The SK01 is a high-power headphone amplifier with tone controls (treble, middle and bass), high and low gain settings, bypass mode, and loudness function. 

Packing List
Product Overview​
Operation Guide
Frequently Asked Questions
Warranty Information
About Fosi Audio


Thank you for purchasing the SK01 headphone amplifier.The SK01 is a high-power headphone amplifier with tone controls (treble, middle and bass), high and low gain settings, bypass mode, and loudness function. The tone-control feature allows you to customize the sound to your preferences and compensate for any shortage in your headphones oraudio system. The high and low gain settings are suitable for headphoneswith different impedance. The bypass mode provides a pure reproduction of the original sound. The loudness function allows you to immerse yourself in the world of music during quiet nights.

Packing List

Product Overview

① GAIN: Initially set to high gain mode, press to switch to low gain mode

② TONE: Press to disable the treble, midrange, and bass adjustment

③ LOUDNESS:Enhances both treble and bass when listening at low volume

④ Power indicator: Red light stays on during charging, turns off when fully charged. Green light stays on during operation with sufficient voltage. Green light flashes when battery is low

⑤ TREBLE: Adjusts the high-frequency range at 10kHz (±8dB)

⑥ MIDDLE: Adjusts the mid-frequency range at 1kHz (±10dB)

⑦ BASS: Adjusts the low-frequency range at 50Hz (±11dB)

⑧ Power/Volume knob

⑨ Signal Indicator: Light flashes in red and blue alternately when the output voltage reaches 2V, indicating the excessive volume

⑩ 3.5mm headphone jack

⑪ 6.35mm headphone jack

⑫ DC 5V USB-C power input

⑬ 3.5mm LINE IN

⑭ 3.5mm LINE OUT


Output Power

HIGH: RMS 2400mW@32Ω; LOW: RMS 300mW@32Ω



3.5mm line in



3.5mm line out, 3.5mm headphone jack, 6.35mm headphone jack






≤ 0.001% @ 1kHz and 770mV


Noise Floor






Frequency Response



Terminating Impedance



Battery Capacity



Power Input Voltage


Operation Guide

1. Connect your audio source to the Line In port of the SK01 using a 3.5mm audio cable;

2. Plug your headphone into the corresponding headphone jack;

3. Turn the power/volume knob clockwise to turn on the device. The power indicator light will illuminate;

4. Adjust the volume, tone, and other settings to your desired level of comfort.



1. If the power indicator light does not illuminate or flashes after turning on the device, please turn it off, connect the DC 5V USB-C port using a USB-C cable for charging, and then turn it on again;

2. If the signal indicator light is flashing, it indicates that the volume is relatively high. It is recommended to lower the volume before wearing headphones and then adjust it to your desired level;

3. Avoid plugging or unplugging the USB cable, 3.5mm audio cable, or headphone cable while the device is turned on;

4. The USB port is for power only and does not support data transfer.

Frequently Asked Questions

1. The tone-control knobs are not working?

Please check if the TONE button is set to PURE mode. PURE is the mode where the signal bypasses the tone circuit. Switching to TONE mode should enable the tone adjustment.


2. Why is the power indicator light flashing continuously after turning on?

The flashing power indicator light indicates low batteries. Please charge the device promptly.


3. Is the SK01 a preamplifier and headphone amplifier? Does the tone knob control only the 3.5mm audio output or also the headphone output?

The SK01 is both a preamplifier and headphone amplifier. The tone knob controls both the 3.5mm audio output and the headphone output. When adjusting the 3.5mm audio output of the SK01, it functions as a preamplifier, connected to a power amplifier, and allows adjustment of the high, mid, and low frequencies of the audio system.


4. Why am I not experiencing the Loudness effect?

Loudness is an equal loudness switch designed to compensate for the frequency response of human ears at low volumes. When listening at low volumes, our perception of high and low frequencies may be reduced. Loudness compensates for this by boosting high and low frequencies, aligning perceived loudness with the actual volume.


5. Why is the signal indicator light not turning on?

The signal indicator light indicates the volume level. It starts flashing as the volume increases (output voltage reaches 2V). The purpose of the signal light is to let you know if the volume is too high before wearing headphones. If the signal light continues to flash and is bright, it is recommended to lower the volume before wearing headphones, and then adjust it to your desired level, ensuring the protection of your ears.


If you have any further questions, please contact our support team at

Warranty and Maintenance

• All Fosi Audio products have an 24-month limited warranty on parts and labor from the date of purchase. Please contact us if you have any problems.

About Fosi Audio​

• Fosi Audio designs and builds innovative audio products with all your music in mind. Great sound, simple but elegant designs, high-quality materials, and truly useful features are what Fosi Audio is about. We sincerely hope you get as much enjoyment from our products as we’ve had to create them!


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