Astell&Kern A&ultima SP4000T Digital Audio Player

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.

RAYTHEON JAN6418 Quad

The SP4000T is the industry's first portable player to feature four MIL-Spec vintage Raytheon JAN6418 vacuum tubes in a dual-channel, symmetrical Quad configuration — a design previously exclusive to high-end home audio amplifiers.

To overcome the challenges of managing multiple highly sensitive tubes, Astell&Kern precisely pairs each valve by noise and gain characteristics, isolating them on independent flexible PCBs to completely eliminate microphonic interference.

Operating in a mirrored differential setup, these four tubes work in perfect balance to cancel out common-mode noise while doubling sonic clarity, delivering the richest, most resonant analogue harmonics ever achieved in a portable player.

Triple Amp System

Powered by Astell&Kern's exclusive TERATON ALPHA technology, the SP4000T seamlessly fuses rich analogue warmth with modern digital dynamics through its signature Triple Amp System.

Users can freely switch between three distinct sonic profiles to match their music and headphones:

OP AMP Mode:
Delivers Astell&Kern's pinnacle signature clarity, massive soundstage width, and distortion-free, high-resolution dynamics across every genre.

TUBE AMP Mode:
Activates the Quad-configured real vacuum tubes, delivering the irreplaceable, emotionally deep, and velvety analogue texture of classic valve amplification alongside groundbreaking low noise.

HYBRID AMP Mode:
A next-generation hybrid engine featuring a 5-stage adjustment slider, allowing you to seamlessly blend the organic soul of tube warmth with the crisp resolution of solid-state playback to dial in your perfect custom sound.

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, which is 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.

HEXA Audio Circuit Structure

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 analogue 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

The SP4000T inherits the acclaimed High Driving Mode from the SP4000, which has been praised 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 creating a deeper, more vivid listening experience.

Proprietary ESA

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

The SP4000 introduces Advanced DAR, Astell&Kern’s second-generation DAR (Digital Audio Remaster) technology, delivering sound that is even more natural and refined.

Rather than sending the audio file directly to the DAR engine, the SP4000 first uses VSE (Virtual Sound Extender) to virtually restore lost harmonics, enhancing sound quality at the initial stage.

The advanced DAR engine then performs a second-stage upsampling process with greater precision and completeness. This two-step approach allows listeners to enjoy a deeper, more immersive sound that comes closer than ever to the original recording.

Astell&Kern Direct Path

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.

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

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 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, where bovine bone powder-derived tallow is added during the vegetable oil tanning process after removing the hair from the hide.

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' colour and wraps snugly around the entire device, offering a perfect fit while featuring a rear pattern cut-out designed to showcase the tube LED glow.

Specifications