Bakoon/Satri SSET AMP-8510A Class A amplifier 💫

 

The First-Ever SSET Design: 

The More You Listen, the More Astonishing It Becomes

SSET stands for "Switching Single Ended Transistor"—a term I coined by drawing an analogy to the "Single Ended Triode" (SET) configuration used in vacuum tube amplifiers. The Bakoon AMP-5570A is one such example: a single-ended amplifier that utilizes transistors—originally designed for switching applications—in a linear operating mode. 

While there have been previous instances of single-ended, non-feedback amplifiers built using a single transistor—such as the SIT series released by Nelson Pass under the First Watt brand—those designs relied on specialized components custom-ordered for audio use. When the original manufacturer went out of business, that technology effectively came to an end. To my knowledge, the AMP-5570A is the first amplifier to achieve this using off-the-shelf switching components. 

The journey began due to a shortage of semiconductors. Many of the linear amplification components that audio manufacturers had long relied upon were discontinued, forcing companies to either exhaust their remaining stock or seek out alternatives. 

CEO Akira Nagai found the answer in switching components. These parts are designed to toggle between "on" and "off" states; if forced to conduct a continuous direct current while only partially "open," they would typically overheat and burn out. It is said that Bakoon Semiconductors played a pivotal role in identifying components capable of withstanding such operation. 

In an advertisement he penned himself, CEO Akira Nagai explained his motivation for the development: he simply wanted to hear what kind of sound each semiconductor would produce. Furthermore, the use of a non-feedback (zero-feedback) design is a principle he refuses to compromise on. He explains that because components other than MOS-FETs lack complementary pairs, a Class A single-ended configuration was the only viable way to achieve a non-feedback design. Thus, the specifications were established: Class A, single-ended, non-feedback, and 10W output. Looking solely at the spec sheet, it appears identical to a 300B single-ended amplifier—not because it was designed to mimic one, but because adhering to certain fundamental principles led to the same result. 

Internally, however, it is entirely different. For voltage amplification, it employs the HBK(I) circuit—an evolution of the "Satri" circuit—which showcases the brilliance of Bakoon’s design. It is a proprietary Bakoon topology that processes signals entirely in the current domain; the signal-to-noise ratio remains constant regardless of the volume control setting. The output stage drives the loudspeakers directly, without the need for an output transformer. 

The power supply is an SMPS (switching-mode power supply)—a somewhat unconventional choice in an audio world that typically favors linear power supplies. Yet, it pairs exceptionally well with a Class A amplifier. Because a high bias current flows constantly, the average power consumption remains nearly constant; even at high volumes, the circuit never demands more current than the bias level already provides. In effect, the load placed on the power supply is predetermined. Bakoon has incorporated a filter to suppress switching noise, and by eliminating the traditional power transformer, the unit is remarkably lightweight for its size. 

The true fascination of this amplifier lies in the ability to choose the output device: one can select from MOS-FET, SiC, IGBT, or ultra-low-impedance MOS-FET (ULI). While the core circuit remains the same and the gain stays consistent at around 19dB, the sonic character shifts depending on the output device chosen. In this instance, I compared the ULI and SiC versions side-by-side. 

Based on measurements from the magazine *MJ Radio & Experiment*, the ULI version features a damping factor of 89.8 and a 0.059% distortion rate at 1W. It offers a distinct sense of clarity and high resolution; a double bass seems to tumble and leap, bringing a vibrant swing to the music—the hallmark of a high-end transistor amplifier. 

The SiC version, by contrast, has a damping factor of 19.2 and a 0.15% distortion rate at 1W. Looking solely at the technical specifications, it might appear inferior to the ULI version. However, playing recordings of Chet Baker or Nathan Milstein changes one’s perspective; the voices and strings gain a sense of physical substance and a rich, supple texture. It is remarkable how much the sonic character shifts, despite the circuit remaining identical and only a single component being swapped. 

I once asked CEO Akira Nagai about the relationship between the 8510A and the 300B vacuum tube. He had long been a devoted user of a 1960s-era Western Electric 300B single-ended amplifier, and he remarked that when he first heard the SiC version, it brought that very sound back to life. He suggested that those who love the sound of vacuum tubes should choose the SiC, while those seeking greater detail and resolution should opt for the ULI. Two contributors to the magazine *MJ Radio & Experiment* also described the SiC as having a mellow, soft quality. 

True to the designer’s intent, the SiC is the closest of the four available component options to the 300B. To my ears, too, it possessed a sonic texture much closer to that tube. Interestingly, the 300B was not the specific target from the outset; rather, while comparing the sonic characteristics of various semiconductors, it simply turned out that one of them leaned in that direction. 

With an output of 10 watts, the amplifier generally performs best with high-sensitivity speakers—90 dB or higher is sufficient. This is why CEO Akira Nagai often brings a small full-range speaker along for his demonstrations. When paired with a full-range driver, the SiC’s vacuum-tube-like character becomes even more pronounced. Conversely, with speakers like high-sensitivity back-loaded horns—which place very little demand on the amplifier—the differences between the various components become significantly less apparent. 

Matching cannot be determined by sensitivity alone; one must also consider listening distance, volume levels, and speaker impedance. In a near-field setup, the range of suitable options expands considerably. A notable discovery during this process was the Dynaudio Confidence 20—rated at 87 dB and 6 ohms—a specification that might otherwise seem too demanding for a 10-watt single-ended amplifier. However, when heard up close, the pairing proved surprisingly excellent. With these speakers, the ULI opened up the soundstage in a refreshing, expansive way. 

Starting now, the SiC version is being officially imported into Japan. The unit I listened to was one of these, borrowed for the purpose of this review. This means one can now choose between the ULI and the SiC under identical conditions: opt for the ULI if you prioritize resolution and elasticity, or the SiC if you seek texture and density. CEO Akira Nagai did not set out to create a "300B" equivalent; he simply sought out components that would function well while adhering to his core design principles. It was as if an arrow had struck the dead center of a target he hadn't even aimed at—that is the SiC.