FX-Audio Tube 02 Pro Review (Headphone Amp)
this is a review and detailed measurements of the FX-Audio Tube 02 Pro headphone amplifier.
It was sent to me by the company and costs 80,- including shipping.
The look and feel of the unit is well above its price bracket. The selector and on/off switches are reassuring to use. And inclusion of both 3.5mm and 1/4 inch headphone jacks are appreciated.
The included tubes are JAN 5725W. I am assuming they are a buffer stage with output being solid state from my rough read of the product spec. They have a nice glow but I think that is courtesy of light underneath them than actual filament. It does look very realistic (sadly I could not capture them in the picture above).
FX-AUDIO Tube-02 Pro Pre-amp Measurement
Since there is also line level output, I decided to test that first:
I usually try to get unity gain by inputting 2 volts and wanting 2 volts out. As you see above, there is a slight negative gain in this mode despite having the volume control set at max (gain switch didn’t seem to make a difference). Tube amps promise distortion and distortion is what you have. I was however impressed by the good signal to noise ratio:
I also liked the very extended frequency response with just a bit of roll off around 20 Hz:
Headphone Output Measurements
I started by measuring the SNR again, including 50 mv:
Not as impressive as line out but still decent at full level.
Measuring power, we see that the gain switch only makes a small difference:
It does however get above my requirement of 100 milliwatts at this impedance which is good. Note that distortion increases above noise at just 0.1 mwatt. I guess the “good news” there is that even your IEMs will experience it.
Distortion rises even more at lower load impedance of 32 ohm:
Stepping through a bunch of impedances we see the proportionality of distortion to inverse of load impedance:
Output impedance can be inferred from above to be very low. So you are not going to get load dependent response that with other tube amps with high impedance (with variable impedance headphones).
I was impressed by how good channel balance was:
FX-AUDIO Tube-02 Pro Headphone Listening Tests
I was wearing my recently reviewed Audeze LCD-XC headphone so chose to test with it first. This is a 20 ohm impedance headphone that is pretty sensitive. Upon the switch form my everyday RME ADI-2 DAC, I immediately notice high frequencies being distinctly distorted. It is hard for me to describe it but they were sizzley, and grungy. I then switched to my standard test headphone, the Dan Clark/Drop Ether CX. Experience was the same and would get worse as I turned up the volume. I found the amplifier unlistenable with either headphone as distortion would be ever so present.
I then switched to high impedance Sennheiser HD-650. Now at low volume the sound was “fine.” Mind you, nothing euphonic was going on but the large amount of distortion with the above two headphones took a back seat there. Crank up the volume and it gets good bit loud but then distortion sets in especially in bass.
Conclusions
I see a price of $79 and read “tubes” and I immediately equate that to junk. Such was not the case here. The casework is very nice, as well as some of the measurements. The amp simply does what audiophiles are asking: deliver them distortion and tons of it. The common misconception is that this distortion is either inaudible or euphonic. Neither was the case in my listening tests. Before you say, “but your test was not blind,” I will remind you that neither are all the remarks about the sound being euphonic! 
If you want a very low cost way to get a tube-based headphone amp/pre-amp that has good effort behind it, here is your chance with FX-Audio Tube 02 Pro.
