Reimagine Analogue Sound, A&ultima SP4000T

A&ultima SP4000T is the culmination of nearly two years of dedication and exploration, beginning from the planning and development stages of SP3000T.

DAC, Tube, and AMP — the essential elements of high-end audio — may be accessible to anyone. Yet even with the same elements, what is ultimately created from them is an entirely different matter. Using the canvas of SP4000, the essence of digital sound, Astell&Kern held onto one defining question over the past two years:
“How can we bring deeper emotion through analogue sound?”

The answer was drawn with sincerity and endless passion, stroke by stroke. Every trace of that journey is distilled into SP4000T. Astell&Kern’s own analogue sound — comfortable to listen to, yet revealing new depth and character the longer you experience it.
Discover it for yourself with A&ultima SP4000T.

DESIGN

“Burst”

The Astell&Kern SP4000T is a model within the flagship lineup that focuses on the very essence of analogue sound. To realise the highest level of analogue sensibility, Real Vacuum Tubes are applied in Quad configuration for the first time in a DAP, completing a deeply immersive analogue sound that transcends the boundaries of digital players.

The SP4000T is the successor flagship to the SP4000, and its design concept keyword is “Burst.” Interpreting through formal language the moment when concentrated energy explodes outward from the centre, the design gives visual form to the source of power from which sound erupts.

The structure tightly centred around the Volume Wheel expresses a sense of focused, tensioned energy, while the sculptural surface composition and contrast of light clearly convey the presence and physical weight of interaction. A design where mechanical precision and sculptural beauty coexist intuitively communicates a sense of ‘power’ through the fingertips. The Volume Wheel on the side is a defining element that symbolises Astell&Kern’s heritage — and the deep groove carved into its centre symbolically represents the rich density of analogue sound and the powerful driving energy of the device.

2nd Generation Anti-Microphonic Noise Architecture

The greatest technical challenge in applying vacuum tubes to a compact portable device is achieving low noise even at high resolution and high output. The SP4000T features twice as many tubes as its predecessor — four in total — and introduces an upgraded 5-stage 2nd Generation Architecture (up from the previous 4-stage design) to suppress microphonic noise (noise generated in vacuum tubes from even minor external vibrations). Drawing on Astell&Kern’s proprietary know-how and years of experience, repeated rounds of design, testing, and verification led to the optimal solution for overcoming the structural limitations of microphonic noise.

1. Separate modular Flexible PCB, isolated from the Main PCB
2. Silicone base cushion for primary shock absorption of the tube
3. A custom-fit silicone tube around the tube leads to absorb the impact transmitted through the leads
4. Silicone damper encasing the tube, suspending it mid-air at the damper tip
5. Copper Shield Can, covering the entire tube, suppresses even the faintest micro-noise

The SP4000T is the industry’s first DAP to feature Triple Tube Mode. Combined with the T Series Signature Triple AMP Mode and the Tube Current level adjustment first found on the SP3000T, it offers up to 54 possible combinations tailored to any genre or preference — a true Analogue Sound Palette.

Because how you operate the same tube fundamentally changes its tonal character, Triple Tube Mode encompasses the full expressive range of a tube amplifier. The delicacy of Triode, the balance of Ultra Linear, the dynamics of Pentode — the extraordinary analogue sound experience once exclusive to high-end tube amplifiers is now yours in the SP4000T.

RAYTHEON JAN6418 Quad — A New Standard in Tube Configuration

The SP4000T is the industry’s first DAP to feature the MIL-Spec vintage vacuum tube RAYTHEON JAN6418 in a Quad configuration.

With independent dual-tube structures dedicated to the left and right channels respectively, the channel-separated design — once exclusive to home audio high-end tube amplifiers — has finally been brought to the world of DAPs.

But the essence is not the number of tubes. Managing multiple highly sensitive vacuum tubes is far more than a simple matter of addition. It demands answers to countless questions: How do you pair tubes with subtly different noise characteristics and gain rates? How do you overcome microphonic noise? How do you minimise output variation between left and right channels?

The four tubes in the SP4000T do not operate independently — they work in a differential configuration, with each pair facing the other in mirror symmetry. In this precise balance, music signals are amplified with twice the clarity, while common-mode noise is mutually cancelled and eliminated.

Each of the four tubes is precisely measured for its noise and gain characteristics before being paired, then fully isolated from noise through independent modular flexible PCBs and a multi-layered architecture — a design that only Astell&Kern, with its long experience in analogue circuits and vacuum tubes, could bring to life.

When four hearts beat as one in perfect balance, the tube’s signature rich harmonics resonate with the deepest, most resonant sound ever heard in the SP4000T.

The Perfect Harmony of Real Vacuum Tubes and OPAMP, Astell&Kern’s TRIPLE AMP SYSTEM

A&ultima SP4000T, built on Astell&Kern’s exclusive Teraton Alpha technology, achieves a perfect fusion of true analogue sound from the Quad-configured Real Vacuum Tubes and the dynamic sound of the OPAMP. SP4000T offers TUBE AMP mode for the authentic analogue sound of a tube amplifier, OP AMP mode delivering Astell&Kern’s signature clarity and expansive soundstage, and HYBRID AMP mode combining the best of both analogue warmth and high-resolution output.

Experience a distinctly differentiated sound through the T Series’ signature amplifier technology.

Overwhelming High Resolution and Dynamics, OP AMP Mode

High resolution that reproduces the original sound without distortion, an unparalleled sense of space, and the ultimate in clarity and dynamics — OP AMP mode delivers the pinnacle of high-quality sound that Astell&Kern has pioneered across every product. The ultra-low noise and high resolution achieved through Astell&Kern’s TERATON ALPHA technology will reveal new dimensions of music you have never experienced before, across every genre.

True Analogue Sound, TUBE AMP Mode

The warm texture and unique soundstage of a Tube Amp cannot be replicated by any modern digital amplifier. Beloved since the very first appearance of tube amplifiers, they remain unmatched in conveying the emotional depth of music. SP4000T’s TUBE AMP mode pairs the deep, rich analogue sound exclusive to its Quad-configured Real Vacuum Tubes with groundbreaking low noise achieved through cutting-edge technology and craftsmanship — letting you lose yourself entirely in the music.

Astell&Kern’s Advanced HYBRID AMP Mode

Beyond just two modes — Tube AMP and OP AMP — a next-generation Hybrid AMP mode lets you handpick your preferred tonal character. Combining the analogue sensibility with the advantages of high-resolution output, the five-stage HYBRID AMP mode allows you to freely dial in the perfect sound for your taste and your music.

The Most Delicate Analogue Sound, Tube Current Option

We verified the safe operating range of the vacuum tubes and developed a system that adjusts the internal plate voltage across 3 levels, each of which changes the amplification of the tube amp — and with it, the tonal texture of the sound. Experience the subtle yet striking shifts in density and soundstage — the most delicate nuances a headphone or IEP can reveal — through tube current adjustment.

HEXA Audio Circuit Structure with Complete Separation of Digital and Analogue Signals

The SP4000T integrates dual AK4499EX DACs—AKM’s latest flagship, proven across Astell&Kern devices—paired with dedicated AK4191EQ processors for each left and right channel, forming a fully discrete audio circuit architecture.

Unlike conventional DAPs that handle both digital and analogue signals inside the DAC, the SP4000T’s audio circuit uses a dedicated digital delta-sigma modulator (AK4191EQ) to reduce noise in the digital signal before passing it to a completely isolated Analog DAC (AK4499EX) for independent analogue signal processing.

Built on this physically complete separation of digital and analogue signal paths, the SP4000T’s signature analogue sound is realised with greater depth than ever before.

High Driving Mode for the Ultimate Sound

The SP4000T inherits the beloved High Driving Mode from the SP4000, acclaimed by audiophiles worldwide. With twice as many OPAMPs arranged horizontally, it delivers even greater sonic density. Like a four-wheel-drive vehicle conquering rough terrain, High Driving Mode channels superior driving power to produce a stronger, more stable sound — precisely articulating even the most easily missed sonic details and designed for a deeper, more vivid listening experience.

Proprietary ESA (Enhanced Signal Alignment) Technology for Minimised Group Delay

The SP4000T dramatically improves Group Delay — one of audio’s most persistent challenges — through its proprietary ESA (Enhanced Signal Alignment) technology. ESA aligns frequency signals so they arrive more uniformly, minimising frequency distortion and delivering sound with greater clarity and purity.

Advanced DAR Support for Enhanced Sound

Advanced DAR is the 2nd-generation DAR technology, delivering even more natural and refined sound. Rather than passing audio directly to the DAR engine, it first routes the signal through VSE (Virtual Sound Extender) to virtually restore lost harmonics and enhance audio quality — then applies DAR technology in a second stage for a more complete upsampling result. This allows listeners to enjoy a deeper, more immersive sound that is truer to the original recording.

Keeping pace with the shift toward streaming-centric listening, the SP4000T runs the latest Android 15 — delivering top-tier analogue sound quality alongside exceptional convenience.

Astell&Kern Direct Path for Bit-Perfect Playback

ADP (Astell&Kern Direct Path) — Astell&Kern’s proprietary customisation technology — bypasses the Sample Rate Conversion (SRC) limitation inherent to the Android OS, enabling lossless Bit-perfect playback even in streaming environments for the highest possible audio quality.

Precision Sound with Any Layer HDI PCB

The SP4000T uses the same Any Layer HDI PCB for high performance as the SP4000. Commonly found in smartphones and high-end electronics, this high-density circuit board enables the precise design of complex circuits within a smaller footprint — minimising signal loss and delivering more refined sound.

99.9% Pure Copper Shield Can

The SP4000T employs a complete Shield Can design to block noise and electromagnetic interference that could affect the audio block. Using a 99.9% pure copper Shield Can, it goes beyond basic shielding to deliver an even cleaner, more vivid sound.

Sound Design Through DAC Filters

Design your own sound style with 7 DAC filters.

Enhanced Battery Protection Mode

The battery protection mode — previously fixed at 85% — now offers a wider range of options. Choose the option that best fits your Music Lifestyle.

Real-Time Information with LED Lights

The LED light surrounding the power button provides information on the Bit Depth of the currently playing song and also adds a visually pleasing element by making it shine and stand out.

The Premium Leather Case

The bundled leather case for the SP4000T was crafted using ‘MINERVA’ leather from ‘BADALASSI CARLO’, a renowned leather workshop located in the San Miniato district of Florence, Italy.

Badalassi Carlo is a tannery that has been producing leather for over 40 years, combining the traditional Tuscan vegetable tanning methods with the theories and techniques of CARLO BADALASSI, a former leather-school professor from the Santa Croce region.

Vegetable tanning is an eco-friendly leather-making process — slow and time-intensive — that tans hides using only plant-derived tannin extracts. Badalassi Carlo leathers employ the Tuscan vacchetta technique — after removing the hair from the hide, bovine bone powder-derived tallow is added during the vegetable oil tanning process.

Pursuing small-batch, high-quality production over mass output, the workshop is highly regarded as the preferred leather artisan for renowned brands that place a premium on material quality.

The case is available in ‘Cognac’ color and wraps snugly around the entire device, offering a perfect fit while featuring a rear pattern cutout designed to showcase the tube LED glow.