NFJ FXAudio FX-Audio Tube-02 PRO Headphone Amplifier – Transistor Big Feelings
FX-Audio Tube-02 PRO is a $79 USD Preamplifier / Tube Headphone Amplifier with a Vacuum 5725 Tube at the core, and a Class A Amplifier supporting Headphones with an impedance between 32 and 600 OHMs, also used for stereo active speakers, or passive in between the DAC and the AMp.
Introduction
FX Audio is known to be an entry-level electronics producer from China, having designed a couple of affordable but well-performing devices in the past. You can purchase FX-Audio products from Amazon, Aliexpress and other Chifi distribution circuits, making those an easy choice if you don’t want to use specific shops.
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Product Link
Amazon – https://amzn.to/4jNL05R
Aliexpress – https://s.click.aliexpress.com/e/_c3g7saw7
Build Quality – Design
FX-Audio Tube-02 Pro has a complicated name, but it basically is a Class A Headphone Amplifier, and is built around the 5725 tubes. It uses transistors for Class A buffer, and promises a strong output power, up to 850mW. Using coupling capacitors, it is designed for a sweet and transparent sound.
There are two sets of line inputs that go through the tube. Ideally, you can use it as a PRE to add some tube character to a system, both active and passive, and it has two gain levels, either zero or +2.5dB. You can replace the tubes with either 6J1 or 6J2, others are currently not compatible. There are two headphone outputs, one in the 6.3 mm format and one in the 3.5mm format, both single ended. It is important to keep in mind that as this is a tube amp, you should ideally not connect two headphones at the same time, as it might overload the amp and will damage the sound. It consumes up to 6W, and has an output impedance of lower than 0.1 OHMs, but also a line output level of 1.7Vrms. This line output level is important because it is quite low, and it will never run into clipping with any system, as most expect a 2V line signal.
Tubes are inherently very sensitive to EMI and RFI, and Tube-02Pro is also extremely sensitive to those by design. Everything is single ended and RCA, while the power delivery is the most basic Switching brick possible, running at 12V. This means that you can easily replace it with a slightly better PSU. Inside the package you can find the Tube-02 Pro, two tubes, and the power supply.
Subjective Usage
You can turn on or off the unit by keeping the button on the left side pressed. It took me a while to figure this out, but it works alright. You can hear the tube noise and background hissing with both IEMs and Headphones, so you will have to use the DAC volume at max, then use the volume wheel of the Tube-02 Pro to control the listening volume. This is actually the way it is intended to be used, but the volume it has also controls the tube gain level which changes the character of the sound with each volume level. Switching between AUX inputs has a pop noise to it, which is not ideal.
Tube-2 Pro also gets really warm during usage owed to it using real tubes in the construction. Tubes are sensitive to noise, including EMI and RFI, you will get a ton of interference, which is not ideal. It has a really loud gain level which means that with most DACs, you cannot use the DAC at max volume, because it will distort, so it is a game of balancing the load input level, or DAC volume and use its own volume to control the tube gain and keeping the background noise at the lowest possible.
Sound Quality
Pairings – I have paired the FXAudio Tube02Pro with a selection of IEMs and Headphones including Kiwi Ears x Z Serene, Sivga M300, AFUL Performer 5+2, ZiiGaat Crescent, Lily Audio Genesis One, Dita Prelude, ZiiGaat Horizon, Letshuoer S12 Ultra, Raptgo LEAF D01, Palma DHS-1, KBEar Cepheus, Crosszone CZ-10 Enhanced, Erzetich Thalia, Erzetich Mania V2024, TinHIFI T7, FiiO FT13, NFAudio NE4, Sivga Peng, Moritz Dragon, Crosszone CZ-8a Enhanced, Sendy Audio AIVA 2, Tangzu TianPeng, Soundz Flame, YanYin Canon PRO, Dunu Vulkan 2, Letshuoer Mystic 8, and HIFIMAN Isvarna.
It has enough power for certain Planar magnetic headphones, including HIFIMAN HE600, Ananda Unveiled, but it falls short for Arya Organic or He1000 Unveiled. Background noise is audible and rather loud with most IEMs and I do not recommend the FX-Audio Tube-02 Pro for most IEMs and Earphones. Using it as a PRE is a good experience, tube noise is less audible, but still you can hear it, so ideally you would set volume at max on AMPs, max on DACs and let the PRE set the volume.
Overall Signature – I was not very enthusiastic about Tube-02 Pro when I saw the shortcomings of its usability, but sonically it is quite impressive, offering a deep, controlled sound with a punchy, rounded bass, and full bass extension, a sharp and crisp treble with a waxy, musical and euphonic midrange. It has less tube character than most tube amps, which keeps the sound mostly free of over distortion.
Bass – At the bottom end, we have a rather rounded and full bass, warming up the midrange a bit too. It is more of a character thing, as it doesn’t really have any kind of extra power and coloration that would be detectable on a measurement. Tubes in general are fairly neutral in the bass too, but with the extra harmonics, bass sounds rich, textured and quite vivid with the FX-Audio-Tube-02-Pro.
Midrange – Voicing is quite sweet, a bit waxy, wide and airy. I would call this a balanced presentation, rich, textured, as you’d expect from a Tube AMP. If you like rich textures and a lot of details, you will enjoy Tube-02 Pro, but it sounds a bit dry, forcing textures to be revealed which can be a bit fatiguing long-term. Speed of sound is very fast.
Treble – At the top end, Tube02pro is less fatiguing than in the midrange, as it has a good extension and air, but it does not sound forced nor harsh or fatiguing, offering a fairly clean experience. If anything, it is a bit on the relaxed and smooth side of things in the treble.
Dynamics And Textures – tube02 Pro sounds really textured, emphasizes everything in the bass and midrange resulting in a sound that can easily be fatiguing and harsh / too grainy on the ears. This happens with all headphones and IEMS and when it is used as a preamplifier as well. This being said, it is quite detailed and dynamic compression is kept minimal. If anything, dynamic compression is quite tightly tied to the loudness saturation gradient of the Tube-02Pro.
Loudness Saturation Gradient – FX-Audio-Tube-02-Pro has a unique loudness saturation gradient, where it really tries to be as loud as possible which compresses the dynamic range slightly, and it also changes the character significantly with volume. Although it seems like the Gain Stage and Volume are connected, it also seems like the higher volume also drives more current through the tube. Which is interesting because the Class of the AMP and the design is not OTL, so there’s more than just the tube powering headphones. At any rate, I was able to see that it will become more distorty, louder, have more gain and also more background noise.
Soundstage – You can explore the wide and holographic sound of the Tube-02 Pro in both the tube amp and preamp mode, it sounds quite nicely layered and airy, but separation is not a central point of the sound as it has a slightly higher scatter factor than most amps. This is also caused by the full single ended design.
Value and Conclusion
FX-Audio were already popular in the entry-level price range, so it is no shocker that they have a very high performance with their Tube-02 Pro, and that it offers a high value, but what is most interesting here is that this might as well be the best performing Tube AMP if you want to spend so little, clean for the price and quite controlled. There isn’t much competition in this range, so it is nice to see that they didn’t slack and kept quality consistent.
If you’re looking to bring a bit of tube character to your system, if you don’t mind a bit of extra distortion, but want a richer midrange and a more bouncy bass, then FX-Audio Tube-02 Pro is one of the best ways to do it. Most people I know who asked me about it wanted to use it as a preamplifier, so if you have a fully solid state system and want to spice it up, it is currently a hot topic.
PROs
- Simple and efficient design
- Bouncy bass and vivid midrange
- Sharp and airy treble
- Versatile and excellent as a preamplifier
- Some of the best value for a tube headphone amp / pre
Cons
- Gets quite warm with usage, typical of tube amps and higher class headphone amps
- Audible hissing and background noise at louder volumes
- Not a lot of control for IEMs and easy to drive headphones
Product Link
Amazon – https://amzn.to/4jNL05R
Aliexpress – https://s.click.aliexpress.com/e/_c3g7saw7
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Full Playlist used for this review
We listened to more songs than those named in this playlist, but those are excellent for identifying a sonic signature. I recommend trying most of the songs from this playlist, especially if you’re searching for new music! The playlists are different for Spotify, Tidal and Youtube, and based on the songs I enjoy and are available on each!
https://www.youtube.com/playlist?list=PL_cjBXGmwSHSdGcwuc_bKbBDGHL4QvYBu
https://open.spotify.com/playlist/5J3oloz8Riy9LxEGenOjQ0?si=979ba4f082414be7
https://tidal.com/browse/playlist/330fd544-8e5b-4839-bd35-676b2edbb3d5
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