The Fosi Audio SK01 review sample was kindly provided free of charge. I didn’t receive monetary or any other kind of compensation and I don’t use affiliate links. The price of the Fosi Audio SK01 is $79.99 and can be ordered from Fosi Audio.
Fosi Audio
Established in 2017, Fosi Audio was created by a group of engineers and music enthusiasts with the aim of providing top-notch audio products. Through extensive research and development, they have successfully perfected the art of compacting HiFi sound, making it available in any environment, regardless of size. Fosi Audio offers a diverse range of products, including speakers, amplifiers, headphones, turntables, and other high-end audio equipment. Their ultimate objective is to make high-quality sound accessible to all individuals and contribute to the progress of music technology. This is my second encounter with the brand after reviewing the Fosi Audio DS1, a USB DAC dongle with excellent price to performance ratio.
Do you like to equalise?
Have you ever found yourself longing for a simple way to adjust your headphones and enhance their sound to better suit your personal preferences? While equalizing is a breeze when using a computer as your audio source, it becomes more challenging with analog sources such as CD players or other devices that lack EQ support.
Fortunately, there is a solution: utilizing an analog EQ with line level input and output. Although these devices are not as abundant in today’s market, there are still a few options available to meet your needs.
Fosi Audio SK01 Review
The Fosi Audio SK01 is an affordable, portable headphone amplifier that runs on batteries. It offers three tone controls to adjust the bass, mids, and treble, making it a versatile option for music enthusiasts.
In addition, the SK01 features a 3.5mm preamplifier output, which allows you to equalize different audio devices such as powered monitors, instrument amplifiers, or power amplifiers.
Appearance
The SK01 boasts a chassis with a sleek rectangular shape, constructed from durable thick aluminum. Its three rotary knobs are also crafted from the same high-quality aluminum material. This unit is meticulously crafted, exhibiting exceptional sturdiness and featuring a sleek black finish. With dimensions of 121x51x43mm and weighing 290g, it is not that suitable for portable use.
Operation
The upper section of the chassis contains the trio of rotary tone controls and the master volume control, all arranged in a vertical manner from top to bottom. These tone controls possess a central point position and offer a smooth rotating action with just the right amount of friction, allowing for precise adjustments. Similarly, the volume control knob operates seamlessly and doubles as the on/off switch.
Adjacent to the tone controls, on the left side and also arranged vertically from top to bottom, are three push buttons: Gain, tone defeat, and loudness switches.
To indicate power/battery status and over-loudness, two small LEDs are utilized. One LED is positioned at the top, while the other is located at the bottom. During charging, the first LED remains red and turns off once the battery is fully charged. When the device is in operation with sufficient voltage, the second LED emits a steady green light. However, if the battery is running low and requires recharging, the green light will flash intermittently. Additionally, when the output voltage reaches 2V, the second LED alternates between flashing red and blue.
At one end of the chassis, you will find the 3.5mm line input, 3.5mm line output, and a USB type-C port for charging the device. On the other end, there are two headphone outputs. This device does not have any digital inputs and operates solely in the analog domain. Both the headphone and line outputs can be used simultaneously. The SK01 offers the flexibility to use it with or without the tone controls, making it a versatile two-in-one device. It functions as both a pure analog headphone amplifier/preamplifier and a three-way active equalizer.
Battery duration
The Fosi Audio SK01 is equipped with a 2500mAh battery, which can be conveniently recharged using the USB type-C port. It takes approximately 2-3 hours to fully charge the battery, and once charged, it can provide up to 9 hours of continuous usage at the low gain setting, with the volume control set to the middle and the tone controls turned off. Although enabling the tone controls may slightly increase battery consumption, the overall battery life is still impressive, allowing for a full day of heavy usage or a couple of days of moderate usage. Additionally, if you prefer to use the device on your desktop, you can keep it plugged into the charger continuously, although please note that the battery will not be bypassed in this case.
Accessories
The package contains a USB charging cable and the manual. A couple of analogue interconnect cables, like a 3.5mm to 3.5mm or a 3.5mm to RCA stereo cable would be a nice addition.
The headphone amplifier
The Fosi Audio SK01 headphone amplifier incorporates the NJM4580M op-amp by JRC, which boasts a maximum power output of 2400mW/32Ω. This amplifier offers two gain settings, 0dB and -12dB, allowing for optimal compatibility with various headphone loads.
In terms of power, the SK01 proves to be quite formidable. Even when paired with the Focal Clear Mg headphones, I rarely found the need to exceed half of the available volume range when using the low gain setting. Additionally, the amplifier maintains an inaudible noise floor, making it suitable for driving sensitive earphones. However, it is important to keep the gain setting at low to avoid any slight hiss that may be introduced at high gain.
For my evaluation, I primarily utilized the Aune X8 XVIII DAC as the audio source. As is customary, I allowed the SK01 to play music continuously for approximately 80 hours before conducting any listening assessments.
Tri-band Tone control
The Fosi Audio SK01 offers three tone controls that allow for precise adjustments to the audio output. The Bass control can be adjusted from 20Hz to 250Hz, with a range of -14dB to +14dB. The mid-range control covers frequencies from 200Hz to 4kHz, with a range of -10dB to +10dB. The treble control spans from 3kHz to 20kHz, with a range of -12dB to +12dB.
In addition to these controls, the SK01 also features a tone defeat button, which allows users to bypass the tone controls without having to reset them. This is particularly useful for those who prefer a neutral sound profile. Furthermore, the loudness function enhances the listening experience at lower volume levels, ensuring a more balanced sound.
It’s worth noting that all these functions are hardware-based, meaning there is no software DSP running in the background. This ensures a pure and unadulterated audio signal. The tone controls themselves utilize three high-quality, ultra-low noise SGM8262 op-amps, guaranteeing exceptional audio performance.
Listening impressions
The Fosi Audio SK01 headphone amplifier offers a neutral, linear, and transparent sound with a subtle touch of sharpness. It provides a musical and engaging experience with vibrant harmonies and a natural timbre, without any artificiality.
In terms of dynamic range, this amplifier is truly impressive for its category. It delivers punchy and impactful sound with a robust and extended low end. The bass is fast, tight, and controlled, while still maintaining clarity and definition. The mid-range is clear, spacious, and well-defined, with good articulation and ample body. The treble has a slight sharpness, but it is not overly bright or fatiguing. It is energetic with fast transients and excellent extension.
The detail retrieval of this amplifier is very good for its category, and the soundstage is wide and expansive. It offers satisfying positioning accuracy and a touch of grandness, although it doesn’t provide much depth.
Overall, the Fosi Audio SK01 is an excellent portable headphone amplifier with plenty of power. It delivers a neutral and uncolored sound signature without audible distortion, even when using the tone controls.
Speaking of the tone controls, they are highly effective in equalizing the input signals without adding any additional sonic character. The amplifier also has ample power to support the loss of headroom, providing a broad safety working zone without clipping or distortion. However, it is important to avoid boosting all three tone controls to their limits simultaneously.
In the end
The Fosi Audio SK01 offers an affordable option to enhance the sound quality of your headphones and other audio devices, while maintaining the integrity of the original signal. Additionally, it serves as an excellent headphone amplifier, providing ample power to drive a wide range of full-sized headphones. Its compact size and battery-powered feature make it convenient to carry along wherever you go.
Copyright – Petros Laskis 2023.
+ Transparent and linear + Powerful headphone amplifier + No distortion or noise + Three tone controls + Preamplifier + Tone defeat button + Loudness function + Two gain settings + Battery powered + Good battery duration + Well made + Affordable - Desktop mode will not bypass the battery - Not really portable - The package doesn't include analogue interconnects
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