Fosi Audio SK01

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​