Aune BU2

corgifall

500+ Head-Fier
AUNE's little Powerhouse
Pros: Great sound quality, easy to carry around, decent battery life.
Cons: 2.5mm over 4.4mm, big for a portable, volume knob not always easily accessible
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I do love portable DAC/amps since I can use them on the go but as a desktop alternative when I go on the occasional trip with the fiance or back home to visit family. I don’t get to do these trips as much these days but I still take my portables to work to use in a lab or at my cube everyday. My most recent portable review was the iFi GO Blu and that has been my go to portable DAC/amp for a bit now. I was excited to get the chance to check out the BU2 from AUNE. The BU2 is using ESS ES9318 DAC and is designed to both be a portable DAC/amp but has the option to use the device as a desktop alternative. It comes in at $350 USD.


Quick shoutout to AUNE for hooking me up with a review unit. While I always appreciate stuff being sent in to test and review, It never affects the rating of my review.


Info and purchase links on the AUNE BU2 can be found below
http://en.auneaudio.com/index.php?s=Home/Article/detail/id/514.html


Gear used​

Lotoo PAW S1, iFi GO Blu, Shanling M3X, IKKO OH10, THIEAUDIO Clairvoyance, UM MEXT, Moondrop Variations and Sennheiser HD560S.


Looks and Feel​

The BU2 is a little larger than what I was expecting for a portable. The BU2 isn’t super thick but is a little heavy since it’s all metal. It has a nice matte black finish with low glare. There is a little display to show basic info such as battery level, volume level, filter and whether it's being used in portable or wired mode. The volume knob is tucked away under a shroud to stop accidental volume increases. This does make it a little harder to do volume adjustments with one finger but I have small hands so it wasn’t difficult for me to adjust the volume. The same volume knob sticks out the front just a little and acts as a button to choose between the options or turning off the unit. I didn’t see a reset button on the device and I haven’t had any issues that would need a reset but I’m not sure how to do a reset should something happen.
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Accessories and unboxing​

There is a nice set of accessories that come with the BU2. Inside the box we get the quickstart guide, business card and a box of goodies. These include two rubber rings, short USB-C to USB-A charging cable, AUNE branded L shaped USB-C cable(fabric but stiff), a USB-C to Lightning male adapter and finally a 2.5mm to 4.4mm pentaconn adapter for those of us who use 4.4mm.
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Sound(overall)​

These final impressions were a mix of Bluetooth and wired via my Apple iPhone and iPad Pro. This will be what the BU2 sounded like with all the headphones I used. Things like headphone pairings or going wired will produce different results and impressions vs what my ears hear on my specific gear.


The BU2 has a really nice detailed but slightly warm sound signature. The lows have a good amount of energy for a lower power output number amp. Bass always felt full and detailed. There was extra weight to the thumps and rumbles which surprised me. The mids were accurate but they sounded a little more neutral to me. It was able to bring out good vocals on IEMs that I rely on to test vocals. Treble is fairly sharp and accurate. I do believe this is due to the ESS DAC being used with the BU2. There is just a little extra bite at the end of instruments from the upper mids and up. I was able to seek out small details I listen for when reviewing gear just fine with the BU2. Not quite desktop level detail but still very good for a portable. Overall I was extremely impressed with the sound the BU2 produces.


Filters​

The BU2 has ESS DAC filters to choose from. It claims the default is the “apodizing” filter so I left it set to that. Plus that’s what I use on my desktop ESS DAC. I will admit I totally can’t hear any difference between filters. This applies to pretty much all ESS and AKM DACs I’ve tried in the past. I’ll think I hear a difference but then realize I can’t hear the same difference later on when switching filters. The option is there which I’ll take over not having any filter options at all.


Imaging/Soundstage​

The soundstage and imaging tend to be headphone specific(at least to me) but DAC/amps can add a little extra on occasion. The BU2 does have what I feel is just a little more space overall when it comes to the soundstage. I would say just a little wider and deeper than a normal portable DAC/amp soundstage. Imaging was accurate which I expect from a source device.


Battery life​

Aune claims 9 hours of battery life when wired. I never got around to doing battery testing but it survived a good 6-7 hours of use throughout the day at work with 3 bars of battery life using bluetooth so I absolutely believe the wired battery claims. I usually charge my DAPs and portable DAC/amps overnight so I only did a 30 min charge test from the half battery point. I got 1 battery bar(out of 4) back from the charge test so not super accurate but something.

Bluetooth/Wired connectivity​

Wired does sound better to my ears but I mostly used this via Bluetooth. I like that the BU2 had both a port for data and one for charging. I know some would rather have a single port but from past experience, I personally have less issues when the ports are separate. I did run the BU2 Bluetooth between my iPhone 12 Pro in AAC mode and off my Hiby R6 in LDAC. AAC was a decent range but after leaving my bedroom I ran into cutting out issues. I’ve never used anything with LDAC that didn’t need the source device within a few inches to stay stable. Same thing here. Only took moving a foot away for the connection to start cutting out. I thought the Bluetooth range was acceptable and since I keep my source iPhone or DAP with me, I had no connection issues with the way I use Bluetooth.
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Personal grips with the BU2?​

I have more nitpicking issues than real issues with the BU2. I don’t really care for the display that slowly cycles through each letter of the AUNE name or Bluetooth codec being used. It constantly catches the corner of my eye if it's sitting near me. Not the end of the world and it’s easy enough to flip the BU2 over.


The volume knob is hard to adjust unless you have small fingers(like me) on a flat surface. I do however like that it’s designed this way to keep accidental volume adjustments from happening. As a portable unit I do think this is a good design decision but I wish the tiny nub at the end of the volume pot stuck out more for easier adjustments. I used an iFi rubber foot from the hip dac to extend it out and I love having it that way.


Lastly, I need to talk about the 2.5mm balanced jack. I would have preferred to see a 4.4mm jack since there seems to be enough space to fit in on the case. I do use IEM cables with swappable connectors but I hate having to swap out unless I have to for testing. I actually use the BU2 in a rotation as a daily portable DAC/amp, so the need for a balanced jack that I use with my other devices is preferred. They do include a 2.5mm adapter to 4.4mm and I use that alot but I still worry about snapping the 2.5mm pin when I have the BU2 in my pocket.


Single ended and balanced power output​

We have a single ended output power of 100mW into 32ohm and the 2.5mm balanced output power comes in at 265mW into the same 32ohm load. Both this and the iFi GO Blu really threw my on paper power number expectations out the window. I was really impressed with the power output on IEMs and just a few not power hungry full size headphones I tried. The BU2 won’t be a good pairing with hard to drive headphones but for IEMs I think the power output is just fine.

IEM pairing opinions​


IKKO OH10​

The OH10 actually did get a slight bass bump but I felt it was well controlled. The bass on the OH10 seemed to be least affected of all the IEMs I used. The mids are somewhat recessed on the OH10 and this stays the same with the BU2. It’s just nothing special, vocals wise. The treble also didn’t seem to be very improved either. There was just a little better detail vs this paired with the GO Blu but I didn’t notice a big difference when it came to the OH10. I also don’t think the OH10 really scales all that well.
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Moondrop Variations​

The BU2 boosts the frequencies the Variations does well without adding too much. The lows get more slam but it doesn’t sound boomy or bloated. The mids keep the same flavor that variations have with a little more of an energy spike in the upper mids. Treble gets a little extra sharpness but it’s never fatiguing to my ears. Staging is just a bit deeper but width stays around the same. I liked this pairing the most out of the three IEMs I listed.
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THIEAUDIO Clairvoyance​

The Clairs get a little extra warmth from the pairing as well as a little more impact from harder hitting notes. The mids continue with the warmer sound but never sound muddy or veiled. The treble has a little extra energy up top from the ESS DAC being used. It gives that extra bit of sparkle up top and I enjoy the little added flavor the BU2 brings to the Clairvoyance. The Staging was about the same as the other portables I’ve used. Possibly a little more width and depth but nothing crazy. A good pairing for sure.
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Over ear pairings​


Sennheiser HD560S​

While the HD560S has grown on me, I still find it really boring sounding. It works well in picking up what a DAC/amp is doing in terms of sound signatures through which I do like. The BU2 might be the first portable to add a little bit of much needed low end thump to the 560S. I was very surprised I got an actual impact/slam when the music called for it on the 560S paired to the BU2. The mids still sounded as detailed and boring as I expected this headphone to sound. The treble had just a little more brightness than I would like but it wasn’t fatiguing on long sessions. Just a reminder the ESS DAC was nipping at the heels of the treble. Staging has a slightly better open sound with this pairing which is fairly nice as I don’t find the 560S very open sounding for how much outside sound you can hear. I wouldn’t actually use this pairing outside of my home but it was still a good pairing for maybe someone looking to use the BU2 as both a portable and desktop setup.
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DAC/Amp comparison​


iFi GO Blu​

Gonna be lazy and paste the comparison from my GO Blu review below

“The BU2 is better at detail retrieval and I personally like ESS DACs for that extra sharpness which is very much noticeable on the BU2. The BU2 also has a nice dynamic bass and crystal clear mids. The big differences here will be the size and unit outputs. The GO blu has a more powerful 3.5mm output and the BU2 has just a little extra power out of the 2.5mm jack. That will be the main reason I recommend the GO blu over the BU2. While I like the overall detail retrieval from the BU2, I think 2.5mm is mostly dead and I prefer my units to use 4.4mm Pentaconn these days. Plus size is a big thing IMO. The GO blu is easy to take around. The BU2 is about the size of some smaller smartphones so it takes up more space.”
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Lotoo PAW S1​

So the PAW S1 was the first thing I thought of when I first heard the BU2. I loved the better bass presentation but with a good detail retrieval and slightly bigger stage I heard with my trusty S1. If that sounds like my sound impressions for the BU2 from earlier, that's because they sound very similar. I think the BU2 takes everything that's great about the S1 just a notch higher in sound quality but the big thing here is that the BU2 has an internal battery so you don’t have to worry about sucking a device dry when using the BU2 wired. Which is better? I can’t choose between the two and I usually use the S1 with my iPad when I want wired and use the BU2 with my iPhone via Bluetooth. Both are great devices but the BU2 has more useability IMO.
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Shanling M3X​

I recently received the M3X for review and decided to sneak in a comparison. While the M3X is a DAP, it does come in just $20 more than the BU2. The biggest difference in sound signatures between the two is the treble. The M3X goes for a warm-neutral sound and doesn’t do anything exciting with the treble or detail retrieval. The BU2 has the extra spice and perceived detail retrieval up top. Staging was about the same and both power output numbers were close to each other(BU2 20mW higher). The big thing here will be battery life and what your source device situation looks like. Battery life is claimed 9 hours wired via the BU2 and the M3X wired is 19 hours from using the balanced jack. You also get android on the M3X so you get a place to store your music should you not have enough storage on your source device. The drawback is that the BU2 is using Android 7. The BU2 5 years from now will work perfectly fine with any source device. The M3X might not have any streaming apps that are still compatible 5 years from now. Both are great devices and come in around the same price. I would choose the BU2 if you have a good source device with the music you want. Get the M3X if you need a place to store your music or don’t want to waste the battery life on your phone throughout the day playing music.
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Overall thoughts​

This was the first product from AUNE I got to experience in recent times and I’m very impressed. Outside of my little nitpicks with the BU2, I really think this is an awesome portable DAC/amp. I will be giving the BU2 a full recommendation. I really liked the sound I got out of the BU2 and while it won’t replace the iFi GO Blu for every occasion as my main daily portable, it will be staying in rotation as one of my portable solutions for a long time. I look forward to checking out more AUNE products in the future. Thanks for reading!
J
jmwant
Thanks for the review. Do you think it can drive the 6xx?
corgifall
corgifall
I imgagine it would if run balanced. Might not be quite enough power to really get those drivers moving well compared to something with a higher power output. You should be able to hit a comfortable volume just fine though.
A
anjaudiofile
No mercy for the 560s!!

SenyorC

100+ Head-Fier
A good companion!
Pros: Build, aesthetics, functionality, sound
Cons: Difficult to use volume knob with one hand, 2.5mm will not please everyone.
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The Aune BU2 was kindly sent to me for review by Aune. They have not requested anything but I will leave a link on my blog and YT channel to the BU2 via their webstore as it is the least I can do.

This means that my review will, as always, attempt to be as unbiased and sincere as possible but you should always consider the fact that it has not cost me anything to try out the BU2.

Intro…

As I mentioned in my recent review of the Aune Jasper, a set of IEMs that pleasantly surprised me, Aune is a brand that has released quite a few DACs and amplifiers that have interested me, although I have not had a chance to try out their products until now.

The original BU1 was (is) a portable DAC/Amp that attracted me aesthetically, as did (does) the B1s, so when they announced the BU2, I was instantly interested in trying it out.

The BU2 is a fully balanced DAC/Amp, using two ES9318 DAC chips along with four independent amplifiers, a R2R ladder volume control and supports up to 32-bit/768kHz PCM with native DSD512 decoding.

All of this sounds great on paper but I am interested in what it sounds (and performs) like in the real world, so I have been putting it through its paces for around 3 weeks.

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Presentation…

The external packaging is a simple white cardboard sleeve, showing an illustration of the unit and a few features, from which a black box, sporting the Aune logo in silver, slides out.

Inside the box we get the BU2 in its own cutout (protected by a plastic bag), along with a smaller box containing a USB-A to USB-C cable, a shorter USB-C to USB-C cable, a 2.5mm TRSS male to 4.4mm Pentacon female adapter cable and a very basic, single page, user manual in English and Chinese.

There is nothing special about the presentation but it is nice that they choose to include the 2.5mm to 4.4mm cable for those who use Pentacon connectors on their headphones/IEMs.

Build and aesthetics…

If you are someone, like me, who likes the aesthetics of the BU1, you will find that the BU2 aims for a much simpler look. A completely black unit, with a small LCD screen on one side and the connections on top and bottom, this does not stand out as much as the BU1 (or B1s) but is certainly not ugly.

The build is completely aluminium except for the two panels around the screen, which are plastic to avoid issues with Bluetooth reception, and even these are well made and do not stand out as being plastic (at first I thought they were aluminium also). This makes the unit very sturdy and feels like it will withstand quite a bit of abuse, especially if we also take into consideration that the volume knob (also metal) is protected by a metal guard (more on that in a moment).

I know that there will be people who complain about Aune opting for a 2.5mm balanced output instead of a 4.4mm, which I can understand. Personally, while I would like the 4.4mm connector, being more robust than the 2.5mm connector (then again, the 6.35mm TRS is also more robust than the 3.5mm but we don’t expect that on a portable unit), I don’t actually have 4.4mm connectors on any of my cables, so I have no real issue with this. As mentioned, Aune also included the adapter which is a nice touch.

At the end of the day, I really don’t have any complaints about the build quality or aesthetics of the BU2. Yes, the BU1 looks more impressive but the BU1 is simple and very well built, with no signs of flaws to my eye.

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Functionality…

The BU2 is very simple to use. At the bottom of the unit there are 2x USB-C sockets. The right hand connector is for data connectivity, just connect a cable between the USB output of whatever source you are using to this socket and you are good to go.

The second USB-C socket, on the left, is for charging. Here there are going to be people who complain that it is a separate socket for charging, meaning that you need to use two cables, however, personally I much prefer this. This means that you can use the BU2 (connected to a phone for example) without draining the battery of the phone, and can recharge the BU2 from a power bank while it is in use.

On the top of the unit there are the two headphone outputs, one 3.5mm TRS unbalanced output and one 2.5mm balanced output. These are pretty self explanatory.

In the center, between these two outputs, we find the volume knob which is my only real complaint about this unit.

The knob is recessed and has a metal plate that surrounds it to protect it. I can understand that this was done intentionally to avoid accidentally turning up (or down) the unit while it is in a pocket but it makes volume adjustments rather difficult. I thought about putting this in the “build” part of the review but to be honest, there are no issues with the build, the knob works great and is very robust (and extremely well protected), it is just a pain to adjust without using two fingers, one on each side of the unit. While you are holding the unit, this is not a huge issue, ok, it takes a little longer to make large adjustments, but once you have the unit flush against something, such as the back of a phone/DAP or on a desk, you can only really access one side and it is difficult to make adjustments at all. After 3 weeks of using the device daily, I have sort of got the hang of it, managing to adjust it with only one finger on one side, but in order to do this, the device needs to be secure, otherwise it will just move around instead of the volume knob turning (like when it is on a desk for example).

This is more irritating than anything else and the actual quality of the R2R volume adjustments far outweighs the pain of the usability, but is still irritating nonetheless.

Speaking of the R2R volume control, it is excellent. After having issues with digital volume knobs skipping and analog volume knobs having channel imbalance at low levels, Aune seems to have hit the nail on the head with this setup. There is no channel imbalance even at the lowest of settings and the volume steps are smooth and consistent, allowing you to select the exact volume level every time.

The volume knob also acts as a button, when pressed from the top. This allows you to power the device on and off, with a long press, and also enter the menu which can be seen on the screen.

The screen is placed on the front panel of the BU2 and is a small simple LCD screen with white letters. I must say that I like the choice of screen very much. There is no need for a huge colour screen that eats up battery life, the small LCD screen shows the necessary information without adding to the power consumption (at least not enough to be relevant).

When powering on the BU2, by a long press of the button, you are greeted by the Aune logo followed by the current volume level which, by the way, is remembered from the last use. After this, and at any other time when not interacting with the device, the screen goes into screensaver mode, which is basically the Aune logo followed by a scroll of the letters (A then U then N etc.). This is again a nice touch as it shows that the unit is powered on but does cut the power usage of the screen even more and avoids screen burn to some extent.

By a short press of the button (with the unit on), the device cycles through the menu options.

The first press brings back up the current volume level for reference. The volume can be set from 0 (mute) to 60. If the device is in screensaver mode, a turn of the volume know will also automatically bring up the volume screen for reference.

Another press takes you to input selection, where you can choose between USB (shows a USB logo) and Bluetooth (shows Bluetooth logo) by turning the volume knob. There is no need to press to select, it will automatically keep whatever selection is shown on screen and revert back to screensaver mode after a few seconds.

Two clicks of the button takes you to filter selection. There are 7 filter modes to choose from, each represented by two letters on the screen. The filters are:

SC: brick wall filter

SU: hybrid fast roll-off filter

SL: apodizing fast roll-off filter (default)

SI: minimum phase slow roll-off filter

SH: minimum phase fast roll-off filter

SE: linear phase slow roll-off filter

SD: linear phase fast roll-off filter


Three presses of the button will bring up an image of the current battery level, represented by an image of a battery with bars showing the remaining power and one final press will take you back to the volume level.

As you can see, this is very simple to navigate and use, making the screen useful without the need for more information.

As far as battery life, the specs claim 9 hours in USB DAC mode and I must say that I have no reason to doubt it. I got more than that but I do listen at low levels.

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Power…

According to Aune, the BU2 offers 265mW @32 Ohms via the balanced output and 100mW @ 32 Ohms via the single ended output. This is obviously not a huge amount, especially compared to some of the alternatives, in fact, the unbalanced output is the same as the S9 Pro and the balanced output is only slightly more than the usb powered dongle.

This means that some of the more demanding headphones, such as the HD6XX or the HE400se, are lacking a bit of power to drive them to their best. However, I must say that I don’t find the BU2 to sound “anemic” even with these headphones. I have found with other devices with low power outputs that these headphones can lack, especially in bass response, but when powered by the BU2 they don’t seem to suffer as much from these issues. With easier to drive headphones, such as the Ananda, I didn’t feel that there was any issue at all.

With IEMs, there is plenty of power, even with those that need more than usual, such as the iSine or the Dusk, with no complaints from me at all.

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Sound…

As you probably already know, I don’t have the necessary equipment to provide measurements of DACs and amplifiers, all I can do is share my experiences with the device. As with all devices, and all people, it is possible that I hear things that my brain makes me hear which maybe don’t exist, but at the end of the day, what I hear is the important part, not what I should hear.

In this regard, I must say that I have spent plenty of time comparing this to various other setups that I have and I find that the BU2 is basically identical to my Modi3+ and Atom that I use daily on my desk at work.

I mean this as a compliment as I find that the Modi+Atom combination is one that I really like for my daily use. It is a clean setup that does not become fatiguing for me, even after many hours at my desk. Obviously the Atom has more power and can drive things like the headphones I mentioned above a little better, but at my usual listening levels, with IEMs and some of my easier to drive headphones, the BU2 could easily replace the Mod+Atom stack on my desk, taking up less space and easily fitting inside my laptop bag for portability.

Even with the Beyerdynamic Custom Studio that I keep at work for when I need a closed back headphone, the BU2 can drive these 80 Ohm / 96dB/mW headphones from its single ended output to volume levels that are plenty for me, without me feeling that I am missing anything.

I mentioned above that the output power of the BU2 is similar to the S9 Pro, however, the BU2 is much smoother and does not become harsh like the S9 Pro can when pushed.

One last thing to mention is Bluetooth. As some of you probably know already, I am not a huge user of Bluetooth and when I do use it, I usually opt for using LDAC either via one of my DACs or via my neckband. The BU2 does not offer LDAC as a codec, being limited to SBC/AAC/aptX and aptX HD, however, I have found that I don’t miss LDAC quite as much on this device as I do on others. Usually I have the BU2 connected to my source (laptop or DAP) via USB and paired with my phone via bluetooth. If I am listening to music and there is something I want to listen to on my phone, with two clicks and a turn, I can swap to my phone. The connection is very quick and once I switch back to the USB input, the BT connection is broken, allowing my phone's BT to go back to whatever else it might be doing (such as the neckband that I use for calls etc.).

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Conclusion…

I don’t think I have mentioned the price of the BU2 yet. It costs around 270€, depending on where you buy it from, which is obviously quite a bit more expensive than something like the S9 Pro, however, I feel that it is also a much better product and does not drain my phone or DAPs battery (something very important for me).

As I mentioned, I find the sound to be identical (to my ears) to the Modi3+ and Atom, which is a desktop stack (although small) that comes out at around the same price (here in Europe) but is obviously tied to a power supply. The BU2 is not as powerful but in my opinion makes up for it with its usability and portability, along with a great build quality.

There are obviously things that could be improved (I have yet to find the perfect product at any price range) but my only real complaint is with the difficulty of using the volume knob with one hand.

As someone that has been looking for a self powered, portable and well built DAC/Amp, the BU2 fits my needs very well. I had the Topping NX4 DSD for some time but it finally broke under warranty and I did not replace it. I feel that the BU2 solves the few complaints that I had with the NX4 and I am happy to have finally come across such a device which, while not cheap per se, is not ridiculously expensive.

This review, along with all my reviews, is available in Spanish on my blog here and on YouTube here.
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captione

100+ Head-Fier
Smooth Sailing...?
Pros: - Smooth neutral sound profile
- Flexible with gears of various sound signatures
- Adequately powerful SE/BAL output
- BT Reciever mode
- Great battery life
Cons: - Buggy UI
- No error/troubleshooting documentation in manuals
- A bit bulky for a portable DACAMP
- No LDAC support
- A one button-knob solution would render the unit useless if broken
Aune BU2 Review

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TL;DR: Portable DAC/AMP. 300 USD. SE/BAL. Adequately powerful. Smooth neutral sound. Features Filter Modes and a BT receiver mode. A bit buggy. No error documentation. No LDAC Support.

Disclaimer: Aune sent this BU2 as part of their tour here and is not mine to keep. I will be fully transparent and honest for most of this review so that Aune can further improve the unit, firmware side, and their other releases.

  • Packaging
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Inclusions:
- Aune BU2 unit
- Short USB-C to USB-C L-plug cable
- USB-C to USB-A cable
- USB-C to Lightning Adapter
- 4.4mm to 2.5mm Adapter
- Rubber Baller for stacking


The Aune BU2 comes in a 2-part box with a sliding cardboard cover that shows its marketing. Inside the velvet-lined box is the Aune BU2 in a silky cover presumably to avoid scratches, a box section containing all the accessories, a manual and a contact card. I like the inclusion of the 4.4mm to 2.5mm for people without 2.5 BAL so usage with it would be more convenient, and is also well built.
I also noticed that some of the accessories are branded with different names (Ugreen for the USB-A, Zephone for the Lightning adapter) except the rubber baller and the USB-C to USB-C cable, just something to take note of.

  • Build & Ergonomics
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The Aune BU2 has a hand-sized rounded silhouette and precision-milled aluminum body with a smooth black finish that is built like a tank. The flush fit button-knob, 3.5mm and 2.5mm is placed on top while the USB-C charge and audio input is at the bottom for easy plug access when stacking on phones. Though I wish the BAL port was of stronger formats like 4.4mm.
The knob is hidden in the silhouette with a cutout on the sides to turn the knob, it helps prevent accidental turns.
The BU2 also has a small OLED screen that shows the volume level, BT/USB mode, filters and battery level.
Handling and portability-wise, the BU2 is a bit heavy to rubber-stack on my phone so I mostly used it on my desktop or the BT receiver mode for phone usage with the BU2s on my pocket, but a phone stack is still fairly pocketable with BU2 if you wear loose pants. The knob placement is kind of bad when you stack it into phones because the knob lays flat and flush on the body so it’s hard to “slide” the knob on the cutouts when you’re adjusting or pressing something. I also worry that the one button-knob solution can turn your unit into an instant brick when it breaks, something to take note of when accounting for long term usage.

  • Functionality
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The Aune BU2 is a DAC/AMP that’s not only capable of having USB input but can be also used as an APTX-capable Bluetooth receiver for wireless convenience which I mainly used them for.

It can decode up to 32bit/768khz PCM and DSD512 if you fancy it. You can control the menu and the volume with the knob, and you can access the volume control, USB/BT mode, battery level and filter mode on the small OLED screen. The volume control has 0-60 steps and sports an R2R circuitry for accurate volume adjustment and perfect channel matching. As for IOS compatibility, I haven't tested since I don't own an Apple product, although you can connect with BT mode albeit limited with Apple's BT codec support.

As for usage, the BU2 usually lasts 7 (DAC/Bluetooth mode, BAL) to 9 hours (DAC/Bluetooth mode, SE) after it shuts down automatically at low battery, presumably to increase longevity of the battery. It also takes 5 hours to charge with a normal non-QC charger.

Here’s the list of the filter modes for various sound fine-tuning (if you can hear the difference):
  • SC: brick wall filter
  • SU: hybrid fast roll-off filter
  • SL: apodizing fast roll-off filter (default)
  • SI: minimum phase slow roll-off filter
  • SH: minimum phase fast roll-off filter
  • SE: linear phase slow roll-off filter
  • SD: linear phase fast roll-off filter

Now for the issues within my almost-one month of testing:
  1. It has an error warning that isn’t described in the manuals. Since there’s no diagnosis on the error sign, I presume it’s referring to the battery level or the USB/BT connection.
  2. Following that error warning, the unit’s OLED display just stops working or glitches out after >30 mins of use. I had to troubleshoot by turning it off and on repeatedly or plugging it in and out of charge just to see if the OLED screen blinks or works. Fortunately it only happens when that error warning shows and the occurrence isn’t common.
  3. No LDAC codec, considering its price and functionality.

Hopefully the issues and needed additions I mentioned here can be fixed or implemented in the future, but for now I take these hiccups as a minus until Aune addresses these issues.

  • Sound, specs and comparisons
Specifications:
  • DAC: Dual ESS ES9318
  • Output level: 1.8Vrms (3.5mm), 3.6Vrms (2.5mm).
  • THD+N: 0.000145%.
  • SNR: -120dB.
  • Frequency response range: 20Hz-20kHz.
  • Output Power (2.5mm BAL): Up to 265mW @ 32Ω.
  • Output Power (3.5mm SE): 100mW @ 32Ω.

The Aune BU2 will give you lots of power on BAL and even in SE. It drived most of my IEMs and earbuds, although with some earbuds I have to crank the volume up without headroom. I don’t think it’ll drive your insensitive full sized headphones (maybe drive them BAL but without proper headroom) but haven’t tried such headphones so I’m completely basing that on the spec sheet.

As for the sound, I would describe the Aune BU2 as a smooth, neutral source that pairs with just about all of my IEMS and earbuds wonderfully. It also synergizes with bright pairs quite well, and controls the brightness with a smooth finish. A good example of it would be my MH755 which needed power and was very picky with the source, but the BU2 in BAL completely “smoothed” out its pesky high mid and provided me with long listening sessions that I am unable to do with other sources.

Comparison:

- Zishan AKM & ESS U1
Here, I can definitely hear the difference in terms of dynamics and transients, and the BU2 having a blacker background/ better dynamic range compared to the two.

As for the sound profile the AKM is a warm and smooth source while the ESS is a deadpan neutral one, which kind of surprises me when the ESS from U1 and the ESS from BU2 are completely far away from each other when compared. With that I still prefer the presentation of the BU2 and just by the sound alone you can see how far a 300 USD source will get you.

Power-wise (SE, since the U1 is only single ended), the U1 wins with its good driving power and headroom for hard to drive stuff like earbuds, though the BAL on the BU2 would reign supreme from the already-stout SE power of the U1.

  • Conclusion
I enjoyed my time with the Aune BU2 for its great sound, driveability and convenience such as going wireless and so on. Unfortunately, the BU2’s great performance was not without its caveats. The lack of proper documentation on the errors, the glitching out screen and the lack of LDAC support, etc. They need to be fixed and implemented for better convenience of the consumers paying a lot of money (300 USD asking price). Until they fix and solve the firmware issue, a deduction will be placed for the rating.

But other than that, I like how the BU2 sounds. It felt like a fit for all my various pairs, almost perfect. I really enjoyed listening to Shinichi Atobe’s Butterfly Effect while I was testing the BU2 with its glitchy, skewed up techno beats with outstanding transient performance. If you think that the BU2 would give you this kind of music enjoyment, then I guess it’s worth paying for. Music is all that matters so, happy listening!

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