January 2025

World Premiere Review!
FiiO S15 Music Streamer, Hi-Res DAC, And Bluetooth Receiver Review
The FiiO S15's ease of use and impressive features is matched by its' excellent sound quality.
Review By Tom Lyle

   I am a proponent of FiiO's portable digital audio players (DAPs). Since I'm an audiophile practically obsessed with music but cannot sit in front of my main audio system to listen to music 24/7, using a DAP is not an option but a must. I've auditioned many DAPs in the past, and since the spring of 2024, I've been using FiiO's top-of-the-line M17 "Portable Desktop-Class Music Player" daily. It has the best sound quality of any sub-$2000 DAP I've ever heard.

When I was offered the opportunity to review FiiO's new S15 Music Streamer / Hi-Res DAC / Bluetooth Receiver I immediately said "yes."

History
After FiiO began marketing their all-in-one desktop system, the approximately 6" square R7, in early 2023, they decided to continue developing this type of product and now offer audiophiles and music lovers the subject of this review: their component-sized S15, a streamer that can also be used as a preamp, a DAC, and a Bluetooth receiver. FiiO calls the S15 a "Desktop High-Res Audio Streamer." But it can perform many more functions than simply a streamer. In their promotional literature, FiiO describes the S15 as a "media streamer-local playback-decoder-preamplifier."

Full-Sized
This is FiiO's first "full-sized" all-in-one media streamer, so in my system, its cabinet's 17" width makes it look at home with the other components on my Arcici Suspense equipment rack. The S15's 7.84" front-panel display is large enough to see album covers and other metadata from quite a distance. What separates the FiiO S15 screen from many other streamers I've used is that the S15's display is a touchscreen.

Even though the S15 has a remote, I found using the S15's touchscreen and the two large knobs on either side of its front panel enjoyable. As a kid, I often sneaked into my local high-audio salon to spin the attenuators of equipment I could only dream of owning. The tactile thrill of using the controls on high-end gear has persisted. This, combined with the fact that I still listen to 7" 45 rpm vinyl singles and EPs, excludes me from any "lazy" audiophile categories! Getting up from my listening seat to use the S15's touchscreen was never a big deal. It was the opposite.

Like many other modern steamers, the S15 is Roon and Airplay-ready. One can also install as many apps as one wants. With its touchscreen and remote, these apps are easily accessible. 

The LAN input on its rear panel works like other streamers, letting one connect the S15 to a home router. However, it can also play music locally with its rear panel inputs for a microSD card, USB flash drive, or equivalent devices. The S15 can also be used as a Bluetooth receiver. One can also load video apps and send a video signal to a screen via its HDMI output.

Innards
The S15 uses a Qualcomm 660 processor. The "powerful" AKM twin (a combo of an AK4191EQ and AK4499EX) handles its DAC conversion section. The power supply is linear, so its processor recognizes the USB port using an XMOS XU316. Bluetooth reception is overseen by the Qualcomm QCC5125 processor, which supports aptX HD and aptX Adaptive and digital signals up to DSD 256.

The FiiO S15 has RCA and XLR outputs, but it doesn't have a headphone amplifier. The digital inputs include TosLink optical, S/PDIF coaxial, AES/EBU, HDMI in, HDMI I²S, and USB 3.0 plus two USB A. As I mentioned above, it even has an SD memory card slot. Its outputs include balanced XLR analog, two pairs of unbalanced RCA, S/PDIF coaxial, TosLink optical, HDMI, and AES/EBU.

The I²s also has a hard disk slot on the upper right corner of its rear panel. It has a high-speed USB3.2 M.2 SSD hard disk cage that FiiO says, "Can efficiently store and manage massive libraries."

The FiiO S15's literature touts its dual "industry-leading Accusilicon high-performance, ultra-low noise" femtosecond crystal oscillators which are featured in no other FiiO products. FiiO claims that these effectively reduce the impact of phase noise on audio signals and result in "high levels of decoding fidelity" and more precise digital output.

For the S15, FiiO developed a proprietary "DAPS," a Digital Audio Purification system. This comprises an Android audio core and "a low-jitter clock coprocessor," which FiiO says took many years to perfect. The clock allows the unit to process the signal without re-sampling it through its innards. It can also more accurately match different sampling rates of both PCM and DSD signals for what FiiO describes as a "purer" sound. 

FiiO also says that the S15 is a fully balanced component, from the DAC to the preamp stage. A balanced circuit reduces noise and crosstalk and improves the output's sound quality in the line and preamp sections.

The output of the S15 can be configured as either fixed or variable. The variable production lets one use the FiiO S15 as a simple preamplifier, so when used in a simple digital system, one would not need the attenuator of a linestage.

System
I set up the FiiO S15 in my main system in an acoustically treated, dedicated listening room with two 20-amp power lines that run directly to our home's circuit box. I employed two lithium battery power supplies during daylight hours, removing the system from our local electrical grid. I used one battery power supply for the power amplifier and another for the rest of the system.

I connected the FiiO S15 to my reference vacuum-tube-powered Nagra Classic Preamp with an external PSU power supply. Although, I sometimes used a two-chassis solid-state Pass Laboratories XP–22 linestage. Early on, I was still using the recently reviewed, top-flight KBL Sound EXtrema interconnects to connect the S15 to the linestage and the linestage to the power amplifier. However, I soon had to return these cables to the manufacturer. For the rest of the review, I used Crystal Cable's Micro Diamond 2 interconnect between the S15 and the linestage and an MIT Shotgun S3.3 interconnect between the linestage and power amp.

The power amplifier used for this review was a 250 Watts per channel Pass Labs X250.8, which fed a pair of Sound Lab Majestic 545 electrostatic speakers. To augment these speakers' bass below the Sound Labs 34 Hz low-frequency limit, I used a pair of SVS PB16-Ultra subwoofers. Running from the power amp to the Sound Labs were Kimber Carbon 18 XL speaker cables, and the subwoofers were connected with long interconnects running from the linestage to the subs on-board power amplifiers.

I connected a generic Ethernet cable from our home router to the S15's LAN input. Our home's Wi-Fi router is in the same room as my system, only about six feet from the equipment rack. Its LAN input was the source I used for much of the review, although I used my music server, connecting Wireworld's Starlight 7 USB 2.0 cable, which ran to FiiO's S15 rear panel USB input.

FiiO's S15 analog outputs were connected to the linestage, usually a vacuum-tube powered Nagra Classic Preamp, but sometimes the two-chassis Pass Laboratories XP–22 linestage.

This system used for review would likely be more advanced than one that purchases a $1000 streamer. However, the FiiO S15 performed so well that it was suitable at home while I auditioned for this system.

I also listened to the FiiO S15 through headphones for much of this review. A very long run of Cardas interconnects terminated with RCAs ran between the analog output of the S15 to my headphone amplifiers. Most of the time I listened through a Woo Audio WA2 headphone amplifier. A while ago I replaced its stock input tubes with a matched pair of NOS (New Old Stock) Amperex 6299s and the larger output tubes with a matched pair of Tung-Sol 6080s. The other headphone amp was a very good-sounding solid-state Pass Laboratories HPA-1 that I acquired in 2016.

Whether listening to the FiiO S15 through my main audio system or headphones, I enjoyed listening to a wireless signal sent from my iPhone using Airplay. I heard no difference in using the Qobuz and Tidal apps in this way compared to using the apps on the front screen of the S15.

Sound
I'll get right to the point – the sound quality of the FiiO S15 blew me away! I did not expect such a solid, faultless sound from a $999 streamer! I used cables in the review system that cost more than the FiiO S15!

One of the reasons why I was so impressed with the S15 was because I've become accustomed to the exceptional sound quality of the pricier Simaudio Moon 681 DAC/Streamer that is in my system. Among other sonic traits, the FiiO S15 was less mighty sounding than the Moon 681. The Moon is also much easier to use than the multifunction knobs on either side of the S15's screen. Perhaps a younger person than I would be more at home using the computer-like settings of the S15. However, all was forgiven when considering the top-notch sound quality of this streamer/DAC. To say I was impressed by the sound quality I heard from the FiiO S15 is an understatement.

As I said, I enjoyed streaming music through the S15, whether sent wirelessly from my iPhone's apps or directly from its Ethernet input. My enjoyment stemmed not from the intelligent design of the FiiO S15 but also from the impressive sound quality. When streaming John Coltrane's monumental Blue Note album Blue Trane through the S15, it was immediately apparent why it has been cherished since its release in 1958.

There are both mono and stereo versions of this album. Many purists prefer the monaural version because of engineer Rudy Van Gelder's penchant for hard-panning instruments in one channel of the stereophonic mix. I'm unsure if I've become accustomed to this hard-panning or if I feel it sounds more like a live event. Nevertheless, for this review, I played the stereo version.

The FiiO S15 reproduced this album so well that I felt like I had entered a sonic time machine once its signals passed through my speakers.

One of my most remarkable audiophile experiences was inviting musicians into my listening room to hear them play their instruments in the same room as my audio system. The first thing I noticed when hearing a saxophone in this situation was that the sound of this instrument was loud. Its volume made me thankful for the attenuator on my linestage.

When hearing Blue Trane played back, the sound of Coltrane's piercing tenor sax was accompanied by a level of talent that other musicians rarely matched. And I was having the FiiO's reproduce what I can only describe as severely lifelike sent shivers down my spine. I was also in awe that I heard this sound sourced by a $999 streamer. After this album streamed through the Moon 681, it had a bit more body and drew me into the music better. But no way did it sound 12 times better!

And it wasn't only the sound of Coltrane's sax that amazed me, but the sound of the other instruments played by the other musicians that he assembled for this session. This included Lee Morgan on trumpet, Curtis Fuller on trombone, Kenny Drew playing piano, Paul Chambers on bass, and Philly Joe Jones behind the drum kit.

The varying styles the band played on this album ranged from mellow blues, ballads, and upbeat bop; holding all this together was the Coltrane's fantastic stream of consciousness solos, along with the FiiO S15's transparent signal that it sent to the linestage, and eventually through my speakers. There is a reason why this album is beloved by so many, even more than 65 years after it was released.

Even though this album wasn't the best one to judge the soundstage and image talent of the FiiO S15, as I said, I listened to all types of music through this streamer, and both the vast, layered soundstage and pinpoint imaging were present, but only when the recording had these characteristics.

Conclusion
For the beginner or young audiophile, $999 might seem like quite a bit of money for a steamer, but this component can easily be compared to units costing much more. This comparison is because of the FiiO S15's functionality and, more importantly, its sound quality.

Some audiophiles of a certain age, like yours truly, might initially find the FiiO S15's layout a bit complex, but after time, I learned how to navigate its Android operating system. I was rewarded with excellent sound quality from a well-built, good-looking, and, best of all, great-sounding component. Highly recommended to all who want to save a ton of money when purchasing a multifunction streamer.

Specifications
Type: Home audio hi-res audio digital music streamer / player
Operating system: Android 12+custom landscape display system
Storage expansion: M.2 SSD; microSD card slot

Frequency Response: 20Hz to 70kHz
Signal-To-Noise Ratio: ≥131dB (A-weighted)
Noise Floor: ＜1.8μV (A-weighted)
THD+N: ＜0.00023%

Hardware configuration
SoC: Qualcomm Snapdragon 660, 4GB RAM+64GB ROM (about 46GB available)
Bluetooth chip: Qualcomm flagship QCC5181
USB chip: XMOS XU316
DAC: AK4191+AK4499EX
Display: 7.84-inch
Bluetooth specifications - 
Bluetooth transmission: Bluetooth 5.0, AAC/SBC/aptX/aptX HD/LDAC/LHDC
Bluetooth reception: Bluetooth 5.1, SBC/AAC/aptX/aptX LL/aptX HD/aptX Adaptive/LDAC

Decoding Specifications
Native decoding: 384kHz/32bit; DSD256 (Native)
USB DAC: Supports 768kHz/32bit (EQ mode supports up to PCM 192k); DSD512 (Native)
Coaxial input: 192kHz/24bit
AES input: 192kHz/24bit
Optical input: 96kHz/24bit
HDMI input: 192kHz/24bit

Digital Output
USB AUDIO: 768kHz/32bit; DSD512 (supports DoP/D2P/Native)
Coaxial output: 384kHz/24bit; DSD128 (D2P/DoP)
Optical output: 192kHz/24bit
AES output: 384kHz/24bit; DSD128 (D2P/DoP)
HDMI output: 192kHz/24bit

Outputs
Preamp RCA output: L+R=4.5Vrms+4.5Vrms (1kHz @ 10kOhm)
Preamp XLR output: L+R=8.9Vrms+8.9Vrms (1kHz @ 10kOhm)
RCA output: L+R = 2.4Vrms+2.4Vrms (1kHz @ 10kOhm)
XLR output: L+R = 4.8Vrms+4.8Vrms (1kHz @ 10kOhm)

Color: Black, Silver
Weight: About 5647 grams
Dimensions: About 430 x 274.5 x 89.5mm (WxDxH)
Price: $999

Company Information
FiiO
No.21, Longliang Road
Xialiang Village, Longgui Street
Baiyun District
GuangZhou, China

Voice: +86 136 605 40625
E-mail: sunny@fiio.net 
Website: FiiO.com