetic components in the power supply, and thus minimize the magnitude of attendant magnetic fields.
Nor are the benefits of high-voltage switching power supplies merely academic, detectable only by measuring. Having last year reviewed T+A Elektroakustik’s PA 2000 R integrated amplifier, which has a class-AB circuit and a single switching power supply, I was surprised at how much better it sounded than my reference Hegel Music Systems H360 DAC-integrated amp ($5700) -- which, like all Hegel amps, has a linear supply. So maybe there is still room for improvement in class-AB amps. Still, at $8500 USD, the T+A isn’t an option for most audiophiles.
Benchmark Media Systems’ products definitely are options. When I asked their director of engineering, John Siau, about the future of class-AB, he told me that Benchmark had long used their own DA-101 line-level distribution amplifier circuit cards for in-house listening tests. “These cards were unity-gain class-AB amplifiers with a 130dB [signal/noise ratio]. They had very low THD and a 0.1Hz-500kHz bandwidth. The only limitation of the DA-101 was power. The DA-101 cards could only deliver 35W into 8 ohms, but they were cleaner and quieter than any other option.”
Such a power limitation can be problematic for audiophiles who want to drive inefficient speakers to ear-splitting levels, and Benchmark wanted to better the DA-101’s noise and THD specs. As Siau told the story, THX approached Benchmark at an Audio Engineering Society Convention with their Achromatic Audio Amplifier (AAA) technology, which uses clever feed-forward error correction. Impressed, Benchmark licensed and implemented AAA for their staggeringly good AHB2 stereo/mono amplifier ($2995), which I reviewed in 2015. As I wrote then:
Lifting the lid on the AHB2 reveals circuitry that is class-AB-ish. As its model name hints, the AHB2 is nominally a class-H amp, which means that when power demands are low, it functions as would a traditional class-AB design. But as the listener asks for more power, extra voltage rails are activated that deliver more power with higher efficiency than can class-AB. . . . The low-power, error-corrected AAA runs in parallel with the high-power main amplifier, allegedly keeping it free of the crossover distortion inherent to class-AB circuits.
While not a pure class-AB amplifier, the AHB2 uses class-AB as a foundation, finessing the circuit to maximize its advantages while minimizing its limitations.
Is there any longer any hope for traditional class-AB amps with linear power supplies? Chris Russell, CEO of Bryston Ltd., seems to think so. While he admitted that class-D -- to which many manufacturers have turned -- holds promise, he concedes that “it is not there yet,” and that Bryston continues to prefer class-AB output stages because of their excellent inherent linearity:
Once we have chosen maximum linearity as one of the most important parameters of an output stage, we can look at the characteristics of the power devices to see where they perform best, and where they may have problems. Power transistors have a rather broad linear region in their transfer function, from just above their “on” threshold, up to the higher current region closer to saturation, or fully “on.” At very low current near threshold, and at very high current near saturation, the transfer function bends toward an overall “S” shape. Thus, it becomes obvious that for the best linearity we want to keep the output devices operating within the optimal linear region of their transfer function. We can do that by operating the output transistors as opposite polarity pairs, switching from one polarity to the other at the zero-crossing region. . . . Bryston began exploring the possibility of giving the two polarities of output transistor exactly “equal and opposite” characteristics. In that way, the zero-crossing region can remain linear.
Bryston has implemented their proprietary Quad Complementary output stage in their new Cubed range of amplifiers, including the B1353 integrated, which Philip Beaudette will soon review for us. I envy him. A few years ago, when I visited Bryston’s factory in Peterborough, Ontario, the Bryston folks hinted with a twinkle in their eye that they’d made a bit of a breakthrough in their amplifier research. Given my affinity for high-quality, high-power, class-AB integrateds, I’m disappointed to have missed out on Bryston’s newest.
The progress made by such companies as Bryston, Benchmark, and T+A makes me optimistic about the future of class-AB amplification. It remains the architecture of choice for integrated amplifiers asked to strike an affordable balance of power and efficiency while remaining friendly to those who continue to revel in the vinyl renaissance. I’ve been tempted by some class-A amps I’ve reviewed, as well as hybrid digital amps like those from Devialet. Their tendency to sound particularly musical with a vibrant midrange dimensionality, make them attractive options. However, like Bryston’s Chris Russell, I don’t think I have it in me to take the leap just yet -- not while there’s still more to come.
. . . Hans Wetzel
hansw@soundstagenetwork.com