Fosi Audio SK01 Review: A Tiny Headphone Amp That Doubles as a Three-Band EQ

Introduction

The Fosi Audio SK01 is a small desktop audio box that sells for around $72 on sale, with a regular price that sits near $75 to $80. It is marketed as a headphone amplifier, and it is one, but that description undersells what the unit actually does.

On the top plate you get four controls stacked in a column: treble, mid-range, bass, and volume. Along the front there is a 3.5 mm headphone jack and a full-size 6.35 mm headphone jack, both unbalanced single-ended. Around the side you find a USB-C 5V power input, a 3.5 mm line in, and a 3.5 mm line out. Three buttons cover loudness, tone, and gain.

That combination makes the SK01 more than a plain headphone amp. It is a compact three-band equalizer with loudness compensation and a headphone stage built into the same chassis. The box is small enough to disappear on a desk, and the knobs have the cool, solid feel of aluminum.

Who This Product Is For

The clearest case for the SK01 is an affordable two-channel system that already sounds good but a little thin at the bottom. Compact Class D amplifiers in the $75 to $80 range, such as the Fosi Audio V3, are clean and detailed, but the low end can lack punch. The SK01 slots in ahead of that amplifier and adds the bass, mid-range, and treble shaping the amplifier does not have.

The same logic applies to affordable small speakers, including models like the Micca RB42 or the Neumi BS5. Any budget speaker that leaves you wanting a little more low-end weight, or a little less brightness, is a candidate for tone controls.

Headphone listeners are the other obvious audience. With 2400 mW into 32 ohm loads, the SK01 has enough output to drive harder loads such as the HiFiMan Edition XS to loud levels. It also has a gain button, so sensitive in-ear monitors can be tamed by reducing the signal.

Because the unit runs on its own internal battery as well as USB-C power, it travels well. It takes up very little room in a briefcase, which makes it a plausible travel headphone amplifier as well as a desk fixture.

Main Strengths

Tone Controls and the Loudness Button

The three-band tone stack is the headline. Treble, mid-range, and bass each get their own knob, with volume sitting below them. Mid-range control in particular is uncommon at this price, and it is the band that decides whether a small speaker sounds forward or relaxed.

The loudness button is the feature that earns the most affection. Loudness compensation boosts bass and treble at lower listening volumes, which makes a small amplifier and a small speaker feel like they have more headroom than they really do. Better still, the tone controls stay active while loudness is engaged, so you can fine-tune the result instead of choosing one or the other.

It is a small change on paper, but in practice it is the difference between a system that sounds thin at low volume and one that stays full and involving.

Headphone Amplification and Build Quality

The headphone stage is genuinely capable. With 2400 mW into 32 ohms, it drove the HiFiMan Edition XS without strain. On a bass-heavy track that causes some dongle DACs to cut out, the SK01 kept going.

Both a 3.5 mm and a full-size 6.35 mm jack sit on the front, so most headphones connect without an adapter. Outputs are unbalanced single-ended, which keeps things simple for the kind of gear this product is aimed at.

Build quality is better than the price suggests. The enclosure feels substantial, and the knobs feel like machined aluminum rather than plastic. The internal battery means the unit does not have to be tethered to a charger all the time.

Things to Consider Before Buying

The SK01 is not a dongle DAC. It has no digital input of its own, so it needs a source, and the quality of that source will shape how good the whole chain sounds. A small desktop DAC or a dongle DAC feeding the 3.5 mm line input is the usual approach.

Cabling is another practical detail. In a two-channel setup you need two RCA to 3.5 mm cables: one to carry the source signal into the line input, and one to carry the line output into your amplifier. Buying both at the same time avoids a second order later.

Because the outputs are unbalanced single-ended, listeners who need balanced connections will have to look elsewhere. The battery also means charging becomes part of the routine if you use it away from a USB outlet.

None of these are dealbreakers, but they are worth planning for before the unit arrives.

System Matching and Cables

Think of the SK01 as the control layer in a chain of inexpensive parts. A compact source such as a small desktop DAC feeds the 3.5 mm line input. The 3.5 mm line output then runs to the amplifier through an RCA to 3.5 mm cable, and the amplifier drives the speakers.

That arrangement works with the affordable Class D amplifiers and small speakers this product is clearly designed around. If you already own a Fosi Audio V3, an Aiyima A7, or a comparable budget amplifier, the SK01 drops in without replacing anything.

The practical benefit is that you can build the system in stages. The amplifier and speakers can come first, and the SK01 can be added later, when you want more control over how the system sounds.

Buying Advice

For anyone running a very affordable amplifier that lacks low-end weight, the SK01 is an efficient purchase. It costs less than a standalone equalizer, adds mid-range and treble shaping as well as bass, and includes a headphone amplifier in the same box.

Headphone users who already own a good source will find it useful too, though the tone controls are likely to matter less in that setup. The loudness button still helps at quiet listening levels, and the gain control is a real convenience with sensitive in-ears.

The main thing to budget for beyond the unit itself is cables, and the main thing to consider is what will feed it. Match the SK01 with a source you already like, and it becomes a flexible control point rather than a compromise.

If portability matters, the internal battery and small footprint make it easy to pack alongside a laptop and a pair of headphones. It is a small addition to a bag and a meaningful addition to an inexpensive system.

Conclusion

The Fosi Audio SK01 is worth its asking price for the tone controls and loudness feature alone, and the headphone amplifier bundled with them strengthens the value further. It is marketed as a headphone amplifier, but its most convincing role is in an affordable two-channel system that needs a little more bass weight and a little more control. Pair it with a source you trust, add the right cables, and it quietly makes a budget setup sound more expensive than it is.