Introduction

Unlike JDS Labs and iFi before, I certainly had to add aune to our list of manufacturers whose products have been covered on TechPowerUp. aune is the Hi-Fi brand of Wuhan Ao Lai Er Technology Co., Ltd., which in itself is only known for the aune brand at this time globally despite being founded in 2004. I suppose it’s much simpler to market « aune, » and the marketing does take advantage of the rounded letters to inspire product design as well. It is a brand predominantly known for its desktop and portable class DACs and amplifiers, often going further in feature set and pricing to come off as more premium than most of its Chinese neighbors we’ve reviewed to date.

Today, we examine the recently announced aune BU2, a premium portable Bluetooth DAC/amp that has quite a lot going on inside, which is why this happens to be the physically largest such portable unit covered, almost to where it is barely pocket-friendly. The full product name confirms there is Bluetooth connectivity, making for a DAC/amp that can be used wired or wirelessly. The latter makes wired portable devices in particular somewhat wireless, and there is a balanced output to power more power-hungry full-size headphones, too. Let’s find out what makes the aune BU2 so imposing, and thank you HiFiGo for providing this sample for us to test and take apart—literally!

Packaging and Accessories

The product box comes with a plastic seal all around and has the customary HiFiGo sticker I’ve seen on samples shipped from the retailer. Removing it reveals a relatively large box for a portable unit, which is not surprising given this is a large unit. The outer sleeve is predominantly white with the company logo and product name on the front going with a simple line render of the product and salient marketing features. I do like the illustration on the back clearly depicting how the aune BU2 is to be used. The inner box can be slid out the top or bottom.

This inner box is all black and made out of thicker cardboard, with the logo in silver lettering on the front. The lid opens up to reveal the contents inside, with a thin foam lining on the underside of the lid for further protection. There is a multi-language quick start guide that goes over the various components of the aune BU2, and I highly recommend going through it since the BU2 has a lot of pre-programmed features onboard and no app support. There is a contact card that has warranty information in addition to finding the social media pages for aune. The BU2 comes packaged into a cutout in thick foam, and a separate box contains the accessories. aune is fairly generous in this regard, and we see an L-shaped Type-C to Type-C cable, longer Type-C to Type-A cable, low-profile female Type-C to male Lightning adapter, and fancy 4-core braided 2.5 mm TRRS male to 4.4 mm TRRS female adapter cable for a balanced connection when you do not have the 2.5 mm plug on your IEM/headphone cable.

Closer Look

I don’t know what I was expecting when I was first asked about whether I would like to review the aune BU2, but it was not this! Never mind the large form factor—this is quite unlike anything else I have used thus far. It’s still portable in that there is a large battery inside for wireless connectivity in addition to the « cleaner » battery power than USB, and the predominantly black ABS plastic enclosure makes for a lighter product than expected even with all the hardware inside. Look at that cutout in the middle at the top, if you will, where there is a knurled multi-function wheel. I do wish the gap were slightly larger, or the wheel extended outward further since it is not the easiest to rotate even with my hands that run slightly smaller than average. But I understand the deliberate design to keep everything contained within the footprint of the enclosure, and it’s a compromise where I just happen to find myself on the other side from aune. Then there’s an integrated display with branding on either side. The branding continues on the back, but in a neat enough manner not to be distracting.

The bottom side is where you will find two separate Type-C ports, including one for charging. This means the other one used for wired connectivity will not charge the internal battery whatsoever. I do like this implementation since you don’t need to worry about running down the battery lifetime with continual charges and, in the absence of an app to change this as with the likes of FiiO and Qudelix, the dual USB implementation is the best way forward. As a 1 A charging port, charging is also faster than with contemporaries. The opposite side contains the two headphone outputs, including a 3.5 mm single-ended TRS jack and 2.5 mm balanced TRRS output. The latter is where I see the aune BU2 unfortunately not lasting the test of time as the larger form factor would easily allow for the increasingly more common 4.4 mm balanced output instead. IEMs increasingly come with cables that can swap between 2.5/3.5/4.4 mm outputs, but good luck finding headphone cables using the 2.5 mm connector for a balanced connection. The provided adapter cable feels like a bone being thrown to soothe an angry dog without actually solving the issue. Between the two is a button tied to the wheel from earlier, clicking which goes through different functions of the aune BU2 akin to what we saw recently with the iFi GO blu that provides both the 4.4 mm output and a better wheel/button implementation despite its smaller size.

After all testing was completed, I disassembled the aune BU2 for a closer look at what’s inside. Doing so means removing the four Torx-head screws on the back, and at this point, you realize that three primary sections are held together by these screws. The back plastic panel has the relatively massive 11.1 Wh battery glued to it, which incidentally makes me feel like all wireless keyboards could be doing much, much better for their size. For a direct comparison, the 11.1 Wh battery is a 3000 mAh battery on the 3.7 V rail it’s operating on, which is also significantly higher than any other portable DAC/amp I’ve tested to date. There is an internal cable to dislodge from the PCB on the back, which is otherwise bare of anything else noteworthy.

The other side is where the real action takes place, including with the rotary knob and display modules connected to the PCB and associated controllers, and an STMicroelectronics STM8S105K4 8-bit microcontroller. Solder quality is exceptional, so much so that I’d be lying if I said a limited edition with a polycarbonate see-through enclosure would be a bad idea. A lot is going on here, although I was somewhat disappointed I could not identify the Bluetooth transceiver under a metal heatsink. A ESS Sabre ES9318 premium-class dual-DAC chip is used, with each DAC tied to a headphone output, and aune is using the same single-to-dual positive/negative voltage power supply output as on the previous BU1 to make for a total of two DACs in parallel and four amplifiers to power everything.

The two DACs are associated with two low-jitter 45M/49M oscillators for clock synchronization. There’s also the same XMOS U30870C10 as in the JDS Labs Atom DAC+—an XMOS XU208 USB bridge with the xCORE 32-bit multi-core microcontroller that has successfully been used for USB audio Class 2 implementations. The aune BU2 is capable of providing up to 32-bit 768 kHz PCM playback, and it supports up to DSD512 native decoding as well. It also boasts a noise floor as low as 3.16 µV, THD+N of 0.0000145%, SNR of -120 dB, channel matching of +/- 0.5 dB across 20 Hz-20 kHz, and has independently verified numbers vouching for this to where it’s effectively an excellent desktop-class DAC in a portable form factor.

Setup and Audio Performance

Regardless of whether you plan on using the aune BU2 in wired or wireless mode, the USB Type-C port on the back has to be connected to a power source for charging the battery first. USB 3.2 Gen 1 (USB 3.0) helps with the 5 V, 1 A maximum intake, but note that the lack of a voltage transformer meant that anything other than 5 V didn’t work when I tried. The separate wired connection also allows for the aune BU2 in wired mode to your PC while paired to a mobile device via Bluetooth, for example. The default operation with the BU2 when switched on is pairing mode to set up the Bluetooth connectivity. I was pleasantly surprised by how well this worked as a desktop-style DAC, prompting me to open it up to see what’s inside. aune does not market it, but there should be the same support for the generic XMOS drivers as with the JDS Labs products to allow for WASAPI/ASIO device recognition with players such as JRiver, as well as providing drivers for, say, Windows 7. No drivers are needed with Windows 10/11, where the aune BU2 is a plug-and-play device in wired mode.

Wireless connectivity is really why I imagine most customers of the aune BU2 would use it, and pairing is simple enough given it enters pairing mode the minute you first power and turn it on, which has it show up as shown in the list above. You can pair the BU2 with multiple sources simultaneously, although only one is active at a time, of course. There is no mention of the SoC used for wireless connectivity, but it is clearly a Qualcomm offering given native aptX support. There is also aptX HD codec support in addition to SBC and AAC. Missing is aptX adaptive, LDAC or LHDC codec support, so take that for what you will. Regardless of wired or wireless mode, the aune BU2 runs off the 11.1 Wh battery inside that is rated for up to 9 hours of playback—at least in USB playback mode at the highest playback rates. This is another case of underselling since I got 10–13 hours easily, and nearly the same in wireless mode. Turn things down in either connectivity mode, including to a reasonable volume level, and things are closer to 15 hours. This makes for the best battery life of any portable DAC/amp tested to date, which it had better given its size compared to the others.

In the absence of an app, using the aune BU2 is all done using onboard settings. The display is extremely useful, and large and clear enough not to hold things back. This is a monochrome display, and the multi-function button and wheel combined cover everything the aune BU2 is capable of. This of course includes powering on/off, changing the input type if both wired and wireless sources are connected, the volume level, and displaying the codec when paired to a mobile device as seen above. There are also seven digital filters to choose from, and this is where the accompanying quick start guide helps identify them since the product page is useless in this regard and there is no online manual available as this is written. These filters are SC (brick wall filter), SU (hybrid fast roll-off filter), SL (apodizing fast roll-off filter, default filter), SI (minimum phase slow roll-off filter), SH (minimum phase fast roll-off filter), SE (linear phase slow roll-off filter), and SD (linear phase fast roll-off filter).

Courtesy the dual ES9318 DACs in parallel and quad-amp layout, the aune BU2 has an output of 1.8 Vrms over the 3.5 mm single-ended output and 3.6 Vrms over the 2.5 mm balanced output. This corresponds to a relatively whopping 265 mW of output power at 32 Ω over the balanced output, besting the previous record of 245 mW for portable DAC/amps from the iFi GO blu, although the 3.5 mm output is restricted to just 100 mW at the same impedance, which is easily bested by the GO blu. On one hand, I expected more from the BU3 3.5 mm output given the tech involved, but, on the other hand, it comes in handy when dealing with sensitive IEMs where a larger power range and, thus, volume output can be less desirable. I did not experience any hissing with even the ridiculously hissy Campfire Audio IEMs, which is quickly becoming my go-to test for noise floor and low impedance or high sensitivity playback. The balanced output provides plenty of power even for headphones, but I am once again shaking my head at the need to use that long adapter cable that will likely not match any stock cables and even requires an additional adapter unless you already have a 4.4 mm headphone cable.

But there is one more trick up the sleeves of the aune BU2, and it played a significant role in making it versatile with sensitive IEMs and more demanding headphones. You see, aune adopted an R-2R resistor ladder network for the volume control. Some of this can be spotted on the PCB photos above, wherein a series of resistors are precisely arranged as a network in a ladder configuration, and the specific layout of the aune BU2 is seen above. I would not necessarily call this an R-2R DAC, but at lower volume levels on a scale of 0-100, this implementation allows for extremely precise steps for subtle volume changes without affecting the sound signature. Those high-sensitivity IEMs can make use of a precise, long volume range of control, which works symbiotically with the maximum 100 mW output off the 3.5 mm output, too.

The aune BU2 portable DAC/amplifier costs $299.99 from retailers, including HiFiGo, as this review, so it isn’t cheap in either sense of the word. All the discussed, available features come together to make for a portable DAC/amp with long battery life, clean and transparent output power with the default filter not changing the sound signature relative to desktop DAC/amp stacks from the likes of JDS Labs and Topping, and plenty of output power, in addition to a very usable volume control range for most headphones and earphones alike. There remain some facets of the user experience that could be improved; however, the only thing I wanted to see was a native 4.4 mm output instead of the 2.5 mm one. This feature set is otherwise an attractive one, and the aune BU2 is a very good portable DAC/amp in my opinion. Just be sure this feature set is for you, especially at this price.