Citied from DavidB
The Fosi SK01 is a headphone amplifier that proffers the user tone and gain control options not seen often in the small portable market segment. It is noted that Fosi audio provided this unit for this evaluation. The SK01 is listed for $79.99 on the Fosi Audio website.
In the box:
The unit is shipped with a USB C to USB C cord that adapts to a USB A to USB C cord. The adopter is attached to the cable with a U-shaped firm plastic piece that offers some security to users like me that tend to lose things. The unit requires a 3.5mm line input from the source device. There is a line out if the user wishes to employ the amplifier as a pre-amplifier. My source was my tablet using a USB C to 3.5mm line out.
The Fosi SK01 contains the battery used to power the device. The charging port is a 5V USB C input. The Fosi website lists battery life as 6 hours.
The top of the unit has 4 knobs. The on/off-volume knob is closest to the headphone connection ports. The standard 3.5mm and 6.5mm headphone outputs are positioned on the lateral surface below the on/off-volume knob. The volume knob has a small light below it that flashes when in use. Moving up the device there are three push-in buttons with knobs next to each button that offer the user the ability to adjust bass, mid-range and treble frequencies.
I listened to several genres of music including classic and hard rock, classical instrumental and symphonic pieces. I used my tablet as the source and DAC. Focal Clear headphones were used for listening. With the tone controls flat, and gain set to -10db about ¾ of the volume was required for a satisfactory listening level. Increasing the gain to 0 will provide the listener with a more than adequate volume for most IEM’s and headphones. The amp provides an impressive amount of separation for a unit of this price range. Bass enthusiasts will be satisfied with the amount of bass that can be elicited by adjusting the bass knob. Users can experiment with the desired midrange and treble for their listening pleasure. BB King and Clapton did not require any frequency adjustment for a satisfactory result.
Fosi provides a power indicator light above the treble control knob. A red color indicates charging and the light is green in use.
The Fosi SK01 is a headphone amplifier only. Uses who require an amplifier with a DAC may want to consider other options. The box does not contain the 3.5mm cord that is required to operate the amplifier.
The user has the option to control the loudness gain and tone. The loudness and gain control options come in very handy in driving some of the more power-hungry headphones. The tone control allows for the desired level of low to high frequencies.
With a price under $100, Fosi has delivered an outstanding amplifier with loudness and tone controls that will allow the user to tailor the listening experience to the desired sound.