Elekit TU-HP01 Hybrid Tube Headphone Amplifier. Made in Japan. Preassembled. 

------ This is a portable headphone amplifier with a hybrid configuration: 2 subminiature tubes for input and OP-amp for output. Enrich your life with smooth and mellow sound anywhere!

Features:

• Battery-operated vacuum tube Raytheon 6418 low consumption current and almost no heat generation so that there is no problem if the holes on the chassis are sealed.
  
• Minimized microphonic noise.
• Microphonic noise, often controversial in tube portable amp, is minimized by setting the vacuum tubes higher than the PCB.
• Unlikely to usual vacuum tube amplifier, TU-HP01 starts up only in a few second after the power is turned ON.
• Exchangeable op-amp: 2 op-amps, JRC MUSES8820 and BurrBrown OPA2604, are included as a standard. (MUSES8820 is mounted by default.) In addition, users can exchange it with many other commercially-available op-amps to enjoy the difference in sound.
  
• Dual supply for op-amp... No capacitor is required to the headphone output.
• Gain switch... Users can select either high or low according to the input level.
• Almost no pop-noise upon power ON.

Specifications:

• Vacuum tube: Raytheon 6418 x 2pcs.
  
• Frequency response: 10Hz - 65kHz.
• Headphone impedance: 16ohm - 32ohm recommended.
• Input terminal: 3.5mm stereo mini jack.
• Output terminal: 3.5mm stereo mini jack.
• Power supply: AAA dry cell battery x 4pcs, or AAA Ni-MH rechargeable battery x 4pcs. (Battery life: Approx.10 hours).
• Dimensions: W78 x H16 x D117 mm (excl. projections).
• Weight: 125g (excl. batteries)

Ref link: Elekit TU-HP01or head-fi.org review. 

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