Having owned a relatively large Yamaha R-S202 stereo receiver for some time, I was looking to downsize my audio setup without compromising on quality. Over the years I had read and heard about the various varieties of Class D mini amplifiers available from Chinese brands such as Aiyima, Nobsound, Fosi Audio, etc. It seems that these hard drive-sized amps most commonly use the TPA3116/TPA3116D2 amplifier chip which is rated for 50W x 2, although in the past few years designs that use the higher-powered TPA3251/TPA3255 (good for 175W/315W stereo, respectively) have become increasingly popular. The Aiyima A04 is one such example of the former, and retailing for only $50 on Amazon at the moment, I decided to pick one up and see what Class D mini amps are all about.
The unit arrived inside a plain cardboard box, accompanied only by some foam insulation and a warranty card. There was no manual included, although pretty much everything you need to know is on the Amazon product page and the device is pretty self-explanatory as we'll get to in a bit. As was made clear on the Amazon product page, there is no AC adapter included with this particular model. Aiyima recommends a 32V 5A adapter, but these are virtually impossible to find in the US (short of jerry-rigging an adjustable open power supply such as this). However, the A04 is capable of taking a DC input ranging from 19V-35V, and thankfully 24V 6A adapters (such as this one) are readily available.
Fit and Finish
The Aiyima A04 wraps its innards in an attractive smooth metal (aluminum) casing, with the front and rear panels secured in place with hex-type screws. The outside controls and inputs/outputs are simple and self-explanatory - a power switch and volume dial on the front, and the DC power jack, RCA stereo input, and L/R banana-type speaker outputs. Of note, the power switch is a robust mechanical affair that has a very satisfying feel.
Using my ALC1220-equipped PC as an input to the Aiyima with a pair of Sony SSCS5 bookshelf speakers connected...wow! Simply put, the Aiyima A04 is in a completely different league compared to my Yamaha. Regardless of genre the Aiyima rendered sound that sounded full and detailed. Even on tracks that I had listened to many times before, there was some additional nuance that I had no idea my speakers were capable of producing. I am no audiophile but even to me the difference was apparent.
Upgrading the Op-Amps
One of the advertised features of the Aiyima A04 is its socketed (and therefore user-upgradable) op-amps, which are visible above in the bottom right corner of the board above the volume dial. To get at them, you'll need a metric hex screwdriver/Allen key set to remove the top two screws on both the front and rear face panels.
We'll not go into the highly complex electronic aspects of what an op-amp does, but suffice it to say (at the risk of gross oversimplification) that in an audio context op-amps transform and amplify an input signal into a form suitable for a speaker to play as audible sound. The stock configuration consists of a pair of TI NE5532P op-amps, which, although it is a 30+ year-old design, is still widely respected due to its low noise profile and relatively neutral/realistic sound signature. As a result of its maturity the NE5532P can be acquired for 50 cents (or less) a piece. A little bit further along on the cost spectrum we come across chips such as the TI OPA2134, a Burr-Brown branded op-amp marketed specifically for high-performance audio applications. A pair of these can be acquired from online electronics stores for about $10-15 shipped. Admittedly this may seem a bit pricey, but if you're looking for a more musical (and potentially less realistic) bias with a warmer soundstage and accentuated bass, the OPA2134 is a worthy upgrade over the NE5532Ps.
The Aiyima A04 is an amazing product, especially for only $50 (plus the cost of an AC adapter), and is an excellent example of what the TPA3251 can do. Although capable of producing impressive sound out-of-the-box, some users may wish to take advantage of the socketed op-amps and upgrade them to suit their own aural tastes.