Astell&Kern SP4000T

Fitting four military-grade vacuum tubes into a chassis that slips into a pocket was a major technical challenge. Astell&Kern pulls it off with this new flagship in the A&ultima series, combining a separate tube stage for each channel with top-tier digital conversion. The stainless steel chassis houses three amplification modes and enough adjustment to finely tune the sound color, from a warm register to the clearest rendering.

Four military tubes in quad configuration

The SP4000T is the first digital audio player to feature four military-grade Raytheon JAN6418 vacuum tubes, mounted in pairs on each channel. This left/right separation is usually found on home tube amplifiers, rarely in a device that fits in the hand. The four tubes operate symmetrically: arranged face to face, they amplify the musical signal while common noise on both channels cancels out.

Astell&Kern measures the noise and gain of each tube before matching them. Then there is the difficulty inherent to any portable tube stage: the microphonic effect, those parasitic vibrations that even the slightest shock transmits to the tubes. The SP4000T addresses this with a five-stage anti-microphonic treatment, in which the tubes are mounted on independent flexible printed circuit boards, isolated by silicone layers, suspended on a damping system, and covered with copper shielding.

Three amplification modes to choose from

Three amplification modes coexist in the SP4000T. OP AMP mode relies on the operational amplifier stage for a clean rendering and firm control. Tube mode routes the signal through the Raytheon tubes for a warmer character and a wider soundstage. Between the two, Hybrid mode lets you adjust the tube contribution across five levels, from a hint of warmth to a distinctly analog presence.

On top of that comes the choice of tube operating mode. Triode mode prioritizes harmonic richness and smoothness. Pentode mode boosts dynamics and attack, with a more energetic sound. Ultralinear mode seeks a balance between the two. A Tube Current setting adds three density levels, from the airiest to the densest, enough to create your own sonic signature without connecting a single accessory.

Dual AKM conversion and Teraton Alpha platform

On the digital side, the SP4000T uses the foundation established by the SP4000: two AKM AK4499EX converters supported by two AK4191EQ processors, one dedicated set for each channel in a fully discrete chain, where the digital and analog paths remain physically separate. Playback goes up to 32-bit / 768 kHz in PCM and handles native DSD512, with no intermediate conversion.

The whole system is built around Teraton Alpha, the in-house block that processes power supply noise and stabilizes amplification. Two treatments are added to it: ESA aligns signals in time to limit distortion, while Advanced DAR attempts to restore lost harmonics before oversampling. High Driving Mode increases power and dynamics to handle power-hungry headphones without raising the noise floor.

The measurements follow suit: signal-to-noise ratio of 124 dB unbalanced and 125 dB balanced, harmonic distortion below 0.0006%, crosstalk down to -145 dB on the balanced output. Jitter on the reference clock drops to 200 femtoseconds, and the audio block is hidden behind 99.9% pure copper shielding to keep interference at bay.

Android 15, streaming, and bit-perfect playback

The SP4000T runs on Android 15 with full access to the Google Play Store, free to install your usual streaming apps. Where many digital audio players struggle, the Android system resamples sound on the fly and degrades playback. Astell&Kern Direct Path technology bypasses this limitation and preserves bit-perfect playback, including from online services.

Wireless connectivity benefits from a first for the brand: a dual Wi-Fi antenna which, according to Astell&Kern’s internal measurements, brings 5 GHz throughput to nearly double that of previous models. The player is Roon Ready and compatible with Qobuz Connect, receives over-the-air updates (OTA), and communicates via Bluetooth 5.2 with aptX HD, LDAC, and LHDC codecs. Connected by USB to a PC or Mac, it also serves as an external converter.

Steel chassis, 2K screen, and connectivity

The stainless steel sets the tone, quite literally: the chassis weighs about 565 grams on the scale, a mass you can feel in the hand and which contributes to the inertia sought in this type of device. The sharp angles and notched volume wheel define a line recognizable at a glance. The 6-inch touchscreen (2160 × 1080) leaves plenty of room for album covers and apps.

The 7450 mAh battery lasts about ten hours in playback (reference: FLAC 16-bit / 44.1 kHz, screen off, low gain) and recharges in five hours via USB-C PD 3.0 fast charging. Astell&Kern includes a Minerva leather case from the Badalassi Carlo tannery, vegetable-tanned according to Tuscan tradition. Its rear cutout lets the glow of the tubes shine through.

The connectivity covers the essentials: 3.5 mm unbalanced headphone output, 4.4 mm balanced output (five-pole connector only), and an optical output housed in the 3.5 mm jack. The 256 GB of internal memory can be expanded via microSD card up to 1.5 TB, enough to hold a music library in high-resolution files without having to ration yourself. A crossfeed function blends part of each channel into the other to bring headphone listening closer to that of a pair of speakers, and reduce fatigue during long sessions.

Technical specifications

General overview

• A&ultima 256 GB audiophile music player
• Model: SP4000T (PPF52)
• Chassis color: silver
• Chassis material: stainless steel
• Dimensions: 85.7 mm (W) × 150 mm (H) × 19.5 mm (D)
• Weight: approximately 565 g
• Operating temperature: 0 °C to +40 °C (32 °F to 104 °F)

Display

• 6-inch (15.2 cm) 2K touchscreen (2160 × 1080)

Audio architecture

• Processor: OCTA-Core
• DAC: AKM AK4191EQ × 2 and AKM AK4499EX × 2
• Triple amplification: normal, tube, and hybrid
• Tube amplification: RAYTHEON JAN6418 × 4 (quad tube)
• Decoding: supports up to 32-bit / 768 kHz bit-to-bit playback
• Native DSD512 22.4 MHz

Supported audio formats

• Formats: WAV, FLAC, WMA, MP3, OGG, APE, AAC, ALAC, AIFF, DFF, DSF
• Sampling frequencies
  • PCM: 8 kHz to 768 kHz (8 / 16 / 24 / 32 bits per sample)
  • Native DSD: DSD64 (1-bit 2.8 MHz) stereo, DSD128 (1-bit 5.6 MHz) stereo, DSD256 (1-bit 11.2 MHz) stereo, DSD512 (1-bit 22.4 MHz) stereo

Audio performance

• Output level: unbalanced 4.2 Vrms, balanced 8 Vrms (no load)
• Frequency response
  • ±0.006 dB (20 Hz to 20 kHz) unbalanced and balanced
  • ±0.06 dB (20 Hz to 70 kHz) unbalanced and balanced
• Signal-to-noise ratio: 124 dB at 1 kHz unbalanced, 125 dB at 1 kHz balanced
• Crosstalk: -136 dB at 1 kHz unbalanced, -145 dB at 1 kHz balanced
• THD+N: 0.0006% at 1 kHz unbalanced, 0.0004% at 1 kHz balanced
• IMD SMPTE: 0.0004% 800 Hz 10 kHz (4:1) unbalanced and balanced
• Output impedance: 0.7 ohm (3.5 mm unbalanced), 1.7 ohm (4.4 mm balanced)

Clock

• Clock jitter: 25 ps (typical)
• Reference clock jitter: 200 femtoseconds

Inputs and outputs

• Input: USB Type-C (charging, PC and MAC)
• Unbalanced headphone output: 3.5 mm
• Optical output: 3.5 mm
• Balanced output: 4.4 mm (5-pole compatible only)

Wireless connectivity

• Dual-band Wi-Fi: 802.11 a / b / g / n / ac / ax (2.4 / 5 GHz)
• Bluetooth 5.2 (A2DP, AVRCP, Qualcomm aptX HD, LDAC, LHDC, SBC, AAC)

Services and compatibility

• Roon Ready and Qobuz Connect
• Firmware update supported (OTA)
• Compatible operating systems: Windows 10, Windows 11 (32 / 64 bit), MAC OS X 10.7

Storage

• Internal memory: 256 GB (NAND)
• External memory: microSD slot × 1 (up to 1.5 TB)

Battery

• Capacity: 7450 mAh, 3.8 V, Li-Polymer
• Charging time: approximately 5 hours (PD 3.0 charging)
• Playback time: approximately 10 hours (reference: FLAC, 16-bit, 44.1 kHz, volume 80, screen off, low gain)

Included accessories

• Premium leather case
• MINERVA leather signed by BADALASSI CARLO
• Handmade in San Miniato (Florence, Italy)
• Made using traditional Tuscan vegetable tanning
• Environmentally friendly tanning process based on plant-derived tannins
• Use of the Tuscan Vacchetta technique for a high-end finish
• Color: Cognac