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.
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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:
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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:
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Performance is distortion limited as noise performance is quite good:
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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:
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Above is at 1 kHz, distortion does rise notably at lower frequencies (and some at high):
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Here is our frequency response in different modes:
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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:
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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:
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The indicator light blinks and shuts the output down. You can see this better as we sweep with different load impedances:
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Channel matching is poor sadly, giving you another reason to avoid using IEMs:
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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:
| Specifications | Details |
|---|
| Model | SK01 |
| Input | 3.5MM line in |
| Output | 3.5mm Headphone, 6.35mm Headphone, 3.5MM line out |
| Amp output maximum power | High Gain: RMS 2400mw/ 32Ω,Low Gain: RMS 300mw/32Ω |
| Frequency Range | 20Hz-20kHz (±0.5dB) |
| THD+N | 0.001% @ 1kHz and 770mV |
| SNR | ≥108dB |
| Headphone matching impedance | 16-300Ω |
| Device Dimension | 4.76*2*1.69inch (121*51*43mm) |
| Net Weight | 0.641b/290g |
| Power Supply | 5V, USB Type-C |
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