Fosi Audio SK01

Sources split, mostly along headphone type. Reviewers using high-impedance headphones and mainstream planars (HD 6XX, HD 650, HE400se, Arya) found plenty of power, often with the bass knob turned up. The bench and some owners found far less than the 2.4 W rating: Audio Science Review got about 0.5 W into 32 ohms before the protection tripped, and a low-impedance Dan Clark Stealth kept cutting out. Owners comparing it with a Douk U3 Mini or a Topping NX7 found less gain and less headroom.

Sources split, mostly one way. With headphones and on low gain, a couple of reviewers heard essentially no hiss. More report audible hiss with sensitive IEMs and some dynamic headphones at higher volumes, along with poor channel matching at the bottom of the volume knob. One owner also heard crackling and a left/right imbalance when a tone button was half-pressed.

Sources split, mostly one way. With the tone controls switched out, nearly every listener heard a neutral amp that adds nothing of its own; one compared the bypassed and amplified signal and heard no difference. The measurement crowd is less impressed: SINAD in the mid-80s, distortion rising in the bass, and 40% of ASR's poll rating it poor or not terrible. Turning the mid knob far up adds audible distortion.

The reason to buy it. Bass, mid and treble knobs with centre detents, a tone-defeat switch, a loudness button, two gain steps, a volume- and tone-controlled line out for powered speakers or a power amp, two headphone jacks and a battery. The criticisms are narrower: the loudness curve is fixed rather than tied to the volume setting, and some find it too mild. The only input is 3.5 mm, there's no DAC, the 'line out' is really a pre-out that stays live with headphones plugged in, and there's no auto power-off.

“This might be the most feature-rich headphone amplifier under $100.”

GREQ · Head-Fi

“it offers an almost unprecedented number of settings for this price range and size”

Djano · Audio Science Review (member review)

“without level dependence of the frequency responses there is no attempt of "loudness control", just the empty label.”

pma · Audio Science Review

Mixed in the details. The tone pots turn with a heavy, satisfying feel, and the top-mounted knobs are easy to reach on a desk. The plastic push buttons feel mushy, and one owner found that a half-pressed tone button crackles. Some knob markers don't line up at zero. Without an auto-off, a unit left on overnight wakes up with a flat battery.

“the pots have a heavy, slow and satisfying feel to them with no backlash or wobble.”

GREQ · Head-Fi

“The buttons are all plastic and the switches feel a bit mushy”

GREQ · Head-Fi

“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.”

amirm · Audio Science Review

Sources split, largely reviewers versus long-term owners. Reviewers handled a solid, heavy aluminium case — "built like a tank" — and Amazon owners rarely fault the build itself. Owners who kept it longer report units that cut in and out or die after 60 days to a year. An ASR member found after two years that once the battery fails, the SK01 won't run even on USB power; a second owner needed the same internal workaround.

At about $80, most sources call it good value for what the knobs do, and Audio Science Review recommended it despite average bench numbers. The counterpoints: owners who moved on to a Topping NX7 found it far cleaner and more powerful, and the reliability reports weigh on long-term value.

“I am going put the Fosi SK-01 headphone amplifier on my recommended list.”

amirm · Audio Science Review

“It's a steal for this low budget with tone controls and loudness”

LTig · Audio Science Review

“The NX7 is a huge stepup in terms of power and cleanliness”

Chibrax · Audio Science Review (owner, after buying a Topping NX7)