Fosi Audio SK01

Fosi Audio SK01

Fosi Audio SK01

Prelude:
Fosi audio is a brand that always puts value for money first. Although the SK01 headphone amplifier was released some time ago, as a proud owner of the SK02 I couldn’t help but review the SK01 model that is compatible with it. Fosi Audio SK01 is a relatively inexpensive device, costing only $98.39, that acts as a headphone amplifier. However, this is not the only function that we can find in this device. Fosi Audio SK01 also has a fully functional hardware equalizer and a built-in loudness option. It is worth noting that the power in high gain mode oscillates around 2400mW per 32Ohm channel. In addition, they took care of a built-in battery that is enough for a quiet 6h of listening and a compact, fully aluminum housing.

Unboxing:
In the classic cardboard packaging for Fosi Audio we receive our amplifier and a USB C-USB C cable with an adapter for classic USB. And a paper user manual. I missed the simplest AUX cable, but in this price range it is not a big problem. The device itself is made really exemplary. The body consists of a uniform aluminum housing, with identical proportions to the one of the SK02. On the back we find a USB C socket for charging, a 3.5mm jack line input and a 3.5mm jack output of the preamplifier. On the front we find a 3.5mm headphone output and a large 6.5mm jack. On the top of the device there is a gain adjustment button, an equalizer switch, a loudness button (built-in equalizer settings) and three knobs for adjusting the bass, midtones and treble. This is a solid and functional set that makes the entire device extremely functional. We can not only use the Fosi Audio SK01 as an amplifier or preamplifier, it will also successfully serve as a hardware equalizer.

Sound:
For my test I used a dedicated Fosi Audio SK02 DAC/AMP and an aux cable made of pure oxygen-free copper.


Bass: The low tones in the Fosi Audio SK01 in the default setting are quite neutral and very smooth. They work great with practically every music genre. They do not lack impact or saturation, but instead of excessive impact we get a clean and clear sound. However, after turning on the hardware equalizer we can quickly completely change the amount of low tones, to the point that the Fosi Audio SK01 can easily become a bass monster with huge saturation and a certain impact in the low tones. The loudness function, on the other hand, greatly colors the sound and makes the bass stand out quite strongly above the rest of the bands. At the same time, in no mode did I have the feeling of muddiness or excessive degradation of the sound.
Midtones: The sound midrange sounded clean, elegant and dignified. In the default mode without the equalizer, the sound stage, although quite intimate, was natural. And the sound itself was presented in a quite neutral and, most importantly, clean way. Here I would like to note that when using ultra-sensitive IEM headphones, noise will be audible, which is why I recommend the Fosi Audio SK01 in particular for use with less sensitive headphones. My Thinksound OV21 did not exhibit any noise. At the same time, such as the iBasso AM05 clearly caught noise. It was not noise that made it impossible to listen, but simply occurred. It is no wonder that with the amount of power that the Fosi Audio SK01 has, this is absolutely normal. Using the equalizer, we can decide whether the midrange should be in the foreground, or whether we want to slightly withdraw it and strengthen other bands.
High tones: Importantly, they were not sharp or unpleasant. In the default configuration, they sounded clean and smooth. At the same time, offering a lot of air and a detailed, open sound. Here, once again, the neutral and natural character of the amplifier itself comes to the fore. I think that Fosi Audio made an excellent decision to tune the SK01 in a way that is close to neutral and add a hardware equalizer that allows users to tune the sound to their own preferences. This gives great flexibility and allows you to tune your amplifier to your own headphones.
Comparisons:

Fosi Audio SK01 (98.39 USD) VS xDuoo xp2pro (149 USD)
Fosi audio SK01 is primarily a less compact and less portable device, although both devices have a built-in battery, the xp2po is additionally equipped with Bluetooth. In terms of power, the xp2pro is definitely inferior to the Fosi Audio SK01. When it comes to sound, the neutrality and additional equalizer functions in the SK01 are a strong advantage. However, considering the built-in DAC and the very musical and transparent sound offered by the xp2pro, I put both devices close to each other. Which one we like more depends largely on our individual preferences.
Fosi Audio SK01 (98.39 USD) VS xDuoo TA01B (169SD)
Here I will immediately point out that I will treat the TA01B only in the context of a hybrid amplifier and connect it for the purposes of comparison to SK02 to have a clear and transparent picture of how the SK01, which is an amplifier, compares with the TA01b as an amplifier. What I noticed first of all is the gain control present in the SK01, which is what I miss the most in my TA01B. In terms of the sound itself, primarily due to the presence of a tube pre-amplifier in the TA01B we have a much warmer and even more intimate sound. In all my comparisons I used a variety of music from Tidal and headphones such as HifiMn arya organic, Thinksound OV21, Thinksound IN20 or the very sensitive AM05 in-ear headphones from iBasso. While the AM05 could be listened to without any problems with the SK01 despite the noise, it is no longer possible with the TA01B. Therefore, depending on what headphones you have and what you plan to use with the amplifier, it depends on which one you like more.
Fosi Audio SK01 (98.39 USD) VS Fosi Audio SK02 (120 USD)
Comparing both devices, I can point out one thing. They are designed to work in a duet and blend together perfectly. Although the SK02 offers a balanced output, it has much less power than the SK01, which may be important in the case of more demanding headphones. In the case of very sensitive headphones, the signal in the SK02 is much cleaner, which translates into less noise. However, I highly recommend using both devices in a duet. Thanks to this, we get a fully functional amplifier and headphone DAC.

Summary:
The Fosi Audio SK01 is a really compact and impressive headphone amplifier. It offers enormous power and a very small size. By default, it is a neutral and naturally sounding amplifier. However, we can quickly and very easily change this using the equalizer settings and practically freely tune our amplifier to any preferences. Adding to this the preamplifier output and a large amount of power, we get an excellent and inexpensive fully analog amplifier. The SK02 already made a huge impression on me, and in combination with the SK01, we get excellent synergy and sound quality at a really high level. All this makes the SK01 an amplifier that I highly recommend for people with demanding headphones who do not want to spend a lot of money on the amplifier itself. Here we have a completely analog circuit and a classic operational amplifier design.

Video version of this review

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Build, Design and Aesthetics


At a glance, it's obvious that this little unit means business.
The SK-01's shell is made out of an aluminium alloy that is perfectly finished aside from a slightly sharper under edge of the top-plate overhang, but fortunately it's not sharp enough to cut.

I'm not big fan of this design choice; it doesn't add anything for me from an aesthetic point of view, but does make it much easier to pick up and move around the desk, which is more important for a portable unit such as this.

There are two wide rubber feet on the bottom which is always a welcome addition, and all audio ports are tastefully finished with a gold coloured metal ring.

The knobs are made of the same aluminium and have an identical finish as the rest of the body, and the pots have a heavy, slow and satisfying feel to them with no backlash or wobble.
Unfortunately for some, the knob markers do not align perfectly in the 'zero' position.



The buttons are all plastic and the switches feel a bit mushy and need to be pushed in completely before the mechanical switch is activated.

I would have personally hoped that these switches were easier to adjust, but if you throw this in your backpack, it would also be annoying if the switches were activated during transport, changing your settings, so there's an argument to be made both ways.

On the back is a Type-C USB power and charging port, 3.5mm line-in and line-out; although this has been mislabelled. This is technically not a line-out but the pre-out. All tone and volume adjustments will be applied through this "line-out".

Finally on the front are two headphone jacks, 3.5mm and 6.35mm.

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Functionality

This might be the most feature-rich headphone amplifier under $100.

It has treble, midrange and bass tone controls, a tone-defeat switch, a loudness switch, a gain switch for toggling between using headphones and more sensitive IEMs, and an internal battery with an advertised rating of 2500mAh or up to 6 hours continuous use.

Let's start with the main event: Tone Controls.
If you've read my previous review on the Fosi K5 Pro, you'll know that I got very excited to finally have an amp with good tone controls, however the unit suffers a few drawbacks including a lack of midrange tone control, somewhat limited range of the tone control’s frequency spread and a slightly limited maximum adjustment range.

The SK01 takes care of all those issues.

Bass control gently bleeds into the lower-mids up to 800hz which is actually better and more balanced than the K5 Pro which reaches up to about 350hz, with a maximum adjustment of 12dB.

The treble adjustment is the weakest of the three, but also provides a solid 9dB adjustment in either direction.


The peak of the 10dB midrange tone control is focused squarely around 1000hz, which is a fair place to add this control.
There are very few headphones which feel weak or strong exactly in this area, but sometimes it's the sum of the tone control adjustments that work in combination that make a good correction.
It's also absolutely worth mentioning that it also gently bleeds all the way up into the treble, and fizzles out around 40hz, which is pretty much all you could ever ask for to ensure a smooth transition between all tone adjustments.


The Gain switch toggles between a high gain mode with a very powerful output of 2400mW at 32 ohm, and low gain mode of 300mW.
I must say; I'm not aware of any other headphone amplifier under $100 with such a powerful output.
It is powerful enough to comfortably drive almost any headphone I own, including large planars, but don't expect to drive hungry vintage headphones or your Hifiman HE-6.

The Tone Defeat switch toggles tone controls on and off, but has no effect on the Loudness Button.
If loudness is on, the tone defeat will not bypass it.

The Loudness button is a very welcome feature on any pre-amp.
Anyone who's ever dabbled with vintage speaker amplifiers and receivers will be familiar with this button.
In a nutshell it gives an extra boost of bass and treble by approximately 30% with exactly the same tone control that is provided the knobs.
Normally there is no reason to use this for headphones, but it works for headphones just as well as it does for loudspeakers.
The purpose of the loudness button is for more 'normal' low-volume listening.
The reason being that on lower volumes, our hearing perception of very low and high frequencies is reduced, so if for example you're having guests over and want to add some quiet background music, just hit the loudness button and the music will immediately sound bigger, fuller and more detailed.
This also works for very low volume headphone listening.

Both the 3.5mm and 6.35mm ports can be used simultaneously which can be a useful for feature if you need to share one source with another; for example cooperative music production or quietly watching a movie with someone.
I wouldn't recommend it for making headphone comparisons, as you cannot adjust the volume of each individual port, so different headphones with different sensitivities will have different volumes.

The Line-Out is actually a Pre-Out

So this is one feature that I feel is good, but is limited or rather could have been expanded on.
I've been using the so-called Line-Out (Pre-Out) to my speaker amp, but the problem I faced is that I have a 2.1 speaker system with a powered sub.

Using the Pre-Out from the speaker amp to feed the sub is just below my preferred volume on the subwoofer, despite the sub being on full volume.
What my subwoofer needs is a separate, strong and proper Line-Out signal, but the only thing I get from the SK01 is the weaker pre-out with it's volume or tone corrections applied.

My only option for optimization is to look for a different DAC or another Pre-Amp with an extra undiluted Line-Out on top of the Pre-Out.

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Sound

As amps go, there is no audible or measureable affect on the headphones I tested.
So in that regard it is linear and imparts no particular tonal flavour of it's own on the sound, but I get the feeling that there is a slight lack of control over the headphone diaphragm.

Compared with my audioGD NFB11.28 TXCO I get the feeling that some bass notes and String Plucks on the double bass, for example, might not have as much texture as other amps, which gives the slight impression of a lack of dynamics, but it's so subtle it might as well be placebo.

So, what's the catch?
In such a feature rich, powerful and well designed device, there has to be something wrong with it at this price, right?
In all my testing, I found only one stand-out issue: with more sensitive IEMs and dynamic headphones, there is audible background hissing on higher volumes.
However, in all my normal testing and listening, this hissing was never present or audible, but it is clearly there when the volume is cranked up while no audio is being played.

So the catch is that you are not getting a perfectly black-background.
However, with my large planars and other insensitive and higher ohm headphones, like the Sennheiser HD650 and Hifiman HE-500, there was practically no audible hissing at any volume.

On Low Gain mode, the problem is reduced but not entirely mitigated.

So for audio production I would not recommend using the SK01 with sensitive headphones like the Sony MDR-CD900ST or the JVC HA-MX100-Z.
Better if you lean more towards a Beyerdynamic DT880 600 ohm or AKG K612 Pro.

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Conclusions

For anyone starting out with headphones or audio production, this is almost a complete no-brainer.
Just couple it with any stereo DAC and you're in business.
Practically whatever modern headphone you throw at it, will be driven properly with the added flexibility of tone control.

It's a unit that I'm more than comfortable adding to my existing system, and can easily recommend beginners building their system around it too.
Obviously purists will find the SK01 a huge turn off.
The contamination to the audio signal caused by tone corrections and potential background hissing on higher volumes is a purist-audiophiles nightmare.

But for anyone looking for a bit of extra spice, this has got ‘the flavour’.

The Fosi SK01 comes in the typical fosi audio box inside one finds a USB-C cable for charging, yes even though this looks like it would be one of those power brick plug in amplifiers, it is in fact internal battery powered and while its small its definitely not a portable. It has an all-aluminum design with both dial and quickset buttons for EQ , and The SK01 buttons that control features such as loudness, tone, and gain. as well. It also has a 2500mA battery with 6 hours of playback time.

Here is a basic assessment of the sound as you can change it so drastically.
As is out of the box the SK01 present with a just north of neutral sound and more toward balanced, it has a nice semitransparent sound with some nice details and separation. It mirrors the source superbly. Using the different buttons one can get to totally neutral or a very rich dynamic sound. It ran my most demanding headphones with good loudness. To be fair I tested it with my SMSL m100 a neutral sounding simple DAC. I really wish Fosi would make something like this as an all-in-one unit no need for a separate DAC, honestly that would be great. There is clarity and power here, I wouldn't recommend very sensitive all BA and hybrid IEM but most single dynamic and planar work well with the SK01. As far as headphones, some of my duller ones become very exciting with just the press of a button. The EQ is well implemented and the BASS, MIDS, AND TREBLE is precise in control.


Afterthought:
The Fosio audio SK01 is one of those rare hidden gems that offers good sound, lots of features and build quality for less than expected. It's built like a tank and is highly customizable sound wise. Truely a cool device.

Specifications​