Product Specifications

Fosi Audio BOX X4 is a Hifi tube phono preamp and headphone amplifier for MM turntable that features volume control and Bass and Treble control with DC 12V power supply. The BOX X4 supports MM turntable input, 3.5mm AUX input and 3.5mm headphone output and RCA line output for powered speakers or amplifiers for your home audio system. Compact ultra-low noise phono turntable preamp that converts phono signals to line level signals. The BOX X4 equipping with 5654W vacuum tubes stands up to the test and will provide you with serious sound performance.

Input Interface:MM turntable input,  3.5mm AUX input and 12V DC input
Output Interface:RCA output,  3.5mm headphone output
Output:Active(powered) speakers, Amplifier
Product Adapter Input voltage Range:AC 100V – 240V
DC Input Voltage:12V
Applicable headphone types:16 Ω – 300 Ω
Output Voltage Level:1kHz and 650mV
Frequency Response:Standard RIAA
Product Weight:0.34kg
Product Dimensions:3.97×4.72×1.37 inch  (10.09*12*3.5 cm)

Product Overview & Control Functions


Package Contents

Fosi Audio BOX X4 *1
5654W Vacuum Tube *2
DC 12V Power Adapter *1
User Manual *1

Quick Setup

Step 1: Install the 5654W vacuum tubes to the BOX X4.

Step 2 (OUTPUT): Connect speakers or headphone (RCA output, 3.5mm headphone output) to the BOX X4.

• RCA Output:

Use RCA cable to connect the RCA line output of BOX X4 to your active (powered) speakers. Or use RCA cable to connect the RCA line output of BOX X4 to your amplifier, then use speaker’s wire to connect the amplifier with your passive speakers.

• 3.5mm Headphone Output:

Use 3.5mm headphone line to connect the 3.5mm headphone output of BOX X4 to your headphone.

Step 3 (INPUT): Connect the audio source (MM turntable input, 3.5MM AUX input) to the BOX X4.

• MM Turntable Input:

Use RCA Cable to connect your MM turntable to the RCA inputs of BOX X4, then connect the ground wire of your MM turntable to the grounding post of BOX X4.

Note 1: If your MM turntable is no need to connect a ground wire, the ground wire can be omitted.

Note 2: MM Turntable Input can only be connected to the turntable, and cannot be used as a normal preampli fier connected to other audio sources, such as mobile phones, MP3, etc.

• 3.5mm AUX Input:

Use a 3.5mm Audio cable to connect the BOX X4 and your device.

Note: Once 3.5mm AUX plug in, the MM input will automatically interrupted, so the input mode will turn to 3.5mm AUX input.

Input priority: 3.5MM AUX input > MM turntable input.

Step 4: Connect the power adapter to BOX X4 and the socket. Turn the Multi function Knob (Volume Control/Power Switch) clockwise to power on the BOX X4.

Note 1: The power adapter does not have a power indicator light, so connecting to the socket means power is on.

Note 2: Before connecting the power supply, please make sure that the BOX X4 is in “OFF” position when connecting to the power adapter. Otherwise there is a “POP” click sound from your speakers.

Note 3: Before connecting the power supply, please turn down the volume control, the bass and treble control to 0 (always to the left) to avoid your hearing from damage.

When the connection is completed, please turn on the BOX X4 first, then turn on the amplifier (or powered speakers), at last turn on your turntable or other audio source, then enjoy.

Note: If you turn on the amplifier (or powered speakers) first, then turn on the BOX X4, there is a “POP” click sound from your speakers, it is normal that all the phono preamp have the same issues, please don’t worry about it, the “POP” click sound will not damage your amplifiers or speakers.


1. Get noise/humming when turn the BOX X4 on?

  • Please know that when no inputs connected to BOX X4, the default input way is MM input mode, in this mode, the gain of the amplify is much large then other input modes, so it’s normal that will have some humming sound when no sound pass through.
  • Please unplug all the inputs from the BOX X4, then turn it on to check if the noise still or not.
  • If the noise disappear, the noise may from the audio source or audio cable, please exchange the audio source and the audio cable to try again.
  • If the noise still, the noise may from BOX X4 or amplifier/speakers.
  • Please exchange the audio cables and amplifier (powered speakers) to have a try and make sure that the noise is from which devices.
  • Please volume down the “Treble” volume on the BOX X4 to check if the noise volume follows or not.
  • Please make sure all the cables completely pushed into the device, incompletely connection will cause this issue.
  • Please make sure that your turntable is MM output and also make sure that it is phono output, some turntables built in phono preamplifier, so it should connect the 3.5mm AUX input on the BOX X4 not the phono input. Also make sure the grounding wire is connected well with the BOX X4, please reconnect it to have a try.
  • Please keep this device away from other electronic devices, like the FM radio or WIFI router.
  • Please make sure the tubes is completely pushed into the socket, incompletely connection will cause this issue.
  • Please reconnect the power adapter to the BOX X4 to try again.


  • If the noise still, please contact us via email at

  • BOX X4 is a preamplifier, not a power amplifier, cannot be connected with passive speakers directly.

2. All cables are connected, but there is no audio output?

• Please check if your audio source, BOX X4, amplifier or powered speakers all are turned on and in working mode.
• Please check whether the light of the vacuum tube is always on after power on, check whether there is a problem with the vacuum tube.
• Please check if the music in your audio source is paused.
• Please make sure that you are chose the correct input mode of the BOX X4, if the 3.5mm AUX input plug in, the MM input mode will be interrupted, at this time if you want to use the MM input, you need to disconnect the 3.5mm AUX input first.
• If the connection is right but the BOX X4 does not work, please try another power supply (Any DC12V power adapter can work with BOX X4, you can use the power adapter of WIFI router or camera) that is suitable for the BOX X4. If BOX X4 works after replacing the appropriate power adapter, it means that the power adapter is probably broken. Please contact us to replace the power adapter; if BOX X4 still not works after replacing the appropriate power adapter. It means that the BOX X4 is probably broken, please contact us to replace the BOX X4.

Note: Make sure that all the cables are completely pushed into the device.

3. How to distinguish Power Switch and Volume Control?

• Power switch means turn the power switch knob clockwise for one time to power on the BOX X4. Adjusting the volume means turning the volume control knob clockwise to adjust the volume.

4. How do you switch between inputs?

• The BOX X4 supports two inputs: MM turntable input, 3.5mm AUX Input. The priority of switching between the two inputs: 3.5MM AUX Input > MM Turntable input.

5. Can I connect both headphones via 3.5mm headphone output and active speakers via RCA output to the BOX X4 at the same time?

• Yes, the BOX X4 can be connected to both outputs at the same time. By the way, it can be connected to both inputs at the same time.
• When it is connected to both 3.5mm headphone output and RCA output, both headphone and RCA can be output. When it is connected to both 3.5mm AUX input and MM Turntable input, the BOX X4 defaults to 3.5mm AUX input.

Note: BOX X4 is a preamplifier, if you want to connect it to passive speakers, you need a power amplifier.

6. My turntable doesn’t have a ground wire connection. Does this need a ground wire hooked up for it to be able to work?

• No, it does not need a ground wire to operate. However, if your turntable has any ground loop, it will be piped and amplified through the BOX X4.

7. How to attach ground wire?

• There is a post to attach the ground wire.

8. How does grounding post work? My turntable has fork spade type Terminal?

• You can connect the fork spade type terminal to the grounding post.

9. Does this have analog to digital conversion?

• No. This product is a phono preamp/tube preamplifier.

10. Is the gain enough to support MC cartridge?

• No. It is for MM cartridge.

11. My turntable has an optional built-in phono preamp or switch to the line. Can I still use this instead of my built-in preamp?

• Yes, deselect the preamp on your turntable.

Note: Please confirm whether your turntable has a built-in phono preamp, if there is a phono preamp, and it cannot be deselected, then you can only connect 3.5 mm input, cannot connect MM Turntable input.

12. Can I hook the BOX X4 to the power amplifier for my system?

• Yes. You can connect the BOX X4 to the power amplifier through the RCA output.

13. Can this work with my turntable and headphones?

• Yes, it has headphones output. The 3.5mm AUX connector on the front panel of the amplifier can be used for headphone.

14. Can I directly connect this phono preamp to the speakers?

• If your speakers are powered speakers, you can directly connect this preamp to the speakers. But you need to add an amplifier or receiver to the passive speakers.

15. What equalization is incorporated in this preamp?

• RIAA equalization

16. Can you hook a subwoofer to this unit?

• Yes, if your subwoofer is powered, you can connect the BOX X4 directly to the subwoofer, but you need add a sub amplifier between the BOX X4 and the subwoofer if your subwoofer is passive subwoofer.

17. Which vacuum tube can be replaced to the BOX X4?

• If necessary, tubes below are substitutions: EF95, 6AK5, 6AK5W, 403A/403B, 6J1, 6J1P, 6J2, 6J3, 6J4, 6J5 vacuum tubes, etc. You can replace the vacuum tubes to DIY your stereo system.

18. Sound quality is not good?

• Make sure all the cables are fully connected.
• Adjust the treble and bass control of the BOX X4 to achieve the right tone.
• Please try to change the sound source and cables to test.

19. Can it be left on all the time?

• The BOX X4 is designed with a power switch. It can stay connected safely without power consumption all the time, you only need to turn off the power switch on the BOX X4.

20. Why the BOX X4 came with a broken vacuum tube?

• Due to long-distance transportation, the vacuum tube may be damaged during transportation. If you find that the vacuum tube is damaged or other problems, please contact Fosi Audio after-sales email immediately, and we will replace the vacuum tube for you.

Any problems please leave a message to the customer support email address:

Warranty Information

  • All Fosi Audio products have an 18-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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