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Flux FA-12 Review (headphone amplifier)

amirm

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This is a review and detailed measurements of the FLUX FA-12 balanced headphone amplifier. It was kindly sent in for testing by a member and costs US $749 from the company direct.

I must say, this is a luxurious and substantial enclosure for a headphone amp:
Flux FA-12 Review balanced headphone amplifier.jpg


Size wise, it is like an enclosure for a 50 watt linear amplifier. Speaking of which, this is a (discrete?) class A amplifier.

Operationally I found having the low gain in the middle a bit odd. Like to see Low, Med, High in sequence. Another oddity is that to use unbalanced 1/4 headphone jack, you have to use unbalanced input. Balanced input only routes to balanced output:
Flux FA-12 Review balanced R2R volume Control headphone amplifier.jpg


Volume control is implemented using R2R relay so channel matching should be perfect for those of you using sensitive IEMs.

I could not find any safety/regulatory marks on the unit itself. When mains power supply is included in an audio device, I like to see third-party certification for safety.

FLux FA-12 Measurements (balanced)
As usual, I start with balanced input and out with such amps, setting the input to 4 volts and adjusting the volume for unity gain (4 volts out):
Flux FA-12 measurements balanced headphone amplifier.png


I must say, I did not expect such excellent performance! Distortion is vanishingly low at nearly -140 dB. It is noise that limits SINAD to 117 dB and change:

best balance headphone amp review 2021.png


Noise is the limiting factor:

Flux FA-12 measurements balanced SNR headphone amplifier.png


most quiet headphone amp review.png


It is paradoxical that there is good volume control for sensitive IEMs but noise floor is rather high for such devices.

Company advertises bandwidth up to 1 Mhz. I don't test that high but the unit has very wide bandwidth:
Flux FA-12 measurements balanced headphone frequency response amplifier.png


Let's test all important power versus THD+N at 300 ohm:
Flux FA-12 measurements balanced Power into 300 ohm headphone amplifier.png


Performance is excellent in low gain but drops below the leaders in medium and high gain, producing less power at slightly elevated noise.

There is a bit more struggle with 50 ohm load requiring more current:
Flux FA-12 measurements balanced Power into 50 ohm headphone amplifier.png


We can see the load sensitivity better when we go through full suite of loads:
Flux FA-12 measurements balanced headphone amplifier power versus impedance vs distortion.png


Flux FA-12 Unbalanced HP Measurements
Switching both input and output to unbalanced we get:

Flux FA-12 Review Measurements Unbalanced headphone amp.png


Power naturally drops:
Flux FA-12 Review Measurements Unbalanced power 300 headphone amp.png


Flux FA-12 Review Measurements Unbalanced power 32 headphone amp.png


Channel balance is as perfect as it gets:
Flux FA-12 measurements channel balance R2R Relay headphone amplifier.png


Flux FA-12 Listening Tests
As usual, I start with my Ether CX 25 ohm headphone using balanced in/out. At first this sounded quite distorted. Then I realized I was playing with the volume control. Doing so causing stepping between levels which results in fair bit of distortion and some static. This happens more or less the same in all R2R implementations but I thought it was maybe a bit more extreme here. Leaving the volume control alone produced superb performance. I could cause the amp to clip but it would be quite loud by then.

Performance with the Sennheiser HD-650 using unbalanced in/out was just as good. Tons of detail, dynamics, etc. I could get the amp to limit out though earlier than others I test with lots of distortion. As with Ether CX though, this occurred at a level you would not use but which you may need with a higher impedance headphone.

As a practical matter, subjective performance was excellent.

Conclusions
By deviating from the current formula of "composite op-amps" I automatically assumed performance would not be good. Flux FA-12 proved me wrong, delivering extremely low distortion and noise. Not enough to land at the top of the class but far better typical boutique amps. You get slightly less performance than state-of-the-art but then you gain R2R volume control for perfect channel matching. Subjective performance is superb if you don't mess with the volume control while listening. Price is high but for that you get a nice package.

I am happy to recommend the Flux FA-12 balanced headphone amplifier. Good to see doing it your way can still result in performant products.

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Balle Clorin

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Nice; but are you saying adjusting volume causes click and pops? My Bottleheqd DIY stepped volume control manages to avoid that somehow, so maybe it could be fixed here too
 

ririt

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Thanks Amir for this nice review. Looking at your measurements, it seems to me that this amp would not fit well with a Headphones like the Dan Clark Aeon RT which has at 13 ohm impedance. Can you please confirm?
 

Doodski

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Makes me wanna buy it and run it without the cover. So cool looking.
Somebody(s) put a lot of effort into this bad boy and the power supplies look like they are good enough for a integrated speaker amplifier never mind a headphone amplifier. :D
 

AnalogSteph

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Some eye candy here for sure and it has discreet outputs.
Not sure about them being discreet but they do look to be discrete. :p

This whole "balanced in --> balanced out" / "unbalanced in --> unbalanced out" thing is weird. They certainly wouldn't have implemented the volume control twice if it's a relay-controlled resistor ladder job?

The power supply board is a bit peculiar... either they've got a built-in DC blocker or it is shared between primary (mostly bottom side) and secondary (most top side). This may or may not pass regulations. I think the power resistor is for soft start... seems a bit overdressed for a presumably sub-100VA xfmr, but hey.
 
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amirm

amirm

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Thanks Amir for this nice review. Looking at your measurements, it seems to me that this amp would not fit well with a Headphones like the Dan Clark Aeon RT which has at 13 ohm impedance. Can you please confirm?
No, it would be fine. It just isn't state of the art with such a headphone. At max distortion, it has a SINAD of 85 dB which should be good enough. It will produce whopping 6 watts of power at that SINAD! So you will be listening at much lower level and hence, distortion.
 

ririt

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No, it would be fine. It just isn't state of the art with such a headphone. At max distortion, it has a SINAD of 85 dB which should be good enough. It will produce whopping 6 watts of power at that SINAD! So you will be listening at much lower level and hence, distortion.
Thanks for clearing that up Amir!
 

Helicopter

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Thanks Amir. Wow. I was not expecting this level of performance. I don't like the aesthetic of this for the price point, but it is a good combination of features. A bigger knob with a nicer finish would look better.
 
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amirm

amirm

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Hemi

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Thanks Amir. Wow. I was not expecting this level of performance. I don't like the aesthetic of this for the price point, but it is a good combination of features. A bigger knob with a nicer finish would look better.

I absolutely agree about the knob. The stock one feels a little bit cheap. That's why I ordered a bigger one.

PXL_20210129_002025562.jpg
 

YSC

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woo this think looks impressive both technically and the class A discrete with this performance is impressive as hell to me! not to say it uses R2R volume
 
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