Fosi Audio SK02

MatAudiophiles

Fosi Audio SK02​

Fosi Audio SK02
INSTAGRAM: LINK

Prelude:
Fosi Audio is quite an interesting brand offering inexpensive and very functional audio equipment. Fosi Audio focuses primarily on excellent value for money and strictly adheres to the set standards. The first product I have the opportunity to test and evaluate is the SK02 model. It offers very good functionality at a very decent price/ SK02 is an all in one device and combines the functionality of a headphone amplifier with quite high power and a quite good digital-to-analog converter. This time the AUX input was omitted, instead offering us an optical input and output, both optical and linear DAC output. This is interesting and important because the Fosi Audio SK02, which costs approximately USD 119, can be a fully functional digital-to-analog converter and work as an output from a console or TV.


Unboxing and ergonomics:
In terms of packaging, it is a fairly standard cardboard box, containing the device itself, the user manual and a USB C-C cable with an adapter for classic USB. There are some compromises and savings here, but thanks to this, Fosi Audio maintains great and competitive prices, offering surprising performance at a very affordable price. Let’s take a closer look at the design of the SK02 itself. The device consists of a matte black block of uniform metal. Despite its armored construction, it is not heavy, or rather I would call it solid. At the top of the device there is a knob with infinite rotation, which also serves as a button to turn the device on and off, and also allows you to adjust the volume or pause the music. It is solid and holds securely to the mount. Overall, I can praise Fosi Audio for its high quality of workmanship. We also have a 3.5mm and 4.4mm ball output at our disposal. But that’s not all, we also get a 3.5mm LINE out and optical input and output. Of course, we also have USB C, which is used both to transmit sound and to power the device. Honestly, the SK02 offers a surprising amount of power (up to 1W into 32Ohm at high gain from the balanced output). At the same time, the SK02 maintains a very small, pocket-sized size. Which makes it the perfect companion for your desk or even on the TV stand. I wouldn’t call it a portable device or a dongle, but I would definitely call it compact and mobile. If I were to notice something, there are many markings at our disposal, but I would prefer a more clear marking of the volume, because it is easy to damage the headphones if the device is set too loud. At this point, I really appreciate EarMen’s patent in which the device resets the volume to minimum every time we turn it on, thus protecting our hearing. However, this is a marginal comment, and at this price it is not a problem. Speaking of markings, on the front of the device we will find a gain button, an input switch, a diode informing us about sampling, and an audio input button. Additionally, we have a green LED when the device is turned on and a volume LED that flashes purple.

Sound:
For my tests, I used very sensitive IEM headphones, the FirAudio E12, and my favorite full-size headphones, the HE1000SE from HiFiMAN. I played music from tidal.


Bass: The low tones are properly controlled and delight with their purity and neutrality. Fosi Audio SK02 is certainly a neutral device, focusing on transparent sound with a slight hint of warmth. The bass tracks were properly enhanced and the lower bands were properly accentuated. Importantly, the lower bands were in no way overwhelming or overly softened. Overall, I rate the sound of the SK02’s lower bands as very balanced and close to neutral. Actually, I wouldn’t lie if I used the term reference in relation to SK02. The tuning used in this device is a reference with a delicate musicality. It is true that it is not yet at the same level as in much more expensive products, but if someone is looking for a linear and coherent device, Fosi Audio SK2 will be a great choice.

Musical midrange: The midtones are linear in the case of Fosi Audio SK02, still retaining a neutrally warm tone. Importantly, the vocals are practically on the same line as the bass and treble. This proves very good tuning and a signature that is very rare in equipment in this price range. Additionally, we can certainly expect a lot of detail and a detailed, full sound. When it comes to the sound stage, it is both wide and boasts excellent depth, which makes the sound of the Fosi Audio SK02 complete and saturated. It may not be a spectacular and entertaining tuning, but instead it offers a neutral and analytical sound, which I personally value very highly. The space and separation of instruments are at the highest level, offering excellent parameters for the price range in which the SK02 is classified.

Treble: Maintains the overall neutral and linear tuning character of the SK02. At the same time, they are well stretched and offer a lot of details and information. In the case of quite crowded songs with a lot of information, Fosi Audio SK02 performs very well, offering excellent sound quality and not getting lost under the mass of information. The upper midrange is smooth and consistent with the overall sound offered by the Fosi Audio SK02. Although such a presentation may be slightly boring, it allows for a thorough analysis of the sound and provides a completely clean linear sound. When it comes to signal purity, Fosi Audio SK02 provides a clean signal, which is an additional advantage.

Comparisons:


Fosi Audio SK02 ($119) VS Fiio K11 ($119)
In terms of software and number of functions, Fiio K11 definitely wins because the digital display and support for cocial input offer great convenience. However, the Fosi Audio SK02 is much more compact and does not require a separate power supply, which saves even more space. Both devices have a quite similar sound signature with neutral analytical tuning. However, in my opinion, the SK02 is slightly warmer, while the K11 offers an even more digital sound signature. Both devices are really great and have a very high value for money. In terms of sound, it’s practically the same level. While the K11 wins with the number of functions, the SK02 wins with its compact and much smaller form. It all really comes down to what sound signature we’re looking for. Both devices are great and offer excellent parameters at very decent prices. If I was looking for a slight warming, I would definitely go for the Fosi Audio SK02, but if I preferred a slightly more ruthless and even more neutral sound, I would choose the K11.

Fosi Audio SK02 ($119) VS xDuoo xp2pro ($149)
I know that this is a slightly different product category, but they have some common features. First of all, both solutions are dedicated as portable products, with xDuoo xp2pro fitting into a small box and being much more compact. However, in terms of sound, Fosi Audio SK02 sounds more resolved and wider. In terms of power after balance, Fosi Audio SK02 certainly has more power, while in the case of the SE output, the leader will be xp2pro. In terms of sound, the xp2pro offers a more entertaining sound, while the SK02 focuses on tonal balance.

Line Out:
In terms of line output, the Fosi Audio SK02 provides a very pleasant sound without adding much to the natural sound signature. This is due to the ess ES9038Q2M DAC chip and chip, which is famous for its neutral and complete sound. Therefore, even for the line output alone, Fosi Audio SK02 is an excellent and very competitive device.


Summary:
Fosi Audio SK02 is a piece of great equipment. It is a budget amplifier and digital-to-analog converter dedicated to headphones. Enclosed in a very compact housing. It offers support for digital outputs and inputs and excellent technical parameters. Its undoubted advantage is also a very good line output and excellent sound quality. However, I would like to have more secure volume control in the future to better protect the headphones and hearing. Apart from that, it’s hard to find any flaws because it is a well-designed product and well thought out in every aspect. This is an interesting alternative for people looking for a small desk amplifier, for example for work. Fosi Audio SK02 offers not only a balanced and linear sound, but also a very transparent presentation with a large sound stage. That’s why I’m quite impressed by the great possibilities offered by such a small device at a price of only USD 119. In my opinion, this is a complete all in one set for all beginner audiophiles, or for people for whom saving space is crucial. Fosi Audio SK02 is very solid and durable.

Introduction!
Friends, I welcome everyone, today in the review we will look at a DAC amplifier from the company FOSI AUDIO, called SK02 and I’m sure you probably saw it in my previous reviews of different headphones and I used it for several weeks to test it thoroughly and share my operating experience.
It comes in a very small standard box with essentially no information about this device other than this sticker SK02.

Let's take a look at what's included!
The set here is very small, we get
1.Headphone Amplifier
2.USB Type C cable two in one with this adapter.
3.User manual in several languages.

Still, I like the way this amplifier looks.
And on the front panel it has 2 buttons, one for switching the gain mode from low to high, and switching the USB mode to optics and vice versa, and under it there are quite bright LED indicators that display the sampling frequency of PCM and DSD and this is 64.128 or 256 , and by the way, the amplifier weighs 380 grams.

And the all-metal round knob here has endless rotation and tactile pleasant clicks that you can feel when you turn the knob, and you can press it like this once to play or pause the music and if you press and hold, you can turn off the power, and from the unusual thing, here is a purple volume level indicator that flashes when playing low frequencies in the track, and this is a rather unusual sight, I’ll tell you.
But on the rear panel there are 3 outputs, namely usb type c for connecting, for example, the supplied cable, as well as one optical output and an AUX 3.5 mm port for outputting an analog signal.

And in front there is a balanced output at 4.4, and a standard 3.5.

And the DAC we were given here is probably already familiar to everyone: ESS9038Q2M with a separate operational amplifier SGM8262.
The output power at high gain is 280 mW at 3.5, and as much as 1100 mW for the balanced output 4.4, so it will swing your in-ear and full-size headphones easily and with a very large margin and excellent control, so you don’t even have to worry about it here, and I advise you right away switch to high gain because even then the amplifier remains only slightly warm even after long sessions, so we get a lot more power, although the first time I got it I only listened to it at low gain, and even then it drive without problems even planar headphones with a large margin.
Well, of course, the DAC works without problems and is detected on Windows, MAC OS, Android, and iOS no additional drivers need to be installed for it everything is picked up automatically.
How about sound?
And now, friends, let's talk about the most important thing, namely the sound of this device, which has excellent measurements, which can be found in more detail on the official website of Fosi Audio. I will specifically leave links in the description so as not to waste time analyzing all this now
And I’d better start with the fact that I tested with it both various planar in-ear headphones and hybrid and dynamic full-size headphones in different price categories!
Since it offers us excellent transparency and excellent localization with a wide stereo panorama with literally crystal clarity and outstanding resolution in no way inferior to my Topping g5, and its frequency response is absolutely linear in measurements, as it should be on modern sources, I hope you agree with me!
And the overall sound signature here does not feel analogue softened and warm, but has a strong bias towards that same cold monitor quality, which I very much approve of.
And I can absolutely say that this amplifier does not try to bring anything of its own into the signal and it has absolutely what it should have: a neutral, honest sound signature with good technique and highlighted attacks, without any brightness or additional warmth, that is, again, like me and said in previous reviews of DAC amplifiers, everything here will depend on your headphones that you are going to connect to this excellent and most importantly powerful source, since it will allow you to use planar headphones such as LETSHUOER S15 or Hidizs MP145 without problems and they will sound very correct on it with amazing control and technicality and in terms of attacks and transient processes, there will also be complete order here, but what can we say about hybrid headphones, which also open up as expected and this DAC does not constrain at all and does not try to blur the sound of the headphones in some super magical way, which is especially cool to hear from some audiophiles, I hope you understand what I'm getting at.
My conclusion on this dac:
Fosi Audio Sk02 is an excellent, very high-quality DAC for a small and pleasant price, with amazing power and the most mature neutral sound, all you have to do next is find good headphones and enjoy the music, and my reviews will help you with choosing headphones.
Link where you can buy this dac!
Fosi Audio:https://fosiaudio.com/pages/desktop-dac-headphone-amp-sk02
I will be glad if you subscribe to my YouTube channel and watch this full review on FOSI AUDIO SK02!

TLDR version on YouTube: TDLR - Fosi Audio SK02

The Fosi Audio SK02 is the latest device released by the brand and has been sent to me directly by Fosi Audio for me to try it out and to share my opinions in this review. Fosi have not made any requests except for asking if I could publish my review around the 26th of February, which is when the product will be officially released. It is currently available (at the time of putting this review together) on Kickstarter as a preorder (for around 65€).

Therefore, as always, I will do my best to be as unbiased as possible in my review.

The SK02 is available from various sources that I will leave links to on my blog.

As usual, links that I share are non-affiliate.



Intro…

Fosi Audio are not a new brand, they have been around for a while and produce audio amplifiers and related products at a fairly budget level. They are not the cheapest you can find on places like Amazon or Aliexpress but they are a brand that is serious about what they do and do it at a very reasonable price point.

I previously reviewed the K5 Pro here on the channel and today we have the SK02, which is another DAC/Amp combo but in a different form factor (and with different specs).

This small desktop all in one features and ES9038Q2M DAC chip (I am actually told it uses 2 of them), which has been used in quite a few devices before by many brands. In this case, Fosi have paired it with an SGM8262 OP Amp, which is also something we have seen before in other devices.

The specified power output (according to the Fosi Audio published spec) is 280mW @32Ohm (unbalanced) and 1100mW @32Ohm (balanced). These are quite powerful numbers and are almost double, in the case of the balanced output, to what I have seen specified from other models in the past using the same OP Amps.

But we will talk about power in just a moment, let’s first take a usual look at packaging and presentation.



Presentation…

There is absolutely no doubt that Fosi Audio spend as little as possible on the packaging in order to keep the price down. Well, I guess they could go with just a generic brown box or even just a padded envelope (it wouldn’t be the first time I have received devices packed just in an envelope), so lets say that the go for the cheapest branded packaging they can.

The box is the same a the one used on other Fosi products and shows the Fosi Logo on the front with the model name added by means of a sticker in the top right corner. There are no specs about the product on the box (which is understandable as it is a generic box for multiple products) although it does mention that you have a 30 day return period, 18 month warranty and lifetime technical support. Of course, if you are buying from Amazon here in Europe (or any other European seller), the warranty will be the legal minimum of 2 years or longer, depending on which country you reside/buy it in (Spain is 3 years).

Opening the box reveals a simple foam cutout that contains the DAC/Amp, the user manual and a USB-C to USB-C cable with an attached USB-A adapter.



Build and aesthetics…

The form factor is different to the majority of small all in one DAC/Amp combos as it is focused around having the volume control and information leds on the top of the device. This is a layout that will work better or worse for you depending on how you intend to set up and place the device.

The shell of the device is aluminium in a black finish and while it is not extremely heavy, there is a bit of weight to it to stop it being easily dragged around the desk when in use.

I am a fan of this style of device for places like my work desk as it provides a small form factor that can sit at the side of my keyboard and USB microphone, with easy to access volume control and easy to read status LEDs.

The overall build quality is pretty good, with rubber strips on the bottom to also stop it from sliding easily. The volume knob does have a slight wiggle when rocked side to side but it is not bad and is not really noticeable when using the device, just when actually focusing on rocking the wheel.

The majority of the status LEDs are orange and are bright enough to see but not bright enough to be distracting, something that I am grateful for when having it within eyesight on my desk.

While I wouldn’t say that the SK02 stands out as an incredible build or has eye catchng looks, I do think that it is well built and really can’t fault it for the price range that it sits in.



Functionality…

On the top of the device we have a large volume knob that also serves as a play/pause button and a power switch, at least on Windows 10. The knob is an infinite rotary style, so you can turn it endlessly and the only real feedback in regards to level is the amount of sound coming from your headphones (or speakers). Well, I guess that is not really true, as there is a volume level LED but that is a single LED that works sort of like a clip LED on a console or other device. By this I mean that the LED only really starts to flash as the volume level starts to increase, flashing brighter and longer as the volume increases. It is a nice touch and I appreciate it being included but it really isn’t something that will give you a real idea of the level.

The volume from the SK02 is independent to that of Windows. By this I mean that the setting of the SK02 in the Windows mixer does nothing. No matter the level set, it will allow the full range of the SK02 to be used and turning the volume knob does not adjust volume in the Windows mixer, it is separate amplification on the device itself. This is something very positive in my opinion as you can use exclusive modes in apps like Tidal or Foobar and not have to worry about it blasting sound through your headphones but it also means that you have to reduce volume with the knob and keyboard shortcuts will do nothing (including mute).

The rotary knob is also a push down switch which plays or pauses the music (or Youtube video etc.) when pressed once. A long press of the knob switches the SK02 off, which means you can disconnect the device without having to unplug it. A long press to switch it back on and the device reappears in Windows almost instantly.

Above the knob, the centre of the top panel features 14 mini LEDs. As I said a moment ago, the majority of these are orange, with only the volume status LED being a purple colour and the power LED being green. To be honest, I don’t really see the need for the power LED as there are always other LEDs active when the device is on anyway, but at least it makes the layout symmetrical.

The LEDs are in two columns of 7, with the left column showing (from top to bottom):

High Gain

Low Gain

384 PCM

192 PCM

96 PCM

48 PCM

44.1 PCM



The right hand column, again from top to bottom, shows:

Optical Input

USB Input

256 DSD

128 DSD

64 DSD

Volume level status

Power



These are all identified by small white text which is simple but easily read and nicely laid out.

Above the LEDs, at the top of the panel, there are two push buttons that sit flush with the device. The left button switches the Gain level, while the right button switches between the two input modes.

On the front of the SK02 we find the headphone connectors. These are a 3.5mm unbalanced connection, with a 4.4mm balanced connection below. The 3.5mm connector is slightly off center in comparison to the 4.4mm directly below, I am guessing due to internal space restrictions. This is only really a minor thing to point out and not a complaint as you only really notice it when looking straight at it.

The sides of the device are empty of connections and moving around to the back panel we find a USB-C input, an Optical input and a 3.5mm preamp output. The output is controlled by the volume knob (hence being a “preamp” output) and is muted by inserting headphones into the headphone output.

Something that I discovered while recording the Spanish video review is that the 3,5mm output is also an optical output, allowing the SK02 to feed other systems digitally. This is something that I always find handy and is nice to see on a budget device.

That is about it as far as functionality, plug in your cables, choose your mode and volume level and that’s it, away we go.

The only real comment I have here is that I think a microphone input would have been a good idea for this kind of device, similar to the one on the K5 Pro or maybe even just a TRRS for the 3.5mm to allow the multiconnector to be used. I am not talking about a high end microphone input, just something simple to allow gamers to use their headsets with this device, even if the mic volume would only be controlled in Windows. This is by no means a complaint, just a comment, exactly the same comment I have made with other similar devices in the past.



Sound…

I have not measured the output power of this device and I am still sceptical about the 1100mW claimed for the balanced output but I have to say that it is a very powerful little device. There is no exact volume meter for me to be able to tell you at what volume level I have been using this amplifier but I have certainly not been anywhere near max level with anything I have plugged into it. In fact, as a quick example, if I set the SK02 to full and then raise the volume in Foobar (as the Windows mixer does nothing), with the KPH40 on low gain, my usual listening levels are around -25dB and -15dB starts to be far too loud for me.

As far as sound, I will make my usual disclaimer that differences in sound between sources are usually very minimal, often exaggerated by reviewers to get our point across and most probably more based on our brain's interpretation than actual differences in sound. Saying that, I find the SK02 to be neutral edging very slightly towards the warm side of neutral. Not warm as in iFi style but certainly not cold and clinical like a THX. I would say it is similar in this style to something like Earmen or maybe Aune, although not quite at that level.

There is no real sensation of it adding anything to the sound, I think it is more that details are ever so slightly rounded, giving it a bit more of a smoother response which means that it is not going to be quite as detailed as some of the other brands and models out there but it is far from being a bad music reproduction.

Testing it with sensitive IEMs I found that, especially as the volume level raises, there is a presence of some background noise. However, I found it difficult to judge if it was the actual noise floor of the amplifier itself or if it was just making the noise of the track more than I am used to. In order to test this further, I used a silent track (basically a recording of silence) and found that I could max the volume with the same IEMs without hearing any of the background hiss, so my conclusion is that, for some reason, it is making the background noise present in recordings a little more noticeable than I am used to.

NOTE: As a safety disclaimer, I strongly recommend that you do not do this test! I was using Foobar with the output via Wasapi to the SK02, which means that “in theory” no other sounds could access the device. However, if something had gone wrong and something that was not silence had managed to play (i.e: another track or a windows notification), then I would probably have damaged both the IEMs and my hearing. Again, please don’t try this at home!

Anyway, back to the sound. As I said, it is neutral edging towards warm and I found that it works pretty well with both IEMs and headphones that are easy to drive, having more power than I would ever need. However, with the KPH40 (60 Ohms & 101dB) and the HD6XX (300 Ohms and 103dB) that, when pushing the volume a little higher, things didn’t seem to be quite as clean, especially when the track had a decent amount of bass. It is not that it sounded terrible, just that they seemed to have a little bit of congestion going on in the upper mids and treble when there is a good amount of bass.

This is something that I did not notice with lower Ohm headphones, even those that have lower sensitivity, nor did I notice it with higher Ohm IEMs, so I am not sure what is going on there. I just get the feeling that things are not quite as clean as they could be in these cases.

I also put some planars to the test with the SK02 and while it did a decent enough job, I did find it struggled to keep up with fast moving bass content on some of them such as the Arya v2. With the Ananda and the Edition X it didn’t suffer as much but I would still say that it lacks enough current to be able to move planar headphones properly, at least those that I have. This is also understandable as it is powered by USB and is a fairly small device.

With IEMs and easier to drive headphones, I found performance to be more than adequate for the vast majority of people who would contemplate having this on their desk.



Conclusion…

Again I find that Fosi Audio offers a device that is going to appeal to a lot of people. I don’t think it is really aimed at the audiophile crowd but more at the general music listener/lover that wants something small and compact for their desktop.

I also think that it is a good option for the gamers out there who use independent headphones and a USB mic. It is plenty powerful and is very easy to adjust on the fly, while taking up about the same amount of real estate as a mouse.

It can also be hooked up to a secondary source via its Optical input, and serve as a preamp to speakers via the aux output, so again, it is something that could serve a lot of people at a very reasonable price.

____
As always, this review is also available in Spanish, both on my blog (www.achoreviews.com) and on YouTube (www.youtube.com/achoreviews)

The Fosi Audio SK02 Dac Amp the latest in Fosi's SK line, it uses the Sabre ES9038Q2M dac chip and SGM8262 opamp same as what FIIO uses on the powerful KA13 dongle and supports up to PCM 384khz, DSD 256 and with an ouput power of 280mw @ 32ohms on 3.5 single ended and 1100mw on 4.4 balance.



It has the same design and built quality of Fosi SK01 amp basically bullet proof metal brick and probably will outlast all my audio gears, on the faceplate it sports 2 buttons for gain and mode usb, optical and below it are the led indicator lights and the solid metal knob with infinite rotation with tactile clicks you can feel when turning the knobs, push the knob once to play/pause push twice to mute/unmute and long press to power on/off, at the back you see 3 inputs, usb type c for input and power since the sk02 in usb powered, there is also 1 optical input and the 3.5 doubles as a analog output and optical out.



Fosi audio did a great job on the implementation of the ES9038Q2M dac chip, its not too warm analog sounding not also cold too bright and technical sterile sound its somewhere inbetween, like they hit the sweet spot. it has a natural neutral clean tone that you get to capture micro macro details in all musical genre you put it through without sounding too digital and a well defined accurate bass though on some tracks i tested it sometimes lean a little bit on the brighter side which also gives the sk02 good details and resolution. with an output of 1100mw on balance it can open up most hard to drive over-ear planar headphones as well as iems. you certainly get more than you paid for sure. Great job Fosi Audio, looking forward to SK03... perhaps making future models more portable and power efficient. SK02 is a fun dac that delivers and performs more that its worth.
True to their word ''HiFi Made Fun''!

Iems i used for testing : Tanchjim Zero, Simgot ea500, Hidizs mp145, ARTTI T10.
Over-ear headphones : Hifiman HE400se.

https://fosiaudio.com/pages/desktop..._xGjSAjHvSJmmFvZ0UGbXyoF5SJUjRb9DPsDhPxkBBCQ4

POOWWWEEERRRR!!!

~~Terry Crews in an old Old Spice commercial.


FOSI AUDIO is one of the few audio companies out there that offers some of their products that truly signifies that price to performance statement on how they manage to construct them meticulously. They usually specialise in producing amplifiers and preamps, and they were able to offer at a very affordable price.



I actually did a review on their product before and I still have a pending item to review from them. What I have here right now to be feature in this review article is their latest Desktop DAC/ headphone amp, The FOSI AUDIO SK02.



The SK02 is a desktop-grade headphone amplifier in which its overall construction is quite solid yet compact as it allows me to carry it anywhere due to its balanced weight for a desktop amp. It has an elongated square shape chassis made of solid aluminium alloy which underwent a process via CNC milling and its surface which were sandblasted to achieve that matte-like finish.



At the top panel, there are some buttons for gain and input modes, some levels of LED indicators for gain display, PCM sampling rates, DSD sampling rates, volume level status and power indicator that were aligned horizontally, and a large multifunctional knob use to adjust for volume control on clockwise/counterclockwise and clicks for basic command functions.



On gain modes, it offers two modes that will be beneficial for headphones and earphones with different power output requirements from very sensitive IEMs up to the power hungry cans with magnetic planar drivers. These modes are low gain and high gain mode, and the difference between two modes are quite a stark contrast in terms of power output delivery, I usually use the low gain mode as the majority of my sets are usually IEM earphones. On the left part, the PCM sampling rate LED indicators are arrayed in a perpendicular fashion and it has some levels of sampling rate from the maximum rating of 384kHz down to a standard 44.1kHz. And on the opposite right side, there are DSD sampling rate LED indicators that were arranged in a similar manner below the input mode indicators like optical mode and USB mode. The DSD sampling rates that have LED display status are DSD64, DSD128 and DSD256 which are more than enough as my collection of DSD files are usually in either DSD 64 or DSD128 format. The volume level status indicator depends on the intensity of flashing lights of magenta colour, the more that we increase its amplitude, the more intensity of blinking lights it occurs. Then below the volume level indicator, there's the power mode indicator that was situated there.



The large knob is used for volume adjustment depending on the loudness level of your choosing. As for click commands that I have mentioned a while ago, a single click is for play/pause, double-clicks for mute/unmute and a long press for 3 seconds on power on/off. It should be noted that command clicks won't work on optical signal connection mode except for the long press one.



At the rear part of this device, there's the power/USB Type-C input port for USB connectivity for PCs, laptops, TVs and other devices and it is also used as a pre-amps pairing it with speakers or amplifiers. The other ports are optical input for TOSLINK-supported devices in which my majority of testing is conducted in this manner and an 3.5mm AUX port for analogue signal output.



And in the front part, there are two types of audio output interfaces that were implemented here, the standard 3.5mm single ended headphone jack and a 4.4mm balanced headphone jack. The 3.5mm headphone jack power output rating is quite powerful and robust as it is capable of delivering a 280mW@32 ohms at high gain mode while the 4.4mm balanced output can deliver up to 1.62W@32 ohms in high gain mode.



Regarding its internal parts, FOSI AUDIO did implement a flagship ESS reference DAC Chip, The ES9038Q2M which is known for its reliability and high performance. The ES9038Q2M is a capable 2-channel stereo DAC with its vaunted proprietary 32-bit Hyperstream architecture with clock jitter eliminator which enables it to deliver a clear, distortion-free audio signal output with a less demanding power consumption. Aside from a high performance DAC, it uses a separate headphone op/amp, SGMicro SGM8262 which is a high speed operation amplifier with low noise and voltage feedback functionality which gives a substantial load of output swing for better linearity and improving its dynamic range performance.



As for product packaging, like all FOSI AUDIO products, the device and with other contents was put in a protective styrofoam in a rather minimalist packaging box.



Here are the contents inside of FOSI AUDIO packaging box:

• FOSI AUDIO SK02 desktop headphone amplifier
• A long Type-C to Type-C USB cable
• Instruction manual

This desktop amplifier will be normally used for decoding audio codecs from PCs, laptops and other multi-media consoles if you use its type-C input as its connection. But if you are using this in your smartphone like a typical USB DAC/amp dongle, it will definitely won't work at all as this device needs a required input voltage of DC 5 volts just to power this one. The majority of the duration of its testing phase of this device will be in optical mode in which I will connect it to QUESTYLE QP2R DAP which has TOSLINK support for analogue connectivity, so it will serve as an amplifier. I will also share some insights on how this device will perform as a desktop DAC when I tested it on my mate’s laptop.



Furthermore, to conduct a test, I will use a variety of IEMs in my collection from warmish-neutral LETSHUOER CADENZA 12, SEE AUDIO BRAVERY RED and BQEYZ WIND, a DF-neutral like LETSHUOER EJ07, reference-neutral sets like KINERA IDUN GOLDEN, HIBY CRYSTAL 6 II and LETSHUOER EJ09, a mild U-shaped AFUL PERFORMER 8, A warm sounding ORIVETI OD200 and lastly, some balanced-neutral sets like LETSHUOER EJ07M, AFUL MAGICONE and BQEYZ WINTER.

The sound impression that this device is able to deliver is a reference-neutral sound which has more emphasis on midrange and high frequencies to give that bright, crisp and detailed sound quality.




LOWS/BASS:

The device is able to deliver a precise, rumbly and incisive sound with a very little colouration as it maintains a tidy bass response that is well-segregated from other frequencies.

While it maintains a sub-bass presence just to exhibit if a certain track requires more reverberation, its mid-bass appears to be quite linear but it is enough to give an ample amount of texture for bass-focus instruments and bass-baritones vocals.


MIDRANGE:

My initial impression on how the midrange presentation is transparent, neutral and energetic as it gives more articulation, clarity and silvery vocals, mainly on female types and some instruments.

If I paired it with warmer sounding IEMs, it just gives more clarity on male vocals to sound more expressive and some detail on its vocal ends in which tenors and countertenors will sound more brassy and musky respectively. As I mentioned that female vocals will benefited more on this one especially if I pair it with reference-neutral tuning sets as it adds more emotive and fiery sound on mezzo-sopranos while the energetic nature of this device will give a more gleaming and shimmering on sopranos as it sounds so open that it project its highest note range.

On instruments, it adds some richness and crispness tones of some instruments particularly on some strings, percussions and woodwinds as it still sounds natural but more detailed on its timbre. Once again, if I pair it with warmer or balanced-neutral sounding sets, it has a very little to none amount of colouration on some brass and percussives that will sound marginally negligible into my ears at all.


HIGHS/TREBLE:

Treble response of this one is definitely bright with a lot of detail, sparkling and quite well-extended.

Pairing it with reference-neutral sets which have more emphasis on upper-mids up to the brilliance part of the treble region will be more accentuated so that it will have more definition and clarity.


OVERALL TECHNICAL PERFORMANCE:

This device is quite analytical in that aspect as it will deliver a more resolving sound in a clean pitch black sonic background. It is able to project a rather spacious stereo imaging with a well-layered of frequency and dynamic tones in a sonic canvas. Separation of instruments is quite remarkable as it has a good gap and spacing on each element.

Resolution capabilities of this one is outstanding as it is able to deliver well-textured macro-dynamics while its micro-dynamics is exceptional as it has a sharper definition on extracting nuances, subtleties and intricacies of note textures from an audio source.


There will be no peer comparisons section as I don't have any desktop amplifier aside from this unit. But I did test some iFi like Zens, S.M.S.L. desktop amps in which I forget its model and Chord Mojo and I find them more coloured sounding due to their warm and analogue-ish nature.


To sum up my assessment in regards to this product, FOSI AUDIO continues to impress me even more that this device performs beyond my expectation. So what are my takes about this one, first one is the desktop amplifier is quite solid for its build construction while maintaining a balanced weight. Second one is pricing which is quite affordable and reasonable for its performance. Third one is its overall sound quality as it has a robust and powerful amplification on its power output while maintaining a neutral and analytical sound. And last but not the least, was its versatility that I can even use it in both modern and analogue audio desktop gears. To whom will I recommend this one? My best answer is for audio enthusiasts who keep pursuing more high-fidelity audio with a very competent technical performance but their budget is rather limited.

FOSI AUDIO SK02 is now available at HIFIGO, ALIEXPRESS and AMAZON. Check out my unaffiliated links that I've provided below:


★★FOSI AUDIO SK02 - HIFIGO★★

★★FOSI AUDIO SK02 - ALIEXPRESS★★

★★FOSI AUDIO SK02 - AMAZON (US)★★

★★FOSI AUDIO SK02 - AMAZON (JP)★★


For my previous review of a FOSI AUDIO product, click the link below to know more about the company.

■ FOSI AUDIO MC101





SPECIFICATION:

Some Tracks Tested: ( * = 16-bit FLAC, ** = 24-bit FLAC, *'* = MQA, '*' = DSD, *'= .WAV)

Alison Krauss -When You Say Nothing At All *
Jade Wiedlin - Blue Kiss**
Led Zeppelin - When The Levee Breaks **
Mountain - Mississippi Queen *
Queen - Killer Queen **
Guns N' Roses - Patience *'*
Eric Clapton - Tears in Heaven '*'
Sergio Mendes- Never Gonna Let You Go '*'
Pearl Jam - Daughter **
Roselia - Hidamari Rhodonite *
Assassin - Fight (To Stop The Tyranny)*
Celtic Frost- Visual Aggression *
New Order - Blue Monday *
The Corrs- What Can I do (unplugged version) *
Jimi Hendrix Experience - Voodoo Child *
The Madness- Buggy Trousers *
Metallica - Motorbreath **
Mariah Carey- Always Be My Baby *
Destiny's Child - Say My Name *
Malice Mizer- Au Revoir *
Mozart - Lacrimosa *
New York Philharmonic Orchestra - Dvorak- Symphony 9 " From the New World." *
Eva Cassidy - Fields of Gold (Sting cover)*
Michael Jackson - Give In To Me *
Exciter - Violence and Force *
Diana Krall - Stop This World **
Debbie Gibson - Foolish Beat *'*
The Sisters of Mercy – Lucretia My Reflection**
Suzanne Vega – Luka **
Lauren Christy – Steep *
Ottoman Mehter - Hucum Marsi *
Diana Damrau - Mozart: Die Zauberflöte*



P.S.

I am not affiliated to FOSI AUDIO nor receive any monetary incentives and financial gains as they provide me a review unit for an exchange of factual and sincere feedback from yours truly.

Once again, I would like to send my gratitude to HIFIGO especially to MS. YUMU SONG and MS. LVY YAN for providing this review unit. I truly appreciate their generosity and trust towards me and other reviewers.

Disclaimer:​

Good morning, readers! In this review, we will analyze Fosi Audio’s latest product. I thank Hifigo, who, together with Fosi Audio, sent me this Fosi SK02 unit to review. I needed something more powerful and this big dongle/AMP has a lot of power.

By now, the quantity of Dongles in my possession has reached a considerable amount. I don’t consider myself a foreigner but hopefully you like my review and find it useful.



The review, as always, will be completely unbiased and uninfluenced.


I would like to remind you that I am just a person who enjoys trying out new items and listening to a lot of music; I am not an expert.


My opinions are obviously subjective.

Tech Specs:​

• Dac: ES9038Q2M
• Op-AMP: SGM8262
• Input USB-C, OPT
• Output: 3.5/4.4mm Headphone
• Out: 3.5mm AUX/OPT
• Headphone Output Power
• 3.5mm PO Low Gain: 90 mW at 32 ohm High Gain: 280 mW @ 32 ohm
• 4.4mm BAL Low Gain: 350 mW at 32 ohm High Gain: 1100 mW @ 32 ohm
• SNR 3.5mm PO Low Gain: 116dB / High Gain: 120 dB
• SNR: 4.4mm BAL Low Gain: 110dB / High Gain: 115 dB
• Distortion ≤0.001% @ 1 kHz
• Noise Floor 3.5mm PO 3.7μν / 4.4mm BAL 12μν
• Crosstalk 3.5mm PO Low Gain: 128dB / High Gain: 137 dB
• 4.4mm BAL Low Gain: 112dB / High Gain: 110 dB
• Frequency Response: 20Hz-20kHz (+0.02 dB)
• Impedance: 16-300 Ohm
• Input Voltage: DC 5V
• Final Price: Around 99 USD (not confirmed)
• Official Fosi Site

Packaging:​

The box is very simple; I’m not even sure that it is the final packaging of the product, as it was sent to me before the end of the Kickstarter. As you can see, it is not tailor-made. Inside, we find the Fosi Audio SK02, the cable with an integrated USB A and Type C adapter (a magnificent accessory), and the manual.


I don’t know the final price yet but theoretically it should be under 100 USD, I think the set of accessories is correct.

Build quality/First impressions:​

Even before opening the package, you can feel its weight. It is certainly not a dongle. Opening the box, its size is immediately apparent; the body is entirely made of aluminum alloy, painted black in a matte finish. The volume knob is also a solid block of aluminum. The peculiarity is that it runs infinitely and allows precise volume adjustment (it has a memory of the settings). The knob also has the function of being pressed and is used to mute, play, or pause.


On the front, we have 2 more buttons, one for the gain and the other for the input mode.


As you can see, the front panel is full of LEDs that indicate the gain. the sampling rate, the input, the power status, and even the volume (or power) level.


In fact, next to the audio/speaker symbol, there is a purple and blue LED that indicates the right and left channels and lights up when the power demand becomes important. Obviously, it lights up to the rhythm of the music. Here, it would have been nice to see two series of LEDs with the musical spectrum, perhaps in the next product.


On the front, we have the two outputs, 3.5mm and 4.4mm balanced, both very powerful.


In the back, we have double input, optical and USB, but also an AUX output if for example, you wanted to use an AUX to RCA cable to connect a speaker amplifier. This output also has optical functionality but its operation is not explained very well, You need a 3.5mm optic cable and in my opinion, it always acts as an audio output so you can connect it to a home theater or a soundbar.


However, it is an extremely versatile product and despite its size, we can also consider it portable as it does not require external or extra power supplies.


Qualitatively speaking, the construction is well taken care of and the assembly is free of defects.

Power:​

In terms of power, considering that it does not require an external power supply, it is a little monster. On the 3.5mm output, we have 280 mW at 32 ohms, which I assume is per channel, This is because, compared to the FiiO Ka11, the volume is much higher. I connected the Hifiman Deva Pro to it with only 18 ohms and it is impossible to reach the maximum volume. The power is sufficient for most headphones and IEMs already on SE output.


On the balanced output, we have 1100 mw at 32 ohm, which I would say are per channel (I have to check with Fosi) but they could easily be the summed values. But seeing the power in the SE of the FiiO KA11, I don’t think it can develop less power. Also because the volume is significantly higher.


Needless to say, it has no difficulty with any IEM or headphones. I don’t have anything with extremely high impedance but I will soon be looking for something with at least 300 ohms.

3.5MM​

For the moment, the heaviest test I can do on a 3.5mm output is the EPZ x Tipsy Star One 64 ohm + adapter with 75 ohm impedance.


I can tell you with certainty that it is not affected in the slightest. Obviously, I have to turn the volume up more but it is still not possible for me to reach the maximum volume. I don’t think you would have any problems with 300-ohm headphones as long as they have good sensitivity.


The Hifiman Deva Pro on 3.5mm seems connected to a balanced output (they also have an impedance of only 18 ohm).

4.4MM​

On balanced 4.4mm output, I have tested all the planars in my possession. As far as IEMs are concerned, it doesn’t have any kind of problem. The KZ PR2 V2, which is the least easy to drive, will likely explode if I try to reach maximum volume. The driver can’t even keep up with the power. As for Hidizs MP145, Letshuoer S12 Pro & S15 and Kefine Klanar, they are managed without any kind of problem.


The Hifiman He400Ses, on the other hand, definitely benefits from the extra amplification. The driver acquires dynamics that I don’t notice with other balanced outputs, including improved soundstage and bass with greater impact.

AUX OUT:​

Given the somewhat particular functioning of this AUX, I asked Fosi for clarification.


I noticed that this output is also amplified and the volume is always controlled via the knob. Theoretically, I would have expected a classic AUX output like I have on my FX AUDIO DAC X6 that I connected to my speaker amplifier. Connecting two amplifiers in cascade, as my colleague Sean pointed out, is not a big deal. Practically, we could also connect headphones to the rear audio output. Now it remains to be understood whether it can also be used at the same time (I doubt it). Furthermore, it is also an optical output and this works normally; we can connect a home theater or any audio system to it.


Update from Fosi Audio:


The rear output has very low amplification and can be used together with the balanced output without problems. I would say that in this case, you can use the output on the back to connect it to a speaker amplifier without any problems. I personally won’t use it that way, as I already have another DAC. The Fosi is extremely convenient for me for headphones and IEMs.

Test/Sound Signature:​

Like all the DACs that use the same chip in my possession, the Fosi SK02 is also characterized by a bright sound. It’s not the brightest but it definitely doesn’t have warm undertones, in fact, if a headphone/IEM is warm, it tends to make it more balanced. You may or may not like this feature but personally, I don’t experience any kind of problem, on the contrary, it doesn’t place an emphasis on the low range like some Cirrus logics. However, in the case of the Hifiman He400Se, the greater amplification gave body to the bass.


So I presume synergy also comes into play, even if it is rare and in some cases, the performances may differ from expectations.

Even the Tanchjim Zeros connected to it demonstrate a certain amount of bass despite being bright.

As far as I’m concerned, the Fosi SK02 will remain on my desk as a neutral, bright, and powerful reference. For a little more analytical and critical listening, it seems suitable to me.

The Fosi Sk02 works differently depending on what I plug it into. On my iMac, I can play-pause tracks, while on my smartphone, I can only mute them. On the iMac, the volume can only be managed by the SK02, while on a smartphone, you have double adjustments. In both cases, it works perfectly, demonstrating high performance even from type C.

Fast Comparison:​

vs iBasso DC04 Pro

iBasso DC04Pro
iBasso DC04pro has something more than the others; they may be small things but although the chip is identical to that of the Moondrop Dawn Pro, the OP-Amp department is evidently more refined. I don’t think the price difference is just a matter of brand but to be sure, they would need a teardown. It comes across as more musical and refined, with a soundstage that sometimes seems wider. The Fosi SK02 is brighter and, in any case, much more powerful and practical for desktop use.


vs Hidizs S9 PRO Plus

Side
Hidizs S9 Pro Plus has its own sound coloring in the DACs. There are those who might think that it is a negative factor but the difference is that even if it doesn’t upset you on some IEMs, it is more noticeable. The sound is slightly warmer and focused on the vocals. It somehow makes even trebles smoother without losing detail.


vs Moondrop Dawn Pro

First look Moondrop Dawn Pro
This is an excellent entry-level DAC that allows everyone to get close to the balanced output. For the asking price, the Moondrop Dawn Pro is definitely the right choice. In terms of sound, it is warmer than the Fosi SK02 but not too much.


Vs Fosi K5 Pro

Fosi Audio K5 Pro
I’ve reviewed it elsewhere so I’ll put a friend’s review in the link. The Fosi K5 Pro is certainly a much less serious DAC, the difference is noticeable but it has the convenience of being able to adjust the highs and lows. This feature is great and would have been very useful on the SK02.

Who do I recommend this DAC to?​

Like all the DACs that use the same chip in my possession, this one is also characterized by a bright sound. It’s not the brightest but it definitely doesn’t have warm undertones; in fact, if a headphone/IEM is warm, it tends to make it more balanced. You may or may not like this feature but personally, I don’t experience any kind of problem; on the contrary, it doesn’t place an emphasis on the low range like some Cirrus logics. However, in the case of the Hifiman He400Se, the greater amplification gave body to the bass.


So I presume synergy also comes into play, even if it is rare and in some cases, the performances may differ from expectations.

Even the Tanchjim Zero connected to it demonstrates a certain amount of bass despite being bright.

As far as I’m concerned, the Fosi SK02 will remain on my desk as a neutral, bright, and powerful reference. For a little more analytical and critical listening, it seems suitable to me.

Where to buy?​

HiFiGo

Aliexpress

Amazon US

Amazon JP

Fosi Audio Official Site + 15% Limited Coupon FASKMKT15

The Fosi SK02 presents itself as a compact desktop DAC/AMP with a vertical design, ideal for space-conscious users. But don’t let its desktop aesthetic fool you – the SK02 offers remarkable versatility, making it suitable for mobile use as well.

The metal housing is both durable and elegant, and it fits seamlessly into any desktop setup or on-the-go powerful audio rig. It is available for $99.

Disclaimer:
This review unit of the Fosi SK02 was generously provided directly by Fosi Audio. While I appreciate their support, all opinions expressed in this review are my own and based on my independent testing and experience. I strive to remain objective and transparent in my evaluations, offering an honest and unbiased perspective for potential buyers.

Unboxing
The Fosi SK02 arrived in a sleek and minimalist box with the phrase ‘HiFi Made Fun’ written on it, which set the tone for what was inside.

Upon opening the Fosi SK02 box, the sleek metal device was revealed, concealed in protective foam. Additionally, I found a user manual and a USB Type-C to Type-C cable with a USB Type-A to Type-C adapter, making it suitable for various devices.


Design and Build Quality
The SK02’s vertical design stands out from the crowd, occupying minimal desk space and making it easy to carry with you when you are on the go. Although you may not be able to put it in your pocket, it is still small enough to easily fit in your bag and be a very strong portable companion. Although it has a distinct visual appearance, it may not appeal to everyone’s aesthetic preferences.

The large volume knob doubles as a power button and play/pause control, offering convenient operation. However, its lightweight feel and susceptibility to accidental adjustments detract from the user experience. Additionally, the volume level light indicator is very inconvenient to use, as it shows the volume level by its changing flashing brightness, and the absence of an LCD display makes checking volume levels slightly harder.


Technical Specifications
DAC Chip: ESS Sabre ES9038Q2M
Operational Amplifier: SGM8262
Maximum Sample Rate: PCM 384kHz/32bit, DSD256
Input: USB Type-C, SPDIF (shared)
Output: 3.5mm single-ended, 4.4mm balanced (high gain 32Ω: 280mW/3.5mm, 1100mW/4.4mm / low gain 32Ω: 90mW/3.5mm, 350mW/4.4mm)
SNR: 3.5mm – Low Gain 116dB / High Gain 120dB. 4.4mm – Low Gain 110dB / High Gain 115dB
Distortion: ≤0.001% @1kHz
Noise Floor: 3.5mm – 3.7μV / 4.4mm – 12μV
Crosstalk: 3.5mm – Low Gain 128dB / High Gain 137dB. 4.4mm – Low Gain 112dB / High Gain 110dB
Frequency response: 20Hz-20kHz (±0.02dB)
Headphone impedance: 16-300Ω
Input voltage: DC 5V
Hardware Controls: Power, volume, high/low gain switching, input switching
Dimensions: 130mm (H) x 62mm (W) x 23mm (D)
Weight: 340g

Versatility and Connectivity
The Fosi SK02 stands out for its remarkable versatility and connectivity options, catering to a wide range of audio setups and preferences. The ability to work as a standalone device that is powered solely by a mobile phone is a key feature, making it portable and convenient for on-the-go setup.

Moreover, the SK02 offers multiple connectivity options to accommodate various audio sources and devices. It features a standard 3.5mm headphone jack, which can be used to connect to a wide range of headphones and in-ear monitors (IEMs), a 4.4mm balanced output, which packs the real punch, and an additional rear 3.5 AUX output.

In terms of input options, the SK02 offers flexibility with its optical input, allowing users to connect digital audio sources such as computers, gaming consoles, or TVs for high-fidelity sound reproduction. This optical input expands the versatility of the SK02, enabling seamless integration into almost any audio setup.

Additionaly Fosi SK02 offers SPDIF output from the rear 3.5mm optical output, effectively functioning as a 2-in-1 feature alongside the rear 3.5mm AUX output. This dual functionality enhances the connectivity options of the SK02, allowing users to utilize either the optical output for digital audio as an “adapter” or the AUX output.

Overall, the Fosi SK02 impresses with its versatility and connectivity options, making it a versatile audio companion for audiophiles, music enthusiasts, and casual listeners alike. Whether used as a portable headphone amplifier or as part of a home audio setup, the SK02 delivers high-quality sound performance and flexible connectivity.



Controls and User Experience
The SK02 prioritizes user convenience with intuitive controls and clear indicators. The volume knob, despite its lightweight feel, offers precise volume adjustments with numerous levels of volume for more personalized adjustments. Additionally, a single button seamlessly toggles between the low and high gain modes, denoted by LED lights, empowering users to optimize performance based on their headphone/IEM requirements and listening preferences.

Further enhancing versatility, the SK02 conveniently allows you to switch between optical and USB input sources at the press of a dedicated button. This flexibility caters to various connection preferences and seamlessly integrates the SK02 into different setups, be it your computer, mobile phone, or a dedicated media source with optical output or even an optical input.

Additionally, there is a sampling rate indicator, which goes from 44.1kHz to 384kHz, and a DSD indicator from 64 to 256. However, the flashing and brightness-changing volume indicator, while attempting to convey loudness levels, may prove distracting and less informative in practice.

Power
The Fosi SK02’s multiple output options give it the possibilities for various audio setups. It features a 3.5mm single-ended output on the front panel, which delivers 280mW (on high gain) clean and detailed sound suitable for a wide range of headphones and earphones. This output provides ample power to drive most IEMs and headphones efficiently, ensuring a satisfying listening experience without compromising on audio quality.

In addition to the 3.5mm single-ended output, the SK02 also boasts a 4.4mm balanced output on the front panel, which brings 1100mW (on high gain). The balanced output offers several advantages, including improved channel separation, reduced crosstalk, and lower distortion compared to single-ended connections. It delivers a more powerful dynamic sound, which can drive more power-hungry headphones and even planar headphones.

Furthermore, the SK02 features a rear 3.5mm AUX output, providing users with added flexibility in their audio setups. While this rear output delivers less power compared to the front 3.5mm output, it still offers sufficient amplification to drive a wide range of IEMs with ease. Whether connecting to speakers, amplifiers, or other audio devices, the rear 3.5mm output ensures reliable and versatile connectivity.


Sound Performance
The Fosi SK02 delivers a sound signature characterized by a more bright, detailed presentation. showing itself as a revealing performer. At its core lies the ESS Sabre ES9038Q2M DAC chip, known for its clean and crisp delivery. This translates into the SK02’s most defining characteristic: a pronounced mid-treble emphasis.

This emphasis results in shimmering cymbals, airy vocals, and the intricate details of string instruments taking center stage. The SK02 excels at unearthing micro-textures and nuances, inviting listeners to delve deep into the sonic tapestry of their music. But be warned, this focus can become a double-edged sword. Harsh recordings or tracks heavy on high-frequency content might sound overly bright or even piercing to sensitive ears.

Vocals occupy a prominent space, delivered with clarity and precision. Every word and inflection carries weight, offering a transparent window into the artist’s expression. However, this analytical presentation might come at the cost of the warmth and fullness associated with the traditional “musical” midrange. Instruments like guitars and brass instruments may lack the organic body and richness found in warmer amps.

Bass presence remains solid, controlled, and tight, providing a foundation for the rest of the spectrum. It delivers enough punch to satisfy most genres but doesn’t overpower the mids and highs. However, bass-heavy listeners or those seeking a more impactful low-end rumble might find it wanting compared to warmer-sounding competitors like the Truthear Shio.

Overall, the Fosi SK02 caters to analytical listeners who prioritize detail and clarity. It shines with acoustic and classical music, where its ability to unveil intricate details is a strength. However, its bright emphasis might not be for everyone, especially those who prefer a warmer, more forgiving sound or listen primarily to bass-heavy genres.


Conclusion
In conclusion, the Fosi SK02 stands out as a versatile DAC/AMP that transcends its desktop-oriented design to offer mobile users a compact and powerful audio solution with the possibility to run only on a single USB. With its bright and detailed sound signature, sturdy build quality, and diverse connectivity options, the SK02 caters to audiophiles seeking a balance between portability and performance. While minor ergonomic concerns exist, they do little to diminish the overall appeal of the SK02 as a versatile and capable audio-on-the-go companion for both desktop and mobile use. This all-inclusive package is priced at $99, making it a great all-arounder for those who want it to do everything on a limited budget.

Fosi Audio takes a giant leap, with their first all-in-one headphone source, with the SK02 Desktop DAC/Headphone Amp.

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If you don't like reading, here's the video version of this review


Build, Aesthetics & Ergonomics

The first thing to note is the all aluminium alloy chassis.
It shares it's fundamental housing design with it's sibling the SK01.

The main human interface is the single full aluminium alloy Multifunctional Knob.
At the far end of the unit are two buttons; a gain toggle and an input Mode toggle between USB and Optical.
The SK02 has a 3.5mm phone out and a balanced 4.4mm out on the front and on the rear you will find USB input for power and digital connection, Optical input and a 3.5mm Aux out.
And finally on the bottom, two flat rubber feet keeps your furniture scratch free.

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Functionality

Aside from the Gain and Input Mode toggle buttons, the Multifunctional Knob is the main way to control this device.
To turn it on and off, simply hold the knob down for 3 seconds.
Clicking the knob once will Play/Pause your music stream and double-clicking will mute the internal DAC.

The knob is the main digital volume control for the internal DAC.
As such it does not take control of your system volume.
The benefit of this is that taking control and lowering system volume can have a small detrimental effect on the sound quality, so this takes that problem out of the equation.

Also this is much more user-friendly for people who deal with multiple sources like me; there will never be a chance of lowering the system volume and forgetting to push it back up to maximum for your other devices.



For analogue fans the digital knob does come with a learning curve.
There is of course no way to know what volume the unit is set to as the knob will spin infinitely in both directions.
In the past few weeks there were a few moments switching between sources and different headphones when the SK02 was pushing out too much volume, so after a few ear-punishing momments I quickly learned to crank the volume all the way, which is just a matter of spinning it a few times anti-clockwise before plugging in a new headphone or listening to music.

At this price point, a volume display would be a lot to ask for, so I'm on the fence about criticizing this too harshly. Either way, there will be a small learning curve for people who have only used analogue controls previously.

The Gain button toggles between:
• Low Gain
- 85mW @32Ω (3.5mm)
- 331mW @32Ω (4.4mm)
• High Gain
- 267mW @32 Ω (3.5mm)
- 1.062 Watts @32 Ω (4.4mm)

The SK02 is designed to be used with more modern headphones from 16-300Ω, so maybe don't go pluggig in your 50 year old AKG studio headphones.
For most users, this range is more than enough and covers almost all bases.



Both the Gain and Input Mode buttons have annotated orange indicator lights.
There are more orange lights for the PCM and DSD sample rate idicators.

It's nice to see Fosi Audio fully embracing this design langauge, as orange accents are also to be found on their other products. It's a small small thing, but this is the kind of attention to detail that I appreciate.

The only exceptions are the power light which is green and above that, the Volume Indicator light which flashes purple depending on the output.
If this light is constant and not flashing, the volume is set extremely high and caution should be taken before plugging in headphones.



If for example you are using the SK02 as a DAC for another, you will use the 3.5mm AUX output to feed that amplifier, however, the SK02 is not a pure DAC, so the most logical next step for most, will be to set the output volume to maximum for the AUX out.
This is a double edged sword, as this is the only way to get the maximum DAC output to another amplifier, but also does not prevent the user from plugging in a headphone and getting a nasty explosive surprise on the front-end.
So if you are planning on frequently using multiple sources, it's a good idea to pay attention to the volume indicator.
I've made a habit of unplugging headphones after every use and always turning the volume down before listening.

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The Sound

Equipped with an ESS 90382QM chip, the SK02 is can handle PCM decoding up to 32 bit at a sample rate of 384 thousand Hz: this unit is able to push out sample rates way beyond human congnitive comprehension and is functionally at the peak of decoding technology.
At no point did I feel that I was getting anything less than a perfectly neutral, high resolution audio experience.
The other benefit of a highly digital device is that on full volume, there was absolutely zero hissing or humming or electrical interferance.
It has one of the absolute cleanest, blackest backgrounds you will ever hear.
From my point of view, this unit guarantees uncontaminated audio.

I tested this with my very sensitive JVC HA-FX1800 IEMs on full volume out of the 4.4mm balanced output and there wasn't even a hint of a hiss.
I even tested it while playing an audio sample of pure silence, just to make sure I was forcing an output.



While the SK02 was not designed as a pure DAC, I have been using it about 50/50, as a headphone amp and as a DAC, with it plugged into my Fosi Audio SK01 Pre-amp and Headphone Amp, for two reasons:
Firstly, it puts out a perfectly clean signal on maximum AUX output, and behaves as a DAC without producing excess heat or consuming huge amounts of power.
Secondly this combination has been a great source and pre-amp for my Stax SRM-313 electrostatic amplifier.
I find that electrostatic amps are generally more sensitive than most to even the tiniest hints of audio/electrical signal contamination, so this is one of the cheapest and most effective ways I can think of to provide a clean, high-fidelity source and good tone control - which many electrostats do require.

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Conclusions

Overall, this little DAC/amp combo leaves little to be desired.
If anything, I think I might be becoming a bit of a digital control convert and evangelist.

If the competent maximum output of 1 Watt on balanced, or the perfectly black background does not convince you, just remember, this thing is practically portable, takes up very little desk space and only requires the supplied 5 Volt USB connection to run.

The only thing I can see that might be missing for some is a 6.3mm socket.
Otherwise, it's just been a joy to use and listen to music with.