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Astell & Kern A&ultima SP4000: Two-minute review

The Astell & Kern A&ultima SP4000 is the brand’s newest flagship digital audio player, and it is priced accordingly. If you measure the worth of a product by how relatively heavy and remarkably shiny it is, though, you won’t be able to argue with the $3,999 asking price.

The SP4000 goes a distance towards justifying its cost in the way it’s specified to perform, too. Numerous technological highlights abound, none of them in any way ‘affordable’, and between the sheer heft of the physical item and the lengthy list of technologies Astell & Kern has brought to bear, the SP4000 seems about as purposeful as these things ever get.

And in action, it is an uncomplicated pleasure to listen to, fully befitting a place in the best MP3 players around. In every meaningful way, the SP4000 is an extremely accomplished device, able to combine brute muscularity with deft insight, rhythmic positivity with outright scale. No matter what you choose to listen to, the Astell & Kern seems to enjoy it just as much as you do – and it’s not about to sit in judgement on your choice of headphones either.

Astell & Kern A&ultima SP4000 review: Price and release date

(Image credit: Future / Simon Lucas)

• Priced at $3,999 / £3,799 / AU$6,599

The Astell & Kern A&ultima SP4000 is on sale now, and in the United States it sells for $3,999. In the United Kingdom the asking price is £3,799, and in Australia you’ll have to part with AU$6,599.

Not cheap, is it? Anyone who takes an interest in this sort of thing will know Astell & Kern has no problem in pitching its products as uber-high-end propositions, but no matter how many times I see one of its products priced this way, it remains difficult not to do a double-take…

Astell & Kern A&ultima SP4000 review: Features

(Image credit: Future / Simon Lucas)

• 4 x AKM4191 and 4 x AKM4499EX DACs in 1:1 architecture
• 4 x opamps per analogue output
• Snapdragon 6125 octa-core processor

Something would seem amiss, wouldn’t it, if a digital audio player costing very nearly four thousand of your US dollars wasn’t groaning under the weight of its specification? Well, when you consider the extensive nature of the SP4000, it’s a wonder it’s not even bigger and even heavier than it actually is.

It follows that I should try to be reasonably brief, otherwise we’ll be here all day.

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At its most fundamental, the SP4000 is built around ‘octa’ audio architecture. The digital-to-analogue signal processing is in a 1:1 structure, with one AKM4191 digital processor paired with one AKM4499EX DAC. This allows digital signals to be delivered to a single DAC, four times over – this is a true quad-DAC design, with the aim of allowing precise signal transfer with a vanishingly low signal-to-noise ratio. The ability to deal with PCM resolutions of up to 32bit/768kHz and DSD512 means any realistic digital audio file is catered for.

There are eight opamps deployed, four attending to the unbalanced 3.5mm analogue output and four dealing with the 4.4mm balanced equivalent. The intention is to increase dynamic range and enhance detail retrieval – Astell & Kern calls this arrangement ‘high driving mode’ and suggests it provides powerful and stable signal output.

A newly developed LDO (‘low drop-out’) regulator in the power supply stabilizes battery voltage in an effort to suppress noise. Proprietary ESA (‘enhanced signal alignment’) technology is designed to improve the alignment of frequency signals (sometimes opaquely referred to as ‘timing’) to minimize distortion and enhance clarity. The PCB is a high-end ‘Any Layer HDI’ design that allows for extremely complex circuitry to be laid out in a very small space, minimizing signal loss.

(Image credit: Future / Simon Lucas)

What else? The audio block sits behind a 99.9% pure copper shielding can, offering significant shielding from electromagnetic interference. The audio block itself is Astell & Kern’s ‘Teraton X’ design, which incorporates HEXA-Audio circuitry along with power-efficient amplification and considerable power noise cancellation, to deliver what the company suggests is the ‘ultimate sound solution’.

The entire show is run by a Snapdragon 6125 Octa-core processor that features a high-performance CPU and 8GB of DDR4. CPU, memory and wireless comms circuitry are configured as a single module, and with the digital circuit components arranged in the same area it’s effectively a system on a chip.

I could go on. There are six digital filters available to allow the user to, in a small way, design their own sound. The ‘crossfeed’ feature allows a little of the left-channel mix into the right channel (and vice versa) and, in conjunction with some adjustment options, tries to replicate the effect of listening to speakers when listening to headphones. The second generation of Astell & Kern’s DAR (‘digital audio remaster’) technology, dubbed ‘Advanced DAR’, uses a ‘virtual sound extender’ as part of a two-stage upsampling process that can convert PCM signals of up to 48kHz to 385kHz or to DSD128, and signals of greater than 96kHz to DSD256, for playback.

Surely, though, the broad point is made by now. Astell & Kern didn’t leave space for the kitchen sink, but it has thrown pretty much everything else at the A&ultima SP4000.

Features score: 5 / 5

Astell & Kern A&ultima SP4000 review: Sound quality

(Image credit: Future / Simon Lucas)

• Epic levels of insight and detail
• Rhythmic and dynamic positivity
• Sounds simultaneously open and unified

Yes, you can fiddle around the edges of the way the A&ultima SP4000 sounds – investigate filters, fool around with EQs, you name it – but what you can’t do is alter its overarching sonic character. Which is just as well, because this Astell & Kern digital audio player is a staggeringly direct, informative and, ultimately, complete listen. Few are the sources of audio information, of any type and at any price, that can match its powers of communication – and I have heard plenty.

No matter if you’re listening to a 16bit/44.1kHz FLAC file of Ride’s Leave Them All Behind, a 24bit/48kHz FLAC file of James Holden’s Common Land or a DSD64 file of The Band’s I Shall Be Released: it’s all the same to the SP4000. In every circumstance it’s a profoundly detailed, rhythmically positive, articulate and energetic listen. There really isn’t an aspect of music-making at which it doesn’t prove itself masterful.

And it’s not as if I can offer a “yes, but…” or two in the name of balance. The longer I listen to the SP4000, the more beguiled I become.

Tonal balance? It’s basically impeccable. Frequency response? Smooth and even from way down at the low frequencies to the vertiginous top end. The Astell & Kern sounds naturalistic and unforced, and it’s completely even-handed in the way it presents the frequency range. And at every point, it’s absolutely alive with detail both broad and fine. The minutiae of tone, timbre and texture are made absolutely apparent, and the player loads all of this information onto the listener without being in any way showy or uptight about it. This fanatical attention to detail is simply a way of ensuring you get as complete a rendition of your digital audio files as possible.

The presentation is spacious and well-defined at the same time, and no matter if it’s a large ensemble all packing the stage or just one voice with a single guitar as accompaniment, the SP4000 lays it all out in confident and coherent fashion.

It deals with rhythm and tempo with similar authority, keeping momentum levels high and observing the attack and decay of bass sounds (in particular) with obvious care. It can ease back if necessary, though – nothing gets hurried along, but rather is allowed to proceed at its own chosen speed. Dynamic headroom is, to all intents and purposes, limitless. From the smallest, quietest event in a recording to the last almighty crescendo, the SP4000 is on top of things – the distance between these two states is prodigious. And the smaller, but no less crucial, dynamics of harmonic variation, the attention to the over- and undertones that surround the fundamental when listening to a solo instrument, are given very judicious weighting. Context is everything, and the SP4000 seems to almost instinctively understand it.

And the Astell & Kern even has the decency not to be sniffy either about the music you listen to or the headphones via which you access it. Obviously it does better work (or, rather, its potential is best exploited) by hi-res files and high-end headphones – but if you want to connect your bog-standard true wireless in-ear via Bluetooth and listen to Spotify’s free tier the SP4000 won’t judge you. Not too badly, anyway.

Sound quality score: 5 / 5

Astell & Kern A&ultima SP4000 review: Design

(Image credit: Future / Simon Lucas)

• Polished 904L stainless steel and PVD-coated ceramic
• 150 x 85 x 20mm
• 615g

Ordinarily, a digital audio player is designed to be reasonably compact, and light enough to be slipped into a pocket. Of course, Astell & Kern sets out for its digital audio players to be anything but ordinary.

So the SP4000 is a fairly large (150 x 85 x 20mm) device that weighs a considerable 615g. Too big and heavy, in other words, to be comfortably carried in any pocket smaller and less robust than that of a military greatcoat. This is its naked weight, too. If you add one of the included screen protectors (which is, admittedly, going to make negligible difference to the weight) and slip the player into its supplied Perlinger leather* protective case, it becomes heavier still. At least that case prevents the player’s sharp, pointy corners from digging into hands or pocket linings, mind you.

(*I’m not a vegetarian. I know people who are, though, and some of them are just as interested in high-quality audio as I am. So once again I find myself wondering why companies like Astell & Kern imagine real leather – in this instance, leather made from “the soft, delicate hide of calves under one year old” – to be the untouchable height of luxury. Surely it’s possible to offer a protective case for the SP4000 that looks and feels upmarket but that isn’t going to alienate who knows how many prospective customers? Or is that just me?)

The four sides of the SP4000 are built of 904L stainless steel (the same stuff the likes of Rolex uses, on the basis that it will accept an extremely high polish), and feature some of the angularity and asymmetry that Astell & Kern established as part of its design vocabulary a good while ago. The front is of toughened glass, 152mm on the diagonal, and is almost entirely touchscreen. The rear panel, meanwhile, is finished in PVD-coated ceramic.

It really goes without saying that the standard of build and finish on display here is flawless. With the design of the SP4000, Astell & Kern has set out to deliver a product that blurs the line between ‘electrical hardware’ and ‘luxury accessory’. Or, as the company’s website rather feverishly has it, “a work of art where technology, design, intuition and performance converge”. You may feel that Astell & Kern has done exactly what it set out to do, you may find the design rather self-consciously opulent. Taste is a very personal thing, after all.

It’s worth noting the grandeur of SP4000 ownership starts well before you peel the protective covering off the player itself. It arrives in a branded box that’s a similar size to that which contained a pair of size 10 Tricker’s boots I bought the other day. Inside there is another, branded, clasp-fastening box covered in what I strongly suspect is a further quantity of leather.

Inside that you’ll find the SP4000, along with compartments that contain that Perlinger leather cover, a case with a flap covering into which the player (in its cover) can be slipped (more leather, I presume), various guides and warranty documents, a congratulatory note from the company, and a reasonably heavyweight, branded USB-C to USB-C cable. I am pretty sure this all comes under the heading of ‘the experience’.

Design score: 4 / 5

Astell & Kern A&ultima SP4000 review: Usability and setup

(Image credit: Future / Simon Lucas)

• 2160 x 1080 touchscreen
• Supports Full Android OS
• Qualcomm QC3.0 fast charging

The SP4000 represents the first time an Astell & Kern product has supported full Android OS. The convenience and all-around common sense of the operating system is intended to help the SP4000 be as flexible and convenient as possible, while some of the Snapdragon 6125 octa-core processor’s responsibilities center around rapidity of the OS response and the smooth, comfortable user interface motion.

Happily, it all works very well. The big 2K (2160 x 1080) touchscreen is responsive and swift, smooth-scrolling and consistent. The operating system will be mercifully familiar to anyone whose smartphone isn’t an iOS device, and it’s just as wide-ranging and usable here as it is in its most successful smartphone applications.

Setting up the SP4000 is no kind of hardship. It’s simply a question of connecting it to your local network (its dual-band Wi-Fi is tenacious when it comes to making and maintaining a connection to your router or tethering to your smartphone if you’re out and about), and from there it’s simple to load the apps you require. The ‘AK File Drop’ function makes transferring files from a PC, smartphone or FTP program on a common network faster and easier than before, too.

The Astell & Kern also supports Qualcomm QC 3.0 fast charging, which means it can be charged more rapidly (and more efficiently) than previous flagship A&ultima models. Mind you, ‘fast’ and ‘rapid’ are definitely relative terms in this instance. From ‘flat’ to ‘full’ takes around five hours, which is about half the time it takes for the SP4000 to flatten its battery if you’re listening to ordinary files at ordinary volume levels.

There are a few physical controls arranged around the edges of the SP4000. As you look at its touchscreen, there’s an elaborate volume control/power on/off on the top-right edge – it’s pleasantly shaped and knurled, and a light behind it glows in one of a variety of different colors to indicate the resolution of the audio file it’s currently playing.

On the opposite side there are three buttons that deal with skip backwards/rewind (accessible via ‘press’ or ‘press and hold’ respectively), skip forwards/fast-forward (same) and play/pause. There’s a ‘button lock’ switch on the top edge, to the right of the 3.5mm hybrid optical/unbalanced analogue and 4.4mm balanced analogue outputs, and on the bottom edge you’ll find a USB-C socket and a microSD card slot, which will accept cards of up to 1.5TB.

Usability and setup score: 4.5 / 5

Astell & Kern A&ultima SP4000 review: Value

(Image credit: Future / Simon Lucas)

First things first: you don’t contemplate ownership of the Astell & Kern A&ultima SP4000 because you’re in any way concerned about value for money. Is it the best-sounding DAP out there? Sure. Is it twice as good as alternatives from the likes of FiiO or Astell & Kern itself that cost comfortably less than $2k? Not a chance.

No, the value in the SP4000 comes from its status as the shiny flagship of the Astell & Kern range. It comes from the knowledge that no one you bump into when in the First Class Lounge has a more expensive DAP than you. It comes from the ability to add ‘DAP’ to the list of ‘madly luxurious accessories I own’.

Should I buy the Astell & Kern A&ultima SP4000?

(Image credit: Future / Simon Lucas)

Buy it if… 

Don’t buy it if… 

Astell & Kern A&ultima SP4000 review: Also consider

How I tested the Astell & Kern A&ultima SP4000

• Tested for over a week
• Tested with streamed and downloaded content
• Tested with wired and wireless headphones

I slotted a microSD card filled with hi-res content (up to 24bit/192kHz and DSD64, anyway) into the SP4000, and I downloaded the Tidal and Presto music streaming apps while I was at it.

I used Sennheiser IE900 IEMs connected via the 4.4mm balanced output, Austrian Audio The Composer over-ears via the 3.5mm unbalanced alternative, and tried out the Technics EAH-AZ100 true wireless in-ears and Bowers & Wilkins Px8 wireless over-ears too.

I listened to lots of different types of music, via lots of different file types and sizes – and I did so indoors and (with some trepidation, I don’t mind telling you) outdoors too.

• First reviewed in September 2025

Astell & Kern A&ultima SP4000: Price Comparison

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