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Fosi Audio SK-01 Headphone Amplifier Review

Rate this headphone amplifier:

  • 1. Poor (headless panther)

    Votes: 7 4.7%
  • 2. Not terrible (postman panther)

    Votes: 58 38.9%
  • 3. Fine (happy panther)

    Votes: 73 49.0%
  • 4. Great (golfing panther)

    Votes: 11 7.4%

  • Total voters
    149

amirm

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This is a review, listening test and detailed measurements of the Fosi Audio SK-01 Headphone amp and preamplifier. It is was sent to me by the company and costs US $80.
Fosi SK-01 Headphone Amplifier with tone control loudness control review.jpg

You can quickly see what distinguishes the SK-01 from myriad of other headphone amps: inclusion of tone control and loudness! Such features were banished from hi-fi gear a few decades ago with the idea that an amplifier would be compromised with them. Never mind that you can and did have bypass switches.

The metal case is solid and rather heavy for its size due to inclusion of batteries. Power for charging is supplied through standard USB-C connector:
Fosi SK-01 Headphone Amplifier with tone control loudness control preamplifier review.jpg

The Line out seems to just be connected to the headphone out but it is nice to have it in case you want to add tone controls to your stereo system.

Back to the battery, I wish there was a time out to shut the unit down. I left it on last night and it was dead in the morning. Fortunately once you connected the USB charger, it seems to be fully operational even with the dead battery. The battery by the way usually provides lower impedance than the charge port so it can improve performance of a headphone amp even if you don't need it for portability.

Overall, this is a solid offering, pun intended both from look and feel and functionality.

Fosi Audio SK-01 Measurements
Using our standard dashboard of unity gain (2 volts in/out) in low gain we get OK performance:
Fosi SK-01 Headphone Amplifier with tone control active measurement.png


Performance is distortion limited as noise performance is quite good:
Fosi SK-01 Headphone Amplifier with tone control SNR measurement.png


Even when turning on the tone controls, you have good headroom with respect to playing back 16 bit content. On the other hand, best to not use ultra sensitive IEMs as the noise performance there is about average:
Fosi SK-01 Headphone Amplifier with tone control SNR 50 mv measurement.png


Best portable headphone amplifier tone control review.png


Above is at 1 kHz, distortion does rise notably at lower frequencies (and some at high):
Fosi SK-01 Headphone Amplifier with tone control THD vs frequency measurement.png


Here is our frequency response in different modes:
Fosi SK-01 Headphone Amplifier with tone control Frequency Response measurement.png

You can see the effect of loudness control at the set volume point in boosting mostly the bass.

Let's see how much power we have:
Fosi SK-01 Headphone Amplifier with tone control Power 300 Ohm measurement.png

I like to see 100 milliwatts here and the SK-01 easily achieves that with 140 milliwatts so it should be good to drive high impedance headphones. Low impedance though is more limiting:
Fosi SK-01 Headphone Amplifier with tone control Power 32 Ohm measurement.png


The indicator light blinks and shuts the output down. You can see this better as we sweep with different load impedances:
Fosi SK-01 Headphone Amplifier with tone control Power vs impedance measurement.png


Channel matching is poor sadly, giving you another reason to avoid using IEMs:
Fosi SK-01 Headphone Amplifier with tone control channel balance measurement.png


Fosi Audio SK-01 Headphone Listening Tests
As usual, I started with my usual Dan Clark Audio Stealth headphone. This is a low impedance headphone that is difficult to drive. That instantly became a problem as the SK-01 struggled to even get to modest volume, constantly cutting in and out if I cranked the volume up at all.

Switching to Sennheiser HD-650 with its high impedance completely changed the picture. There was plenty of power to drive them to as loud as you wanted. Better yet, I was now able to turn up the bass control and compensate with lack of that in HD-650. There was now super enjoyable bass that survived my cranking up the volume to almost extreme levels! It put such a smile on my face. :)

I also tested the Loudness feature. It worked just like what I remembered from my system back in 1970s. You mostly notice the bass boost. At first you might think it is too much but then you turn off the control and the sound becomes so lackluster, at least on HD-650. Definitely a fun feature.

Conclusions
If this were an ordinary headphone amp, the objective performance would indicate a pass. But add tone controls without much noise penalty and that loudness control and you subjectively have a superior product to headphone amps with far lower noise and distortion. Many headphones need equalization to sound good and while having three pots won't give you the precision you want, you can still fix gross issues as I did with fixing the bass performance with HD-650. And since these are analog controls, you can quickly play with them unlike changing parameters in an EQ system which may glitch, have delay, etc.

I am going put the Fosi SK-01 headphone amplifier on my recommended list. I hope the company produces a higher priced one with better performance and that, would be a fantastic product to have.

Manufacturer Specifications:​

SpecificationsDetails
ModelSK01
Input3.5MM line in
Output3.5mm Headphone, 6.35mm Headphone, 3.5MM line out
Amp output maximum powerHigh Gain: RMS 2400mw/ 32Ω,Low Gain: RMS 300mw/32Ω
Frequency Range20Hz-20kHz (±0.5dB)
THD+N0.001% @ 1kHz and 770mV
SNR≥108dB
Headphone matching impedance16-300Ω
Device Dimension4.76*2*1.69inch (121*51*43mm)
Net Weight0.641b/290g
Power Supply5V, USB Type-C

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Bruce Morgen

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Any chance you could measure its performance as a preamp -- IOW, its output at the "LINE OUT" jack rather than one of the two headphone jacks?
 
OP
amirm

amirm

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Any chance you could measure its performance as a preamp -- IOW, its output at the "LINE OUT" jack rather than one of the two headphone jacks?
I did and noted it is identical to headphone out.
 

Bruce Morgen

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I did and noted it is identical to headphone out.

My bad -- thanks. My headphones are very easy to drive and pretty good without EQ, so my main interest is in adding tone controls and the loudness button so I can adjust my system for recordings with good music but (IMO) less than optimal tonal balance by putting this gadget in the tape monitor loop of my old Yamaha DSR-100 Pro so it can be completely bypassed when not needed -- which is most of the time. BTW, I was going to buy one and send it to you -- glad to see it so thoroughly reviewed without my help. :cool:
 

DanielT

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Thanks for the test Amir. :) Nice features and ok performance for $80.

Loundness functionality I think is a great feature to have. At least if you occasionally listen at low volume. It would have been even better if the Fosi Audio SK-01 had variable loudness, but that's perhaps too much to ask for the $80 it costs.

Loudness or not is discussed in this interesting thread::)


I think that Amir also created some thread about the Fletcher Munson Curve, but I couldn't find it now.
 

Hipster Doofus

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I posted this on another Fosi product, at least Amir you did not quote me directly ;)


GET ME STARTED ON TONE CONTROLS

OK...i am an audiophile by osmosis only, with no real knowledge of electronics. But I have met a few who do.
My good friend was in business with Dick Olsher from Stereophile and another Stereophile reviewer lives up the street. I have been in on testing Ongaku amplifier clones. and on and on.

I have seen people, including myself spend hours and hours and thousands of dollars all trying to discern minute differences, in test that took way too long to even remember what the first tone sounded like.

Things like... (all quite fun by the way)

upgrade capacitors in amps and speakers
roll tubes!
change cables.
change cartridges.
mix and match so much equipment it would make your head spin.

And every once in a while, I would say in my wise-ass way, if you want a different sound why don't you use the tone controls.

oh yeah ... audiophile equipment does not have tone controls, I would hear.

SO, 3 CHEERS FOR TONE CONTROLS
 

Robbo99999

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My thoughts on this headphone amp - it's good value & useful functionality with the tone controls, (but just how useful are they?). Tone controls would be particularly useful for headphones that don't require any sophisticated parametric EQ - as Amir said with his HD650 just putting in a bit more bass elevates that headphone. I suppose if you've got a headphone that really requires parametric EQ then basic Tone Controls are not all that useful as you'll already be running parametric EQ to sort out the headphone. There is the argument that you need Tone Controls for different music based on "Circle of Confusion" and some music being recorded too bright or too dull, but I've found with the right headphone EQ then it's not necessary to fine tune between different tracks, so I'm not really a proponent of "Tone Controls necessary for Circle of Confusion in music". I think ultimately a simple portable dongle combined with parametric EQ on your phone is probably the best way to go rather than this semi-bulky portable headphone amp, but this amp will give you more power if you need it. Good product for the money, but how many will need it or want it vs just a simple inexpensive & highly portable apple dongle, I'm guessing not many.
 

LegionOfHell

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what is the output impedance for hp out ?
 

Ericgeers

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The design looks familiar. In Europe, there's a similar-looking amp by a company called InLine and in the US, something like it is sold by a company called Syba.
For a much higher price on Amazon that one is around € 120,-. There is one without the battery and one with.. So check that before you buy. They look the same and cost almost the same.
 
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Aperiodic

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I would be interested in a desktop amp (even better, a combo) with some of these features, especially the tone control. I actually saw one once but forget what it was called. It would be useful to compensate for things like differences between the mastering engineer's monitor system (and taste) and the listener's. It doesn't seem as useful in a portable, where I don't listen as 'critically' as I do at home. The form factor seems awkward for a portable. Anything with SINAD falling short of Redbook spec tops out at 'Not Terrible' for me. It seems compromises were made to get the feature set they wanted at the target price. I think a slightly higher price with better performance might be more attractive to the audience they seem to be aiming at. But the form factor, while imaginative, is kind of a non-starter for me.
 

Joe Smith

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I remain hopeful that Fosi will make a "pure" stereo class D amp (similar in power to a Schiit Vidar) and a matching preamp with many inputs and a remote (like the Schiit Saga S). And all for about $300 - 350.

This is a pretty nice unit, but being designed as a headphone amp, I don't have much use for it. Unusual form factor too, good for fitting into a desktop I guess.
 

Kevinfc

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Sorry, I’m no doubt less sophisticated, and certainly less knowledgeable than most of you, but I’m at a loss when trying to figure out how this product is useful.
 

staticV3

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@amirm
This graph says Max voltage 6.5V
Fosi SK-01 Headphone Amplifier with tone control Power 300 Ohm measurement.png

This one says 18.3V??
Fosi SK-01 Headphone Amplifier with tone control Power vs impedance measurement.png

This one says 4V @32Ω:
Fosi SK-01 Headphone Amplifier with tone control Power 32 Ohm measurement.png

And this one only 3V @32Ω??
Fosi SK-01 Headphone Amplifier with tone control Power vs impedance measurement.png

Some time ago I posted graphs like these:
Topping AIO_ Maximum Output Power before clipping (knee) vs Load (2).png
and you asked me to post them regularly:
Wow, that is super nice and useful! Would be great to post these graphs in every review if you have time.

I need reliable data to create them. Please work on making your data more reliable.
 

pablolie

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given the position of the headphone out and line/in-out/USB ports, the orientation of the button descriptors seems very counter intuitive. so this has to be laid on its side to get to all inputs/outputs...?
 
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