This is a review, listening tests, and detailed measurements of Fosi Audio SK02 USB powered DAC and balanced headphone amplifier. It was sent to me by the company and costs US $99.99 (€99.00).
The SK02 is excellently designed. Having it horizontal gives comfortable access to the rotary volume control. I was going to complain about it not showing the current level but then noticed an LED which blinks with music signal and becomes brighter at higher volumes. Brilliant! The rest of the LEDs provide the exact kind of information you want to know about the status of the device and the format of the incoming signal. Again, wonderful. The gain button is right there to nicely control that setting. And mode selects between Toslink optical and USB-C inputs:
This is as ideal of a design as one can imagine for a compact dac and headphone amplifier.
Fosi Audio SK02 DAC Measurements
I started with balanced HP output, setting it to 4 volts using the volume control (high gain):
This nicely lands the SK02 in the middle of our "competent" rating of all DACs ever tested:
Unbalanced 3.5mm turns in similar performance:
Company spec is 106 dB which is essentially what I am measuring.
Noise performance is excellent for class:
Jitter test shows some interference but they are below audibility:
Linearity is near perfect:
IMD test shows the class sign of "ESS DAC IMD hump:"
I put most of the blame for this on ESS for not issuing a bulletin on how to fix this, leaving every company to have to fix it themselves.
The filter has good response in audible band but not enough attenuation in ultrasonics:
This naturally hurts the wideband THD+N measurement:
If your player can upsample to higher frequencies, you may want to use it (set it to 192 kHz).
Multitone performance is again excellent for class:
Even the worst case intermodulation is at or below threshold of audibility.
Fosi Audio SK02 Headphone Amplifier Measurements
We have surprising amount of power for a USB powered device:
Note that the gain switch simply rescales the volume control and hence, won't get as loud. It therefore has no effect on actual performance. So I focused on high gain for the next tests:
We see that a lot more power is delivered with high gain. And that unbalanced has lower noise floor so use that for IEMs which don't need a lot of power anyway.
Sweeping impedances we see that the ultimate limit of USB power kicks in:
Unfortunately output impedance is non-zero so be mindful of that with headphones that have variable impedance vs frequency.
Fosi Audio SK02 Listening Tests
I started my testing with Dan Clark E3 which is low impedance and difficult to drive. I was pleased that the SK02 could drive it to reasonable levels with very nice fidelity using balanced output in high gain. Cranking it all the way up did cause it to shot down however.
I switched to Sennheiser HD-650 which in high impedance could be pushed to any level you wanted. The sound was beautiful with all the detail, bass impact, etc. that you would (almost) want. I also tested it with unbalanced output and while max volume was lower, it was still enough for everyday use.
Conclusions
Fosi innovates in user interface in this form factor producing best in class functionality and usability. Objective performance ranges from excellent to areas that need addressing. Subjective testing results were quite positive for this class. It is not everyday that I get excited about "yet another dac and amp" but I am that way about SK02. Given how responsive company is to user input, I am hopeful that the next revision will iron out the weaker points and produce even better performance.
I am going to recommend the Fosi Audio S02 based on its practical performance and usability at very reasonable cost.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
The SK02 is excellently designed. Having it horizontal gives comfortable access to the rotary volume control. I was going to complain about it not showing the current level but then noticed an LED which blinks with music signal and becomes brighter at higher volumes. Brilliant! The rest of the LEDs provide the exact kind of information you want to know about the status of the device and the format of the incoming signal. Again, wonderful. The gain button is right there to nicely control that setting. And mode selects between Toslink optical and USB-C inputs:
This is as ideal of a design as one can imagine for a compact dac and headphone amplifier.
Fosi Audio SK02 DAC Measurements
I started with balanced HP output, setting it to 4 volts using the volume control (high gain):
This nicely lands the SK02 in the middle of our "competent" rating of all DACs ever tested:
Unbalanced 3.5mm turns in similar performance:
Company spec is 106 dB which is essentially what I am measuring.
Noise performance is excellent for class:
Jitter test shows some interference but they are below audibility:
Linearity is near perfect:
IMD test shows the class sign of "ESS DAC IMD hump:"
I put most of the blame for this on ESS for not issuing a bulletin on how to fix this, leaving every company to have to fix it themselves.
The filter has good response in audible band but not enough attenuation in ultrasonics:
This naturally hurts the wideband THD+N measurement:
If your player can upsample to higher frequencies, you may want to use it (set it to 192 kHz).
Multitone performance is again excellent for class:
Even the worst case intermodulation is at or below threshold of audibility.
Fosi Audio SK02 Headphone Amplifier Measurements
We have surprising amount of power for a USB powered device:
Note that the gain switch simply rescales the volume control and hence, won't get as loud. It therefore has no effect on actual performance. So I focused on high gain for the next tests:
We see that a lot more power is delivered with high gain. And that unbalanced has lower noise floor so use that for IEMs which don't need a lot of power anyway.
Sweeping impedances we see that the ultimate limit of USB power kicks in:
Unfortunately output impedance is non-zero so be mindful of that with headphones that have variable impedance vs frequency.
Fosi Audio SK02 Listening Tests
I started my testing with Dan Clark E3 which is low impedance and difficult to drive. I was pleased that the SK02 could drive it to reasonable levels with very nice fidelity using balanced output in high gain. Cranking it all the way up did cause it to shot down however.
I switched to Sennheiser HD-650 which in high impedance could be pushed to any level you wanted. The sound was beautiful with all the detail, bass impact, etc. that you would (almost) want. I also tested it with unbalanced output and while max volume was lower, it was still enough for everyday use.
Conclusions
Fosi innovates in user interface in this form factor producing best in class functionality and usability. Objective performance ranges from excellent to areas that need addressing. Subjective testing results were quite positive for this class. It is not everyday that I get excited about "yet another dac and amp" but I am that way about SK02. Given how responsive company is to user input, I am hopeful that the next revision will iron out the weaker points and produce even better performance.
I am going to recommend the Fosi Audio S02 based on its practical performance and usability at very reasonable cost.
------------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
