Astell & Kern SP4000 Review: An Ultimate Audiophile Upgrade

After you've gotten the best headphone, the best amplifier, the best DAC, and the best cables, where do you go next in your audio adventure? The answer: the source. A high-quality Digital Audio Player (DAP) enhances your listening experience by providing music directly from a dedicated source, eliminating distractions from other features. Astell & Kern is a company that has impressed in bringing audiophiles exemplary devices to listen to their favorite tracks. 

Their newest release has turned heads and stirred excitement for the next ultimate upgrade. The A&K SP4000 is the latest and greatest in digital audio players. Years after the release of the SP3000, Astell & Kern has listened to users and adapted their device to create the next flagship for audiophile setups. I was lucky enough to test out one of these devices to share a full, comprehensive review of all the finer details of the SP4000. 

Simply put: the SP4000 is stunning. It's certainly a heavy unit, made from stainless steel that weighs in at 615g. Visually, the all-black aesthetic is sleek and professional, catering to more incognito setups. Its shiny exterior is prone to fingerprints, but a quick wipe with a microfiber cloth clears them away in an instant. The leather case that comes with it lightens the aesthetic slightly, bringing in a nice light brown color against the black. The leather case itself is expertly crafted to fit perfectly onto the SP4000. 

The SP4000 is heavy and sturdy, which weighs down in your pocket but definitely stays stable on a desk. The edges are sharp and have geometric accents for a fun design choice. The corners are rather sharp, but the included leather case smooths those out quite well while still keeping the main shape. The build alone tells you the SP4000 is a quality device, and that's before diving into any of the features. 

Astell & Kern DAPs are jam-packed with features and different functionalities to make them a truly universal device. At its core, the SP4000 is meant to play music. This can be done in a couple of ways, either via streaming or with downloaded files. There is a space for an SD card for downloaded files, holding up to 1.5 Terabytes. You have 256GB of memory on the device itself to save music. Additionally, if you want to log into your preferred streaming service, you can download any app from the Google Play Store and log in to your accounts. Even if you choose to use a different source, you can Bluetooth connect to the SP4000 or broadcast via AirPlay for a wireless connection. Once you connect to WiFi, you'll be all set to listen wherever you are.

For outputs, you're given the two standard options of a balanced and an unbalanced connection. Any 3.5mm or 4.4mm termination will be compatible with the SP4000. This makes pretty much any headphone or earphone compatible with the SP4000, which is great for any audiophile with their unique gear configurations. 

Even when you start listening, there are ways to customize your sound. With a few EQ filters and space for customizable filters, you can adjust the frequencies with more than 10 adjustable bands. The SP4000 has an option for advanced customization, where you can adjust the frequency EQ parametrically as opposed to just boosting or cutting. You can save these presets to the SP4000 or turn them off entirely, so that each genre you listen to can have its own prescribed filter. You can also choose between some DAC filters, with 6 options for you to test out. 

The power coming from the SP4000 is enough to drive some pretty daunting headphones. With a 6,780mAh 3.8V Li-Polymer battery, this DAP is able to push power to some demanding headphones like the HD 600 and the Dan Clark series. I was impressed by the volume this device put out, from sensitive IEMs to high-impedance headphones. You were able to reach high volumes with ease and still have headroom to grow higher. 

The battery on the SP4000 lasts about 10 hours, and it takes 5 hours to charge to a full battery. This is perfect for all-day listening, and the USB-C charger makes it universally compatible with a lot of stock cables. 

GENERAL SPECIFICATIONS

• Model: SP4000 (PPF51)
• Body Color: Black, Silver
• Body Material: Stainless Steel 904L
• Display: 6inch 2K (2160 x 1080) touch screen
• Supported Audio Formats: WAV, FLAC, WMA, MP3, OGG, APE, AAC, ALAC, AIFF, DFF, DSF
• Sample rate: 
  • PCM: 8kHz ~ 768kHz (8/16/24/32bits per Sample)
  • DSD Native: DSD64(1bit 2.8MHz), Stereo | DSD128(1bit 5.6MHz), Stereo
  • DSD256(1bit 11.2MHz), Stereo | DSD512(1bit 22.4MHz), Stereo
• Output Level: Unbalanced 4.1Vrms │ Balanced 8.2Vrms (Condition No Load)
• CPU: Octa-core
• DAC: AKM4191 x4 + AKM4499EX x4 (Real Quad DAC)
• Decoding: Support up to 32bit / 768kHz Bit to Bit playback
• Input: USB Type-C input (for charging & PC & MAC)
• Outputs: Unbalanced Out (3.5mm), Optical Out(3.5mm), Balanced Out (4.4m, only 5-pole supported)
• Wi-Fi: 802.11 a/b/g/n/ac (2.4/5GHz)
• Bluetooth: V5.0 (A2DP, AVRCP, Qualcomm® aptX™ HD, LDAC, LHDC, SBC, AAC)
• Dimensions: 3.34" (85mm)[W] x 5.89" (149.8mm)[H] x 0.76" (19.5mm)[D]
• Weight: About 21.7oz (615g)
• Feature Enhancements: Firmware upgrade supported (OTA)
• Operating Temperature: 0℃ ~ + 40℃ (32℉~ 104℉)

AUDIO SPECIFICATIONS

• Frequency Response: ±0.006dB (Condition : 20Hz~20kHz) Unbalanced │ ±0.006dB (Condition : 20Hz~20kHz) Balanced±0.06dB (Condition : 20Hz~70kHz) Unbalanced │ ±0.06dB (Condition : 20Hz~70kHz) Balanced
• S/N: 129dB @ 1kHz, Unbalanced │ 131dB @ 1kHz, Balanced
• Crosstalk: -136dB @ 1kHz, Unbalanced │ -145dB @ 1kHz, Balanced
• THD+N: 0.0005% @ 1kHz, Unbalanced │ 0.0003% @ 1kHz, Balanced
• IMD SMPTE: 0.0004% 800Hz 10kHz (4:1) Unbalanced │Balanced
• Output Impedance: Unbalanced Out 3.5mm (0.7ohm) │ Balanced Out 4.4mm(1.7ohm)
  • *Audio measurements are based on the Normal mode.
  • *Auto Mute is not used during audio testing.

CLOCK SOURCE

• Clock Jitter: 25pS (Typ)
• Reference Clock Jitter: 200 Femto Seconds

STORAGE CAPACITY

• Built-in Memory: 256GB [NAND]
• External Memory: microSD x1 (Max. 1.5TB)

BATTERY

• Capacity: 6,780mAh 3.8V Li-Polymer
• Charge Time: about 4hours (QC3.0 Charging)
• Playback Time: about 10hours (Standard : FLAC, 16bit, 44.1kHz, Vol.50, LCD Off)

SUPPORTED OS

• Supported OS: Windows 10,11(32/64bit) MAC OS X 10.7

SOUNDSTAGE & IMAGING

Across headphones and in-ear monitors, the SP4000 delivered exceptional sound that stayed true to the given sound signatures. I was impressed with the depth of the sound and how the music was able to occupy more space in the stereo field. "Lush" is the first word that comes to mind when I think of the sound signature as a whole, since everything sounded elevated and given extra body. I heard this in thinner and more transparent headphones like the HE1000 Unveiled, and even in IEMs where the soundstage runs narrow. 

Pairing the SP4000 with different devices was a fun game to see which had the best synergy. I used a lot of different IEMs and over-ear headphones, including things from Noble Audio and Dan Clark. Throughout my listening, I preferred the sound of headphones with the SP4000 as opposed to the sound of in-ear monitors. I think the fullness from the SP4000's signature complemented an over-ear experience better than it did an in-ear one. This isn't to say I didn't enjoy listening with IEMs: I found the Kronos and Campfire Alien Brain to be amongst my favorites during my listening.  

BASS FREQUENCIES

The bass response in the SP4000 is not as overwhelming as you'd expect. Yes, the sound is certainly fuller and more hefty in body, but the bass response doesn't get an over-the-top booming sound that takes over an entire sound signature. Instead, it stays refined and subtle, taking lower frequencies from all instruments to give the illusion of extra body. I liked how the bass in the SP4000 stayed punchy and tight in earphones like the Campfire Alien Brain and the headphones like the Meze POET. In earphones where the bass wasn't overly present, the SP4000 didn't boost the response artificially; instead, you can hear the natural sound from the bass more present and a bit more forward than you could previously. Let it be known: if you're looking for the SP4000 to boost the bass in your headphones, it likely won't be as effective as you'd like. You'll have to use the EQ features or get a more bass-forward headphone and let the SP4000 pick up that strong bass response. 

MIDRANGE FREQUENCIES

The SP4000 made sure you can still hear all the midrange clarity and detail you love in whatever headphone/earphone you use. In earphones like the Kronos or headphones like the Abyss, the midrange is the highlight and holds all the character of the sound signature. The SP4000 doesn't add any distracting color or flavor to interrupt those profiles. You get to hear the same great clarity and texture with an extra boost to the front, so you know you're not going to miss a thing. The midrange is full of natural color and timbre from the different instruments, so everything sounds exactly as you'd imagine. With the different DAC filters, you can experiment with sound reproduction a bit more, but it's beneficial that your baseline listening can be with a sound signature you recognize. In headphones like the Abyss JOAL, the relaxed tonality gets a jolt of energy that brings more life into your listening. 

TREBLE FREQUENCIES

At the top range, the SP4000 doesn't bombard with a ton of sharp detail that makes headphones unlistenable. Instead, you have space at the top end that stays honest to what the headphones started at. You have a good sense of space that lets frequencies ring out, all paired with an airy quality to make sure the listening isn't overwhelming. The brightness and color from the SP4000's treble are great in IEMs, especially ones like the Kronos, where the treble is the main characteristic of the sound signature. Even in more bass-heavy signatures, there is a good balance between the top and bottom ranges. It's nice that the top range doesn't get piercing or overly analytical, so you can use this to listen for longer without fatigue. 

If you're someone who already has a DAP, you may be debating whether or not the SP4000 is worth the upgrade. In my opinion, this DAP serves as a clear and obvious upgrade for anyone looking to finalize their audio setup. So what sets this DAP apart from the rest? 

Looking at the SP4000 against previous A&K models like the SP3000, the newer device is slightly increased in size and weight. You get a wider frequency response and decreased distortion rate. You get a wider range of sound reproduction in the SP4000 that can reach higher volumes without risk of distortion. Both come with the same features of playback options and connectivity, though the SP4000 has nixed the 2.5mm balanced port. Since 3.5mm and 4.4mm are more universal, these are the only two to stay on the device. Functionality and interface design remain the same, so Astell & Kern users will appreciate the familiarity. 

FiiO is another company that makes high-end DAPs for budding audiophiles. They sit in the more affordable range for DAPs, but still give high-quality sound for excellent value. I was able to test the SP4000 against the M23, and hearing the difference between the two was a fun experiment. Physically, the devices have very different designs and styles, and their sound reflects that. Where the SP4000 sounded full and had more weight, the FiiO M23 had a thinner and crisper sound that made tracks weightless. It was a more transparent reproduction, which made me lose a bit of detail when compared to the SP4000. 

The Astell & Kern SP4000 proves itself to be a high-quality device with flagship-seekers in mind. If you're looking to upgrade your listening experience to an all-in-one device, the SP4000 will check a lot of your boxes. Space to download files, WiFi and Bluetooth compatibility, sleek design, and plenty of power: all of these are great things to have in a flagship device. If price isn't a concern and a little weight isn't an issue, getting the SP4000 is an easy buy that will definitely elevate your listening experience. 

The Astell & Kern SP4000 is available at Audio46.