Hifiman EF2A Usb Headphone Amplifier

i bought the ef2a as a dac/amp for my work rig, hoping it would represent a nice step up from the portable dac/amp i was using at the time - the ibasso d4.
 
immediately out of the box, i was struck by its looks.  its an attractive little unit.
 
however, after a couple weeks of use, the dac/amp really started revealing its weaknesses. 
 
first, the dac section of this unit is nothing special.  comparing it with the d4, the d4 is superior, providing more instrument separation, more clarity, and less congestion in the lower mids and upper bass. 
 
and so i kept the d4 in the chain, and decided to use the ef2a as an amp only.  but, the amp section has problems as well.  the first of these is an unruly amount of gain.  so much gain, in fact, that the volume knob is rendered pretty much useless.  
 
by way of explanation:  let's say i'm using grado cans.  and i'm listening to mp3 using foobar.  i have my computer volume and foobar's volume set to max, as i should.  i turn on the ef2a, and begin turning up the volume until i am comfortable with the volume level.  the problem is, this volume level doesn't exist on the ef2a.  because comfortable listening level is blown past easily in the region where channel imbalance is still an issue on the volume knob.  and when i say channel imbalance, i mean it.  no signal in one ear until 9 o'clock on the dial.  and then suddenly the channels balance out, with raging volume at 10 o'clock on the dial.
 
so, what does this leave the listener to do other than lower the volume of the source?  and, as computer-savvy head-fiers know, this doesn't do any favors for the sound quality.
 
and that is a problem with the function of the amp.  i haven't gotten to the sound yet.  it has a few problems as well.  however, in fairness i haven't tube rolled the unit, so i can't say if some of the weaknesses of the amp can't be overcome with a little more money and effort.  (personally, i didn't feel tuberolling a $170 amp  would be fruitful.)
 
mid-bass, all the way through lower mids seem somewhat closed-in and congested, and the treble seems a bit brighter than neutral to my ears.
 
and, the amp doesn't really have a lot of power.  in fact, it's frustrating to me that it tries to disguise its lack of power with gobs of gain.  this makes for a frustrating listening experience, as i'm constantly toying with the volume level in foobar and on the amp to reach a listening level that doesn't seem fatiguing, and doesn't have that strident sound of cans being underpowered. 
 
in fairness, the ef2a sounds pretty nice with grado sr125 and sr225.  this is, in my opinion, a happy accident of headphone synergy more than anything.  so, if you're willing to cut the volume on your source by half, then the ef2a does play nice with those cans.
 
and so, overall, it really sounds as if i'm bashing the ef2a.  but, i can't help but expect a little bit more from it.
 
also, did i mention that the rca inputs are reversed as well?  yep, when feeding the amp from another source, i have to plug in the rca's in backwards to correct this.  however, that's just a problem with my particular unit.  but it's indicative, i think, of what you get when you go this route with hifiman...
 
a route i won't be taking again.

Yeah, I agree with The Wuss.  I am rather new to the headphone amp universe.  So, don't choose to (or not to) buy baseed on my review alone. 
 
I listening to the HiFiMAN EF2A through my Grados and a pair of Sony Studio Monitors I use for live music mixing.  The Sonys were instantly revealed as crap for critical listening.  The Grados came to life and quickly taught me what the term "muddy" was all about.  This amp reproduces the upper lows through the lower mids as though they are on top of each other.  These frequencies are fuzzy and sound as if the phones are being asked to play too loud without enough wattage, even at very low volumes.  Turn them down low enough to begin to resolve this issue and you sacrifice balance.  The left can is much louder.
 
This problem leaves you adjusting, constantly, the gain on the amp as well as the volume on the computer.  Unfortunately, each song is different so the search for the proper balance goes on and on.  But, every now and then, your music player shuffles you to a tune that matches the perfect recording with the correct volume levels and you get perfection!  The soundstage is huge, the highs (although usually bright) pick out the vocals and strings delivering a fleeting glimpse of what you were looking for when you bought this thing.  But, the lucky song ends and then it's back to back to your mixing job.
 
I am going to stick with it and try some new tubes to see what I can wring from this beauty.  The best comparison I can make is to a super-model stereotype.  Very pretty on the outside, but her personality leaves you wanting.  The more time you spend with her the more frustrated you get, knowing the relationship is doomed almost from the start.  But, you definately want your friends to see her before you trade her in.  (Maybe one of them will want her!)
 
 

Sorry if my english is not so good.
I bought this several days ago  (200 euro) so I cannot post an opinion about durability. 
I use this dac/amp just on  pc for everyday use: movies, voice, games, music, around 14-20 hours and I can say it's ok.
Looks well built, not heavy and not really hot. The ac adapter is a regular one, kind of big and unfortunately it came with US plug (the store Stereoplanet/Bucharest/Romania didn't tell me so it's a minus for them. When I called them, they try to sell me an 80 euro power adapter, telling me that it will sound better). No big deal, but annoying when you arrive at home and see that you cannot use the product. Luckily I have an us/eu travel adapter.
Windows 10x64 recognized the device instantly, no driver needed. I have a Gigabyte x99 gaming G1 motherboard which has 2 usb (marked with yellow) special powerd for dac/amp. If you have some, use it. I have compared with a normal usb (2 and 3) and the volume and sound quality is much better with the special one. It really has plenty of power.
I use this amp with Akg 712pro and Sennheiser 650hd. I use the amp volume set on 1 (it has 9) and the pc  set on 88-90%. The better sound I get on sennheiser then the akg where i find that the separation of bass and other instruments is not so good sometime, although the akg seems more warm and with some more bass. The highest bass and less treble I have it with Sennheiser momentum wireless 2.0 over detachable cable. I will try it with  Grado r2e (not owned yet). I mentioned treble. Yes, with the first two headphones is a little too much, but not so annoying.
If you feel that the amp doesn't have much bass you can add some more from "playback devices/properties/enhancements"where you can found bass boost, virtual surround, etc. I like it better without enhancements.
In terms of looks, it's average. The tubes are inside the box so it doesn't light much. No night view. I hate the silver front panel. Black (like the rest of the box) it would have been more interesting. The on/off led is blue, so it doesn't match the tubes light. Orange was the right colour for me. Black and orange.But this is question of taste.
Until now,I have no issues with the volume knob or sound crackling.... It workes ok.
I did some comparison with woo wa7, carrot one fabriziolo (own and sold) and some solid state ( matrix quattro 2, matrix hpa3, fiio k5/e17k-I own). The woo wa7 (next to buy) was much, much better in terms of everything, but is 6 times more expensive and I think it worth. Carrot one is a small italian (china)1 tube dac/amp class d, if I remember right. It has an interesting looks (better than hifiman),nice and powerful sound, but ef2a is much powerful and the sound quality is better. The separation between voice and instruments is better. Carrot one does have more bass but the treble is the same. Price is the same.
The Matrix (both of them) doesn't impress me at all. Although more expensive (2-3 times), their sound is plain and dull, like a regular pc soundcard. So I think ef2a is better. More colourful.
The combo Fiio k5/e17 (better than matrix) is kind of similar in terms of price (50 euro more). Fiio is not that powerful but it has a very nice sound, warm, some bass and with just the right amount of treble. Ef2a sound is slightly different, probably is the tube flavor. I own them both and cannot say I can replace one with another.
 
 P.S. the wa7 doesn't provide the plug cord from their ac power source to wall plug so you have to use a pc one ( at least the model I've seen). Annoying for that kind of money.

Changed the tube to mullard ones. Sounds better IMO.

Hifiman EF2A Review
 
Overview:
This is my first headphone amp review and I'm just going to review the amp section of this headphone amp as the DAC is nothing special, a bit better than most computer or portable device DACs but is easily outclassed by any decent dedicated DAC or soundcard. I've had this amp for a while and I tried it with a variety of AKGs, Senns, Beyers, as well as a few other miscellaneous headphones. I fell in love with this amps sound from the get go and it was my first desktop amp. This is an amp I have owned twice, the first one died after a couple months of use but I later found it was a returned and defective unit and I was able to return it for a full refund with no shipping cost to me. I later got the Schiit Magni and I disliked how it sounded in comparison, the Magni was obviously a level below this amp and it became more obvious with a better DAC and headphones and this was confirmed when I got my second EF2A. I am using this headphone amp with the HRT Music Streamer II+ USB DAC set at 24/92 connected with Auric RCA interconnects. And the amp is mainly used with my AKG K712 Pro, AKG K612 Pro, and the Sennheiser HD 545. I have noticed there is a channel imbalance where the left become louder than the right at levels below my preferred listening level. This review is a bit of a work in progress, I'll add more later.
 

 
Tubes/Sound:
I did some experimentation with tube rolling. Much of how this amp sounds come from the tubes, so stating what tube I am reviewing based on and talking about the tubes traits in the amp is important for a review like this. I came to the conclusion the Radiotechnique(RTC) 5654RT tubes are the best sounding tubes I tried on the amp for most of the headphones I tried with the amp. The tubes generally run about $20 for a matched pair. The tubes have big and powerful dynamics, if you use a more intimate headphone like the AKG K240 MKII the dynamics may be too strong on some songs. There is a real sense of physical presence and tactility with the tubes, you don't just hear the sound/instrument, you feel and hear the sound/instrument with a life-like realism I haven't heard on any other tube or amp I have owned. There is a roundness, body, and depth to the sound that is something really special, especially in an amp this price. The tonal balance is also spot on but there some grain in the sibilants(I think this is more amp related than tube related). These tubes are clear and transparent with just a touch of pleasing warmth. These are incredibly musical and enjoyable tubes, especially with the AKGs I paired it with, the K712, K612, and K240 MKII. They also sounded good with the Sennheiser HD 545, Sennheiser HD 555, Sennheiser PX-100 ii, NVX XPT100, Beyerdynamic DT 990 Pro, but the AKGs are just something special with this tube/amp pairing. The Sennheiser HD 380 Pro with these tubes in the EF2A is a horrible match, excessively closed-in, collapsed soundstage, and bad dynamics and was the first and only bad pairing I experienced with this tube/amp combo. The K712 is the best pairing I have heard on this amp, it's simply beautiful sounding with big powerful dynamics, a strong physical presence, transparent sound with a black background. It doesn't seem underpowered or held back at all given the RTC tubes are used.
 

 
 
Conclusion:
This is a great and versatile little amp given you are willing to invest in different tubes. It's a fun and musical sounding amp that doesn't lack power and pairs exceptionally well with some headphones. If you own AKGs similar to what is mentioned in the review and likely others this is probably one of the best amps out there for them under $200.