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.

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