The Atma-Sphere Class D Monoblocks
In this month’s Roger’s Corner, we are going to talk about Class D amplifiers and introduce a “new” Class D amplifier that I doubt any of you reading this have heard of. Class D technology and amplifiers have been around for longer than you might think. In fact, the designation of Class D happened many years after the underlying technology was first introduced. The core technology behind Class D is pulse-width modulation (PWM) which is accomplished by switching output transistors fully on or off rather than operating them in their linear region. This wastes far less energy as heat. Related work in pulse modulation and sampling was developed during the early twentieth century. Alec Reeves, working at International Telephone and Telegraph, pioneered pulse-code modulation (PCM) in 1937.
The actual designation of “Class D” was introduced in the late 1950s to describe switching amplifiers and distinguish them from Classes A, B, and C. The “D” in Class D is sometimes incorrectly thought to stand for “digital” when in fact the “D” was assigned to follow the other amplifier classes alphabetically. Early designs used vacuum tubes and then discrete germanium transistors, which were too slow and lossy. It was not until the 1970s that Class D made a technology breakthrough in the audio world. The Infinity SWAMP (SWitching AMPlifier) also marketed as the Infinity DSP Switching Amplifier and designed by John Ulrick debuted in 1974 at the Consumer Electronics Show (CES). In addition, in 1977 Sony introduced the TA-N88 amplifier using V-FET transistors allowing for fast clean switching and making higher carrier frequencies practical. While both amplifiers represented notable engineering achievements, they were prevented from achieving lasting commercial success due to reliability problems, high manufacturing costs, and mixed reviews.
In the mid 1990s Bang & Olufsen (B&O) filed patent applications for what became known in 2001 as ICEpower which was among the first real commercial breakthroughs in Class D design. In addition to B&O products the ICEpower modules were licensed by other audio companies including Bowers & Wilkins, Legacy Audio, Martin Logan, and Jeff Rowland Design Group. Also not to be overlooked is Tripath’s trademarked variation of Class D amplification that also appeared in the mid-90s and used a proprietary switching and control system to combine the high efficiency of Class D with the sound quality associated with linear amplifiers. Known as Class-T, these chips were used in amplifiers that were often referred to as “chip amps”. Bel Canto and Audio Research were notable audio companies that used Class-T chips in their designs.
From here Class D took off, leading to further engineering breakthroughs and module designs with Bruno Putzeys playing a leading role. Improved feedback topologies reduced distortion and other engineering improvements reduced sensitivity to power supply noise. Higher switching frequencies and better output filters improved linearity. While I will not go into detail regarding the history and technology of various Class D modules, it is worth noting several that most audiophiles are now familiar with, including Purifi Eigentakt, Hypex NCore, Hypex UcD, Pascal, and the previously mentioned ICEpower. In addition, Gallium Nitride (GaN) switching devices represented another important development.
However, as mentioned earlier there is a “new” Class D amplifier I want to talk about. This is the Past-Stable Amplifier, which may be a first among Class D amplifiers to fully realize the potential of Class D operation. In the Past-Stable Amplifier, every transistor within the feedback loop is operated as a high-speed switch. The overall linearity and frequency response then depends only upon two resistors. The issues of bias, Miller effect, feedback compensation, device linearity, and temperature drift are greatly reduced as design concerns. The amplifier can easily drive reactive loads, and its damping characteristics are much like that of a tube amplifier.
The greatest advantage of the Past-Stable Amplifier lies in its claimed rail-to-output efficiency of 90%. The reported overall efficiency, including the power supply, is 81%. According to the manufacturer, this high efficiency eliminates the need for heat sinks, and 12 V battery operation is both possible and recommended. When powered by a 12 V storage battery, the amplifier is independent of powerline fluctuations and noise. A trickle charger is employed to keep the battery fully charged and may be unplugged during listening if desired. As storage batteries are known for their very low internal resistance and can deliver large currents, they provide a stable, high-current power source. This was another key advantage emphasized in the amplifier’s design description.
The switching frequency is approximately 500 kHz, with pulse-width modulation. The signal, after very thorough filtering, is then applied directly to the speaker with no further feedback. The power supply is a push-pull DC-to-DC converter operating at 100 kHz. Total power consumption at idle is under 5 watts. Beyond idle, power consumption is 5 watts plus approximately 1.2 times the average power output. The manufacturer notes this is much lower than conventional amplifiers. Power output is 40 watts per channel into 8 ohms, and 52 watts per channel into 4 ohms. Total harmonic distortion is approximately 0.1% at all power levels and all frequencies from 20 Hz–20 kHz. Unlike other amplifiers, which often exhibit rising distortion with rising frequency, the specifications describe distortion as remaining essentially constant with frequency. Therefore, the Past-Stable Amplifier can reproduce any frequency as easily as any other. Dual volume controls and high input sensitivity allow for direct connection to a line level source.
So, I imagine you are curious to know who manufactures this “new” Class D amplifier. The amplifier is the Arnoux Model 7B. It was designed by Mac Turner, but more notably co-designed with Roger A. Modjeski in 1995. While Roger needs no introduction here, Mac Turner's expertise as an inventor is largely recognized in the aerospace industry. He is known for his elegantly simple solutions in electrical and electromechanical systems. Turner’s past inventions in the audio field have been the Servo-Sound speaker (1966), the Turner Phono System (1970), the Articulator (1978), and the Remote-Sensing Amplifier (1979). While Mac Turner was a talented engineer, he did not have a vast pool of audio designer resources available to him. So, he brought in other renowned high-end audio designers such as Roger to consult on his solid-state switching amplifier designs.
So, my apologies if I gave the impression this was a modern-day amplifier. Understand that like me, I am sure this information is new to all of you. Perhaps someone reading this may have heard of the Arnoux Model 7B, but in all my years of knowing Roger, he never once mentioned this project. While he was certainly aware the technology existed, Roger never really liked to discuss Class D amplification. More importantly I never saw him bench test a Class D amplifier. I find that odd for a man who was known for bench testing any audio component regardless of the manufacturer and keeping extensive notes on everything he tested. Perhaps it was no longer an exciting subject to Roger having already met the challenge of designing a Class D amplifier a couple decades or so earlier. However, I can say with a high degree of certainty that had I not come across publicity material for the Arnoux Model 7B in one of Roger’s files, none of this may have come to light.