I finally found the time to translate about 60% of the article that is the most relevant:
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Although this machine can be a class-A headphone amplifier, a tube preamp and a class- A integrated tube amplifier, my test is focused on its performance as a headphone amplifier. The emphasis will be on the difference between the triode and the ultralinear modes.
While planning for a special issue on headphones, the HA-1A happened to be one of my reference amplifiers. What I liked about this unit is the impedance selector. I have spent a good deal of time matching it with many brands of headphones, and became quite acquainted with its sonic character. First, my impressions of HA-1A include: an incredible dynamic range, the ability to drive any headphone, a smooth bass extension and being able to deliver powerful punches. However, the background noise due to the power supply is less than ideal. In the absence of input signals, at the normal volume setting, the noise is clearly audible. This weakness is reflected in our measurement graphs--harmonic distortions related to 60 Hz AC frequency are somewhat large. This issue needs to be addressed in the future version or upgrade of HA-1A.
To help the reader understand the characteristics of HA-1A, I have chosen K501 and HD650, two headphones with rather different sonic attributes, as references for listening tests.
When HD650, which has a rounded, bottom-heavy sound, is driven in the triode mode, its performance is full-bodied and pompous. The low-frequency impact immediately rocks the head, but sounds perhaps a bit too full. The high resolution helps bring out what was originally recorded. The density and quality in the mid-to-high frequency range is especially oustanding, producing a noble tone. However, the expansion of the soundstage and the separation of the instrument seem to be compromised in the triode mode, because the sound is too centralized and mixed-up. On the other hand, under the ultralinear mode, the aforementioned shortcomings can be improved. The thickness at the bottom end and the restrcited ambience is improved. Moreover, the brilliance in the high frequency is enhanced, the sound becomes more three-dimensional, and the midrange retains the warmth that HD650 is known for. The balance across the entire frequency range is more ideal in the ultralinear mode than the triode mode. Unfortunately the ultralinear mode also increases the background noise, which can be relieved if a lower out impedance is chosen.
Next, I tried AKG K501, which has a leaner sound, closer to a studio monitor headphone. In the triode mode, I find that HA-1A add more “beef” to the leaner K501. Thus, when K501 is trying to express the weight of the piano or pop music with an emphasis in the mid-to-low range, there is a more lively sense of involvement. Also, K501 went from coolness to the warmth and passion characterized by tube amps. But it maintains its transparency in the treble as usual.
As for the ultralinear mode, K501 does not have the background noise issue of HD650, which is related to the sensitivity of the headphone. The layering and spaciousness of the soundstage is better than the triode mode. Unfortunately, K501’s brilliance in the treble (especially in instrumental recordings featuring cymbals) turns into slight harshness under the ultralinear mode. Similar drawbacks become more pronounced on headphones with even more treble emphasis, such as Audio Technicas or Sony MDR-SA5000.
Based on the listening comparison using the two headphones under both operation modes, we find that HA-1A can enhance low-frequency impact and add body and texture under the triode mode. In this case, headphones with weaker bass performances may benefit while others don’t. The ultralinear mode does not emphasize the bass but the treble. Matching with the right headphone, this carries a sweet, metallic timbre; in other cases it may be too much shrill.
From the measurements we acquired, under the ultralinear mode, there are higher harmonic distortions in the treble range, which may lead to a shrilling sound. When playing large-scale orchestral music, the treble may appear unstructured or chaotic. The triode mode avoids such problems and provides better transparency and extension in the treble. The triode mode provides tube warmth and a sweet sound which is easy on the ear. Moreover, the triode mode offers a better dynamic range, suitable for high-efficiency headphones under low-power operations. Under high-power operations the ultralinear mode will outperform the triode.
Based on the listening tests, can we say that the triode mode provides more flavor or sophistication than the ultralinear mode? In some sense we may say so. But I have to emphasize that each headphone has its own character. How to accentuate the merit of each headphone and compensate for its weakness, and pay attention to all details, is the most important and the most difficult part of picking a headphone amplifier. There is no headphone amplifier on the market that could perfectly match every single headphone. HA-1A provides selectable output impedance and the choice between triode and ultralinear modes, providing more versatility and matching options. The multi-function potential is the greatest asset of this machine.
Moreover, the pros and cons of the triode versus the ultralinear mode is only a “self-test” on this machine. If we compare HA-1A to other brands of tube headphone amps, its tube flavor is what numerous tube fans would consider “addictive,” holding its own against European and American competitors. More importantly, when we consider its generous internal components, carefully designed circuitry and multi-functionality, it appears to be a bang-for-buck headphone amp. Taking into account its price, function and appearance, there is no reason for this well-made headphone amplifier to remain neglected. Especially for head-philes with multiple pairs of headphones, or an audio shop, the HA-1A is a superb value because it can match headphones of various impedance values and sonic attributes.
