Yamamoto Sound Craft
HA-02 Headphone amplifier

Only one Western Electric 408A miniature vacuum tube is used for amplification, one for each side. The output of 300mW is sufficient for a headphone amplifier. The 408A also has excellent noise characteristics, and has excellent resolution at low volumes. Therefore, for background music, it is advantageous that the sound is clearly reproduced even when the volume is turned down. The 408A is configured with only one stage of amplification circuitry, taking advantage of the large amplification factor of the pentode tube, and succeeds in maximizing sound clarity.

The output transformer of this model uses a high-performance output transformer equivalent to those of high-end models and Teflon insulation paper to achieve the best sound quality. Two types of terminals are provided on the secondary side of the transformer to draw output for 50-ohm headphone amplifiers and for 8-ohm speakers.

The input section of the HA-02 has two terminals to allow connection of a CD player or music player, which can be switched using the switch on the top of the chassis. Most portable devices such as i-pods can be connected using the included cables. When the HA-02 is used for headphones, the optimum load impedance is 50 ohms, but a wide range of headphones with impedances from 8 ohms to 600 ohms can be used. The HA-02 is also recommended for use with headphones.

The input terminals of the HA-02 are our original chrome copper Teflon-insulated pin jacks. The speaker terminals use gold-plated high-grade terminals that can be connected to speaker cords up to 5 mm in diameter, and the headphone jack is a highly reliable type manufactured by Neutrik.

The chassis of this unit is made of COMPLITE, which has been well received in the A-08S and other models, and is made of wood impregnated with phenolic resin at high pressure, a material with excellent electrical and strength characteristics. The legs are made of rounded African ebony wood, which is also popular as a setting base and is highly effective in damping vibrations. In addition, a high-performance R-core transformer is used for the power transformer. For rectification, first soft recovery diodes are used on the plate side and the screen grid side separately to bring out the sound quality of the pentode tubes. Film capacitors are used at key points in the power supply for a high-quality sound power circuit.

Tubes used	WE408A 2 tubes included
Maximum output	Headphone jack 300mW (at 50Ω load) Speaker jack 240mW (at 8Ω load)
Frequency Response	30~21,000Hz (headphone jack) 45~17,000Hz (speaker jack)
Residual noise	100μV at headphone output 70μV at speaker output
Input impedance	50k ohm
Gain	Headphone output: 9.5dB (at 50Ω load) Speaker output: 1.5dB (at 8Ω load)
Size	150(W)325(D)125(H)mm
Weight	4.2kg

The HA-02 is as ultra-transparent as water, as if it were essentially non-existent as a playback device. Listening sensitivity usually declines during a session. To compensate for this loss, which is why movie soundtracks get louder toward the end, the HA-02 is forced to respond in the opposite way. The music seems to get louder as the track progresses. Why is this so? My experience, based on what I have heard so far, leads me to what is usually referred to as the low-frequency weighting of amplifiers and speakers. What does this have to do with the midrange and low-high frequencies?　The amount of low-frequency weight and density projected into the vocal range is unusual. However, it is different from unnatural energy to enhance presence, so even such vocals do not tire the listener or take away any heat. Whether the bass is light or not is not the determining factor of whether the midrange is colorful or not. It has to do with the color temperature of the musical colors, and the HA-02's palette is incredibly rich, densely saturated, and perfectly in tune with the aforementioned watery transparency.
*sixmoon.com