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Hardware Teardown of Hifiman EF2A DAC and Headphone Preamplifer

amirm

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#1
Pre request in my review of Hifiman EF2A and kind permission from its owner, this is the teardown review of the unit.

Let's start with the main board's top view:

Hifiman EF2A DAC headphone amplifier teardown main board.jpg


The heart of the unit is the now quite obsolete TI/Bur-Brown PCM2702 DAC chip. It came out circa year 2000 and only supports 16 bit audio at 32, 44.1 and 48 kHz.

The unit has analog input which I think goes through the OPA264 op-amps.

untitled-192814.jpg


Surprising to see is the Philips TDA1308 headphone amplifier.

untitled-191529.jpg



It is being used apparently to then drive the tubes. In other words, half-way through the signal flow we already have our headphone output but then we choose to mess with it with later stages. :) The headphone amp likely has nice gain and hence the reason it over-drove and clipped the input stage when using USB DAC.

The output of the headphone amp goes into pair of pentode tubes. The one that I have reportedly has aftermarket 5654RT pentode tubes.
untitled-192403.jpg


A few beefy TO-220 2SC/2SA complimentary power transistors drive the headphone output:
untitled-190844.jpg


No wonder this thing has lots of power.

Capacitors appear to be top brand names from Nichicon and United-Chemi-Con:

untitled-191042.jpg


untitled-191752.jpg


The markings seem genuine. And so do the markings on other parts. The could still be counterfeit but if they are, it is not easy to see.

Not so good is seeing hand modification instead of spinning the PCB board and fixing the design issue:
untitled-190659.jpg


I am sure pulling the leg of that transistor way to the right over the other parts is against the Geneva convention for humane treatment of electronic parts. :D

Outside of that, the top side seems fine. It is the bottom side that gives me pause:
untitled-190720.jpg



untitled-190732.jpg

This thing screams dirty, sloppy, cheap, PCB and construction. You can see those white stains. And lots of mods with resistors and such hand soldered there.

Worst example is hand soldering tiny surface mount parts to what is a through-hole design:

untitled-192904.jpg


untitled-192922.jpg


All of this can be cleaned up with revising the PC Board and including the small few changes needed to mount things properly. Instead, they are paying people to sit there and hack the thing manually. Cheap labor is no doubt the enabler.

Here is the transformer feeding the thing:

untitled-182933.jpg


It produces AC which is then rectified and regulated internally.

Conclusion:
The design here while following the typical hybrid tube architecture (tube buffer but transistor output) but also odd choices like headphone amplifiers driving the tubes. The choice of super obsolete, 16-bit DAC is pretty bad. They must be getting these for pennies or something.

The level of hand modification is excessive and the back of the board quite ugly.

Clearly this is not something to be proud of. On the other hand, it is what it is for the price.

As always, questions, comments, corrections, etc. are welcome.
 
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#2
Looking at them soldering compared to that NASA rulebook makes me drop a tear or two.
 

amirm

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#3
I should note that there were copious amount of hair and other foreign objects in there. Due to use of tubes and the heat they generate, these are open-case designs allowing all kinds of junk in there. At least I am hoping that is the case and the unit was not shipped with all that in there!!! :eek:
 
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#4
Hifiman should just stick to headphones, though this is one of their very early products.

Make you wonder how is the amp for that fancy ole Shangri-La.
 

amirm

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#5
Pretty sure this is what we call a "phoned in design." You call a Chinese companies to make you a tube amp and they do. No supervision is made to make sure it is quality product or that it has good design.
 

Valorum

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#6
The heart of the unit is the now quite obsolete TI/Bur-Brown PCM2702 DAC chip. It came out circa year 2000 and only supports 16 bit audio at 32, 44.1 and 48 kHz.
Looking at the Amazon reviews, the first is from 2009, so I think this unit is already quite old itself.

I appreciate you taking it apart and analyzing the components and build quality. I'm impressed by how many high quality components they appear to have used. It's a shame they aren't implemented more intelligently.
 

Jimster480

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#7
Pretty sure this is what we call a "phoned in design." You call a Chinese companies to make you a tube amp and they do. No supervision is made to make sure it is quality product or that it has good design.
It could indeed be a phoned in design. Its pretty weird to see those after-production corrections?
 

Blumlein 88

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#8
Can you tell how they are using the pentodes? Are they connected as triodes?

I don't have one, but for those who do, could you lower gain of the headphone amp in the middle or install some attenuating resistor pairs to prevent the overload that is happening.

Oh, and your photos seems to have very high contrast, it makes the ugly bits seem harshly edgy, cold and extra ugly vs some nice softening of the images. :cool:

Oh, and a 2702 DAC chip isn't obsolete. It is a classic DAC design befitting of being paired with tubes. ;)
 
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amirm

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#10
FYI I was about to ship the unit back to the owner but had to do one more measurement. Powered the unit up and one channel was not working! :(

Cost me a couple of hours tonight to track it to really, really crappy soldering job in the volume control and the connecting cable to the main PCB. It is some cheap material (both PCB and cabling) that solder almost doesn't want to adhere to it even with added flux. I got it working but looking it over, there are tons of pins where there is inadequate solder.

Please, please stay away from this device. It is way below acceptable quality bar.
 
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