SB Luna – 2A3 Single-Ended Integrated Amplifier

The 2A3 is one of the most celebrated vacuum tubes in the world of audio, a true icon among tube amplifier enthusiasts. Used mainly in single-ended configurations, this tube is renowned for its warm, natural, and incredibly musical sound. Its simplicity of construction and minimalist design make it ideal for achieving a perfect balance between elegance and high-quality sound performance.

In a single-ended amplifier, the 2A3 stands out for its ability to express extraordinary sonic detail while maintaining a linearity that envelops the listener in an engaging and authentic sound experience. With its 3 watts of power, the 2A3 is not designed to “dominate” the room, but rather to deliver a delicate and refined sound that allows every nuance of the music to be appreciated.

If the heart of the true audio enthusiast lies in the pursuit of purity and the ability to elevate music to a higher emotional level, the single-ended 2A3 is a choice that represents the very essence of high fidelity.

Integrated amplifier with 2A3 output tubes

For this amplifier, I chose to adopt the “STC” or “Super Triode Connection” topology, a design conceived by Shinichi Kamijo which, although not widely used, offers interesting potential. The circuit is entirely DC-coupled, without the use of capacitors along the signal path.

The basic configuration consists of a pentode input stage, a feedback triode, and an output tube that can be either a triode or a tetrode/pentode. However, implementing this schematic is not easy, as finding the right balance among the three tubes is challenging, ensuring that all of them operate within dissipation limits and in a linear region.

One of the most interesting aspects of this configuration is its ability to achieve a high damping factor without using a closed-loop feedback taken from the loudspeaker terminals.

In the STC circuit, local feedback is applied from the plate of the output tube to its own grid through the upper triode of the SRPP stage, while the input element should ideally be a pentode, since its very high internal resistance allows for minimal distortion in the circuit.

Readers of this website already know my opinion regarding the damping factor of an amplifier, which I consider a fundamental element for sound quality. One of the features that contributed to the success of the STC as a reference tube circuit is precisely its above-average damping factor. In an audiophile context where many prefer to avoid feedback for fear that it might negatively affect the sound—and where the importance of the damping factor is often overlooked—it is interesting that a circuit which actually incorporates feedback and exhibits a high damping factor is appreciated precisely for its sonic quality. This success may stem from the fact that, for many, the feedback is not easily perceptible, while others, more attentive to detail, are fascinated by the non-linear theories applied by designers, thus discovering a sound that surprises even those who initially would not have considered this solution.

Returning to the SB Luna, it is an optimized STC circuit employing a 6SJ7 pentode at the input, the triode section of a 6SN7 per channel as the feedback element, and a 2A3 output tube. In my version, the circuit uses fixed bias and does not include a cathode resistor under the 2A3, as this would have caused excessive heating. Instead, the cathode of the output tube is raised above ground by a stabilized +200 V supply provided by a tube regulator circuit. This voltage is more than sufficient to feed the few milliamperes needed to lift the output tubes and to supply the screen grids of the 6SJ7.

The SB Luna represents the evolution of one of my earlier amplifiers, the SB-IT 2A3, now considered “obsolete,” which also used the STC circuit. Although the output stage design has remained unchanged, the differences between the two models are significant. In the Luna, the large heatsink that in the previous model cooled the transistors supplying stabilized voltage to the 2A3 filaments has disappeared. The filaments are now powered by a passive CLC cell, equipped with 33,000 µF of filtering capacity and inductors specifically designed for 2A3/300B tubes.

In the new model, bias adjustment has been simplified. Previously, one had to insert a tester’s probes into two sockets on the rear panel and adjust two trimmers that varied the voltage supplied by the PCL84 (one per channel) to lift the output tube from ground. Now, with the new configuration using a single stabilizer, the total current is easily managed by one PCL84. Furthermore, by eliminating the heatsink, I gained space to add two milliamp meters and two trimmers directly on the chassis, greatly simplifying bias monitoring and calibration operations.

The output transformer of the Luna was designed using the experience gained from building the previous IT 2A3. Thanks to this evolution, the Luna now delivers one more watt and offers a wider bandwidth. If the old IT 2A3 was already pleasant to listen to, the Luna takes it further, with an even more refined high-frequency range and micro-detail.

An archive photo of the old SB IT 2A3 from the early years of SB-LAB

Inside the SB Luna there is a circuit that manages bias delay on startup and handles volume control and input selection via infrared remote control. The potentiometer is motorized, while the right knob allows input selection without using the remote.

I personally developed a custom control board and wrote the software to implement features not found in commercial circuits, such as the delayed application of high voltage (B+) with corresponding LED indication on the power button. Other features include remote-control signal indication and disabling of unused channel buttons. Luna provides 3 inputs in total.

The audio signal from the inputs is not switched by simple relays, but by hermetically sealed relays with silver contacts, which are at least 20 times more expensive than those used in preassembled Chinese modules. The latter often employ low-quality relays and cheap brass-contact switches that tend to oxidize over time, causing capacitive crosstalk, especially when the input stage impedance is high. Even in my previous IT-2A3 project, I used rotary selectors solely to remotely control the same high-quality relays now adopted in the Luna.

A Sound Without Compromise: Transparency, Control and Emotion

When you power up this amplifier, you are immediately welcomed by a soundstage that can only be described as magical. Its sound is incredibly open and airy, with a stereo image that envelops the listener without ever becoming confused or overlapping. The vocals are crystal clear, with a transparency that lets you hear every nuance, every emotional outburst, as if the performers were right in front of you. The reproduction of high frequencies is refined and delicate, never excessively bright yet never dull; every detail, from the finest violin breath to the gentle shimmer of a cymbal, emerges with astonishing naturalness.

The real surprise, however, lies in the handling of low frequencies: here the amplifier shows incredible control. The bass is deep and rich, yet never bloated or muddy. The speaker response is precise, as if each note were sculpted in sound itself, with impeccable timing that allows even the subtlest tones to be perceived—those often lost with less refined amplifiers. There is no trace of unwanted resonance or the heaviness that sometimes plagues overly powerful amplifiers; on the contrary, the low frequencies are managed elegantly, giving an impression of absolute control.

The overall result is a sound that not only fills the room but invites the listener to lose themselves in the details, immersing in musicality that is both captivating and emotionally engaging. Every track, every wave of sound is conveyed with a fluidity that seems almost effortless. Ultimately, this amplifier is a jewel for those seeking reproduction that is not only accurate but stirs the soul, elevating every listening session to new levels of understanding and emotion.

Luna: A Custom-Built Amplifier, Made for You

The Luna amplifier is handcrafted to order with the utmost care, ensuring an exclusive and tailor-made product for every audio enthusiast. Each unit is assembled in full compliance with electrical safety standards, guaranteeing long life and uncompromising performance. Moreover, Luna comes with genuine CE certification, attesting to its compliance with European safety and quality standards. Every purchase includes a detailed user manual and warranty, offering complete peace of mind to our customers. For those interested, more information and orders can be made through our contact form.

Measured Data