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FX-Audio Tube 02 Pro Review (Headphone Amp)

Rate this product:

  • 1. Poor (headless panther)

    Votes: 73 40.3%
  • 2. Not terrible (postman panther)

    Votes: 63 34.8%
  • 3. Fine (happy panther)

    Votes: 37 20.4%
  • 4. Great (golfing panther)

    Votes: 8 4.4%

  • Total voters
    181

amirm

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This is a review and detailed measurements of the FX-Audio (KGUSS) Tube 02 Pro headphone amplifier. It was sent to me by the company and costs US $80 including shipping.
FX-Audio KGUSS Tube 02 Pro Review Preamp Headphone Amplifier.jpg


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.

FX-Audio KGUSS Tube 02 Pro Review back panel Preamp Headphone Amplifier.jpg


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:

FX-Audio KGUSS Tube 02 Pro Measurements Preamp Headphone Amplifier.png


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:
FX-Audio KGUSS Tube 02 Pro Measurements SNR Preamp Headphone Amplifier.png


I also liked the very extended frequency response with just a bit of roll off around 20 Hz:

FX-Audio KGUSS Tube 02 Pro Measurements Frequency Response Preamp Headphone Amplifier.png


Headphone Output Measurements
I started by measuring the SNR again, including 50 mv:
FX-Audio KGUSS Tube 02 Pro Measurements SNR Headphone Amplifier.png


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:
FX-Audio KGUSS Tube 02 Pro Measurements Power 300 ohm Headphone Amplifier.png


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:
FX-Audio KGUSS Tube 02 Pro Measurements Power 32 ohm Headphone Amplifier.png


Stepping through a bunch of impedances we see the proportionality of distortion to inverse of load impedance:

FX-Audio KGUSS Tube 02 Pro Measurements Power vs load Headphone Amplifier.png


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 KGUSS Tube 02 Pro Measurements Channel Matching Headphone Amplifier.png


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. I am not in that camp so can't recommend it.

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As always, questions, comments, recommendations, etc. are welcome.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 

ta240

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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! :)

So, that makes both statements valid or both invalid? How exactly does the math there work?
 

mhardy6647

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So, that makes both statements valid or both invalid? How exactly does the math there work?
Based on @amirm's listening comments he found the distortion objectionable in several cases, and didn't note any euphonic distortion when the not-euphonic kind was not audibly/noticeably present. :)

I apologize for jumping in and responding to an implicit question to someone else... but I, too, initially misread the comment, but I went back & read the listening comments in the review and all was made clear.
 

Billy Budapest

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Based on @amirm's listening comments he found the distortion objectionable in several cases, and didn't note any euphonic distortion when the not-euphonic kind was not audibly/noticeably present. :)

I apologize for jumping in and responding to an implicit question to someone else... but I, too, initially misread the comment, but I went back & read the listening comments in the review and all was made clear.
I think the characteristic tube sound that some people describe as “euphonic” has less to do with the distortion measurements and more to do with high frequency roll off and the much lower damping factor than solid state, especially on the high and low frequency ends of the spectrum. If the harmonics are high enough to be audible, they will contribute too, but I don’t think they are the main things people are hearing when they describe “euphonic tube sound.”
 

Billy Budapest

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The irony of mocking one group for doing sited tests and doing the same?
The difference is the other side doesn't claim to have science on their side.
Yeah, exactly right! That’s the point.
 

Dzhaughn

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Well, I admit this is WAY cheaper than using a MiniDSP Flex to create this particular distortion effect.

But, only with the MiniDSP Flex or something better can you demonstrate just how dysphonic other reproductions (such as accurate ones) of the same recording can be.

Euphonists should recognize that each distinct recording (in fact, each rendition of that recording) is optimized by different parameters, and only something like the Flex can provide that in a space efficient way.

And we could share our euphonia! Imagine everyone publishing their 10 favorite distortion settings for Dylan's "The Complete Basement Tapes" on twitter! I mean on the blockchain. Whatever kids are doing these days, the MiniDSP Flex makes it posisble! The Future Beckons!
 

Billy Budapest

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Euphonists should recognize that each distinct recording (in fact, each rendition of that recording) is optimized by different parameters, and only something like the Flex can provide that in a space efficient way.

And you could share your euphonia! Imagine everyone sharing their 10 favorite distortion settings for Dylan's "The Complete Basement Tapes" on twitter! I mean on the blockchain. Whatever kids are doing these days, the MiniDSP Flex makes it posisble! The Future Beckons!
Your proposal sounds a lot like MQA. :p
 

Walter

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I have no interest in this product, but damn, they need to fire their front panel designer. He/she took a basically very attractive product and made it cheap looking and butt ugly with all the white or light gray coloring. Use much smaller fonts for the brand and model number, omit the stupid marketing description and horizontal bar, and you have a winner from an appearance standpoint.

Edit: Also, since every single element is circular, center-align them vertically. This looks like the kind of designs Chinese companies were putting out 20 years ago, when industrial design courses were just starting to be introduced in the universities there. There is really no excuse for it now.
 
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