Our only amp review so far was about the $7 FiiO E3, so it might be time to step things up a bit. Enter the Three Stones MiniBox-E+.
Peculiar brand name… After a bit of sleuthing I’m still not quite sure who is behind the brand “Three Stones”. It seems to be a Chinese one-man project that’s being distributed internationally by US-based Hi-Fi vendor Head-Direct. Three Stones have been in the business for quite some time with several portable amp models, and the MiniBox-E+ I’m writing about in this review is their current top-of-the-line model.
Read on after the jump – especially if you feel the need to drive your 300 Ohm phones properly on the go.
The cute anachronistic looking cardboard box contains the amp, the AC charger, and a sturdy looking interconnect cable.
The AC adapter is only available in an US-pronged version, but it’s capable of handling 100-240V, so it works anywhere on the planet – provided you get a prong adapter for your specific location. The included cable appears to be well made, perfectly usable if you don’t already have another one (or don’t believe that it makes a difference to use some “oxygen free silver, braided by elves” nonsense over a length of 10cm between an MP3 player and the amp).
Enough about the peripherals, on to the amp. There’s no denying it: look and feel of the MiniBox are very cheap and DIY-ish. Well, that’s actually a bit unfair towards DIY builders like Gary Ali (PA2V2) or Robert Gehrke (Penguin Amp) – their amps appear to be more professional quality than the MBE+. For the not so low price of the MiniBox one would expect some significantly better construction than this. This is nowhere near the build quality of a Corda, Headamp, or Ray Samuels amp.
The white front and back plates of the amp appear to be some kind of epoxy, the material printed circuit boards are made of. The plates are attached to the sheet metal housing with self-winding hex screws. Since there are user replaceable parts inside the MBE+ (op-amps, more about that later), it should be easy to open and close the housing. However, I found it almost impossible to get the two parts of the case to fit together again after I opened it up. One would have to remember which screw belongs into which hole, since they’re all somewhat askew, and even then it requires quite a bit of force to get them in (but not so much as to damage the fragile screws and windings).
The amp’s case is quite wide. It’s a good match for hard disk players like the Cowon X5, Zune 30/80, or various HDD iPods. Paired with modern slim-line flash memory players it’s a bit on the bulky side, though. Measurements of the MBE+ are 80 x 67 x 16mm, by the way.
Inside the amp we see, well, at first a huge 18V Li-Ion battery taking up most of the space, and then the dual-mono layout of the circuit board. If I’m not mistaken, the MBE+ is very similar to Tangentsoft’s old MINT amp design. Maybe it’s even exactly the same, with just a few cosmetic differences. There are some stacked Burr Brown BUF634 buffers and Analog Devices AD8610 mono op-amps in DIP8 sockets for each channel.
The op-amps are of course the fun part of the MBE+, something like Lego bricks for adults. They are swappable – what the cool kids call “op-amp rolling”. I tried a few varieties and found the National LME49710 op-amps to be superior to the stock AD8610 in most aspects (less background hiss, somewhat punchier bass response). Burr Brown OPA627 were pretty much exactly the same as the AD8610, and Linear Technologies LT1115 didn’t work at all in the MBE+ (loud oscillating sweeps, very noisy). Other op-amps that should work well are the Analog AD797, but I didn’t try those myself.
Another important issue to mention is that the MBE+ has absolutely no shielding against electromagnetic interferences. One time I was listening in public with the MBE+ in my pocket, some guy about 5 meters away got a call on his cell phone, hilarity ensued. The interference from the cellphone signal was extremely loud and annoying, louder than the music – mind you, I was listening at rather quiet volume levels. Needless to say, the issue gets much worse when you carry a cell phone on your body.
I didn’t manage to get anywhere close to the claimed battery life of 60 hours – after about 20 hours of normal listening the amp’s battery ran dry. That’s fine by me, 20 hours is respectable enough – especially considering the sheer power this amp is capable of. By the way, the original battery of my MBE+ was faulty; it got “pregnant” and died after the first charging cycle. Fang from Head-Direct however was kind enough to send me a new battery right away, no questions asked.
Does an amp have a certain “sound”? Well, yes – usually if it’s a sub-par amp. A good amp is generally considered as neutral – it neither adds to nor subtracts anything from the sound. That’s what headphones are usually for – they’re the best suited part in a setup to “color” the sound according to one’s personal preferences. That’s more or less what the MBE+ is – an amp that is neutral.
Nevertheless, there actually is one sound shaping feature built into the MiniBox – the bass boost button. It does what it’s supposed to do; it boosts the bass by about 5dB below 100Hz. The slope of the bass boost is unfortunately rather wide, it starts already at about 1000Hz, so the resulting bass boost is a bit muddy in the midrange.
Of course there are people that hear (or imagine to hear) the tiniest, most subtle differences between amplifiers, but I’m not one of them. I can only tell you that the MiniBox has a perfectly linear frequency response, acceptable channel separation, and low harmonic and intermodulation distortion. It powers headphones with authority, restoring much of the dynamics and “punch” in mu
sic, important audio cues often missing when demanding phones are underpowered by wimpy MP3 players.
It’s a good amp regarding most technical aspects – as long as your phones have rather high impedance and a low-ish sensitivity rating. There are no strict rules as to what specs phones need to work well with the MBE+. As nicely as the amp works with Sennheiser HD650, Yuin G1A, or Phonak Audéo PFE – as badly does it synergize with Ultimate Ears UE11 (or any other UE phones I tested), Shure SE530, or even something like the Sennheiser CX300.
The MiniBox is far too powerful for these efficient IEMs; one can hardly touch the volume dial before it goes to ear-splitting levels. There’s also lots of background hiss with these kinds of phones, and a dangerously loud click/pop when the amp is turned on. If it doesn’t destroy your ears, it might at least damage the IEM’s drivers. I’m serious – the “explosion” at turning the MBE+ on with high-sensitivity IEMs plugged in is nothing to joke about.
This is actually where the “P-to-S” button would come in handy. It adds 75 Ohm resistance to the output. It got its name from the Etymotic ER-4P IEMs (27 Ohm), which can be turned into the (presumably better sounding) ER-4S model (100 Ohm) with this added resistance. In theory this would get rid of the background hiss from most low-impedance phone, and would make the volume control less sensitive. However, in reality the P-to-S switch makes the overall sound noticeably worse. Phones like the UE11 and SE530 become very muddy and veiled sounding with the added resistance. Furthermore, stereo crosstalk goes through the roof, no matter what phone attached, resulting in much worse soundstage. I don’t have any Etymotic ER-4P to test, but for the ultra-efficient IEMs I own the P-to-S switch is unfortunately useless.
The following will be considered blasphemy by some radical “audiophiles” who firmly believe to drive demanding phones like the Sennheiser HD650 you need at least a tube amp that consumes 50 Watts of power (to spit out about 1 Watt on the other end) for the phones to sound right – but the MBE+ drives the HD650 just fine, nothing to complain here. Seems the 18 Volt battery does make a difference compared to other, lower-powered portable amps.
The MBE+ might not pack quite the same punch as my desktop amp, the Woo Audio 6, but for what it is it performs admirably well with medium-to-high impedance phones. With these phones it does make music on the go more lively and dynamic – and therefore more enjoyable.
On the opposite end of the headphone spectrum – as mentioned above – with low-impedance and high-sensitivity IEMs, the results are less than stellar with the MBE+. It’s simple: if you own Ultimate Ears, Shure, Westone ES, JH Audio, or similar IEMs – stay away from the MiniBox. There are much better suited amps available for these phones, amps with much lower gain settings.
If you crave serious power on the go for your full-sized cans or high-impedance IEMs and don’t mind the DIY-ish flaws of the amp, the MiniBox-E+ might be what you’re looking for.
- Very powerful 18V amp, perfectly suited for driving high impedance phones
- Neutral, detailed, precise sound
- Op-amps can be swapped (“rolled”, as the kids say)
- Good battery life, considering the power of the amp
- Questionable DIY build quality and design, no EMI/RFI shielding
- Not suited for driving low impedance, high sensitivity phones/IEMs
- P-to-S switch engaged makes the sound noticeably worse with many phones
You can buy the MiniBox-E+ from Head-Direct, for $229.