Some time ago I got a message from Miu Audio asking me if I would like to build one of their headphone amplifiers for free. Thrilled to receive my first free review sample in my entire time on head-fi, I accepted the offer.
Initially there were some problems with the device. They made a very stupid mistake in the diagram with the part numbers and switched up the values of two resistors. Because of this, the left channel had a 20dB lower gain compared to the right channel. It took me some time to get it fixed, and there were multiple people with the exact same problem. The issue is fixed now, and they have corrected the diagram.
The Mini3 was my very first DIY project, and I have this little amp for more than a year now. Back then I wasn't very good at building this sort of stuff (still ain't), so it doesn't look all that pretty, and the soldering is safe to say horrible compared to my current skill level. To save money I made the front plate myself, but the holes are not in the right places and it's covered with tape. It's a horrible sight, but it's fully functional at least.
Since I now have two amps of similar size and price, I thought it was an excellent idea to make a comparison for those who are interesting in building either of the two. The sound quality comparison is rather short, and I'll get to the reasons why later on.
I meant for this to be an as thorough as possible comparison, so if you feel I left something out I'll be glad to add it in later.
Pictures and size comparison
Both weigh about 60 grammes (my scale is incredibly inconsistent, so I'm not sure). They feel more or less the same weight.
The MRB is bigger than the Mini3. The Mini3 is credit card size if you exclude the knob. My knob is rather huge, because I judged the size wrong when I ordered. However, a smaller knob is definitely possible. The advantage of this huge knob is that I can adjust volume through the cloth of my jeans, even while biking.
Size measurements: (between brackets is length + knob)
Mini3: WxLxH = 54 x 86 (/109) x 23mm.
MRB: WxLxH = 56 x 96 (/108) x 24mm.
The Mini3 comes in both a black, and alumnium-colored version. The MRB only comes in an aluminium colored version.
Both amps are very easy to build, and are a good first DIY project. The MRB is slightly easier because the Mini3 requires you to solder two small surface mount opamps. In the MRB these are in a socket and can be soldered through-hole. However, you will find that soldering the opamps really isn't that hard if you follow proper instructions.
Additionally, the Mini3 has more directional parts (diodes and some capacitors), so there are more chances for mistakes. If you take your time, you shouldn't make any mistakes on these kind of things though. Don't be scared to ask for help when in doubt either.
Finally, the Mini3 is slightly too big for its case. It depends on which specific battery and capacitors you build it with, but in my case it requires a lot of force and even tools to get the amp out/in of its case. What I do is open it on both sides, grab some non-conductive rigid object (e.g. A pen or marker) and put it vertically on the table with the object resting against the back of the battery. Then I push with all my might to get the amp out. It's the same thing for putting it in.
Although the Mini3 is slightly more difficult to build, it is easier to fix in case something does go wrong. This is because if you have problems with the Mini3, you can ask for help on the AMB forum. I have found that the people over there are quick to respond and very helpful.
Features and build quality
Both amps have a very solid build quality and use a body made completely out of aluminium. The Hammond enclosure of the Mini3 comes with plastic front plates, in which you need to drill the holes yourself. However, if you want, you can order pre-drilled aluminium front plates for $40 per set from the AMB shop. I chose no to get these myself, as I was rather short on money at the time.
Both amps have one 3.5mm input, one 3.5mm output, and a DC hole for charging. Where the amps differ is that the MRB has a seperate on/off switch, whereas the Mini3 has a combined volume/power knob. The disadvantage of the latter is that you need to seek for the right volume every time you turn it on, whereas with the MRB it will be on the same volume as you used it last time.
The only other difference is that the MRB uses a lower voltage charger, and includes an adapter cable for a a thicker DC plug to the thinner one that the MRB uses. This may not seem like such a large difference, but it turns out that I had a suitable charger for the MRB laying around somewhere in my house (I think it was an old laptop charger), whereas I had to go out an buy one for the Mini3. This might just save you thirty bucks or so. This is because 12V chargers are very common, but the ones the Mini3 uses are less common.
For clarity, the MRB uses a 12-16V charger. The jack is 3.5mm with an adapter for 5.5mm plugs included.
The Mini3 uses a 15-24V charger. The jack is also 5.5mm. Both amps require a center-positive charger, which is standard in any case.
The Mini3 has a possible upgrade to using Li-po batteries. This will both increase battery life, and reduce charging time. If you want optimal battery performance you might want to do this upgrade. I haven't done this myself, so I don't know how easy it is to do.
With the Mini3 you can configure the gain yourself by changing the value of a couple resistors. The exact values are listed on the AMB site, and the recommended gain values lie between 2x and 8x.
With the MRB it should be possible to do the same, but you'd have to supply the resistors yourself. You could probably ask AMB for the exact values needed (or anyone with some electronics knowledge). If you require the schematics of the amp, I can give them to you.
The standard gain of the Mini3 is 5x, and 11x on the MRB. However, the max volume on the Mini3 is significantly higher. The MRB is barely able to drive my HD650's, but the Mini3 does it with ease. Higher sensitivity cans (like my HD 25-1 II) are driven easily by both.
For this review I specifically built an A/B comparison box. I can swap the amps with the flip of a switch. I matched the volume by ear, but I could do it more precisely with a multimeter if needed.
Switching between the amps at many different volumes and songs I could not hear a difference. Even getting out a different source (the Audio-GD NFB-12 instead of my phone) or different headphones (tried both the HD 25-1 II and the HD650) did not help.
Now, this does not neccesarily mean they sound exactly the same, but I do think that the differences wil not be huge.
Funny thing is that when I did some A/B comparisons the old fashioned way (plugging and unplugging the connectors, instead of flipping a switch), I swear I found the Mini3 more smooth sounding. Hard to describe the exact difference, but the Mini3 really did sound a little better. Whether or not this is a figment of my imagination, I'll leave the reader to decide. Especially since I'm not sure myself.
Another important poiny is the amount of background noise. Both are audible on my HD 25-1 II when nothing is playing, but both are soft. When music is playing I can't hear it, even during really soft passages. The MRB's is a little louder than the Mini3, but both are quite soft. If you're using very sensitive IEM's, then I think the Mini3's lower noise floor could be important.
The Mini3 also has a very interesting quirk I should note; when no music is playing for a couple seconds when using my phone, the Mini3 suddenly starts to generate a fairly loud hum (conversation level SPL). When playing music, or even during mute passages, this is not the case, so it doesn't really bother me that much.
Member Bowei006 has reported that with his headphones he hears a very weak if not muddy sub bass with a lack of mid bass punch, when playing bass heavy songs loudly on the MRB. I have not found such problems myself, but it may be worth noting. Overall he is not very impressed by the sound quality, but I personally don't really see any problems with the MRB's sound quality.
It should be noted that the MRB has a very high output impedance. Through measurements I found it to be between 28 and 30Ω. With a lot of cans that is going to have an effect on the frequency response. This is not desirable.
Before you ask why I didn't experience such changes myself, I will have you know that I wondered the same thing. Then I got out the measurements of impedance of the HD 25-1 II, and calculated that such an output impedance will cause a broad -0.3dB dip at about 150Hz, as well as a slight roll off of -0.3dB from 10kHz to 20kHz. That is under the threshold of audability, so it's not weird I did not hear anything. For the HD650 I didn't even bother to check, since the nominal impedance is ten times as high as the MRB output impedance, so the changes will probably be far smaller than those in the MRB.
If you're going to use the MRB with an IEM or low impedance headphone, then the output impedance will definitely form a problem. It could possibly alter the sound in a 'fun' way, but it's usually best to just avoid high output impedances altogether.
I later checked the schematics of the MRB again for an unrelated reason, and I noticed that the output impedance is caused by two resistors directly at the output.
For the left channel this is R14 and R16, and for the right this is R5 and R7. With the standard values this corresponds to 27.5Ω, which corresponds to my measurements nicely.
You could, however, lower the value to reduce the output impedance (or increase it if you want). This shouldn't cause any problems, but it hasn't actually been tested, so I'll leave it to your own judgement whether it's a good idea or not.
If you want to change the output impedance it's fine to just lower R14 / R5. Reducing or shorting R16 / R7 would also reduce output power, so it's best leaving it as is.
For the left channel, the output impedance is: R14 * R16 / (R14 + R16). Lowering R14 from 30Ω to e.g. 10Ω would give Zout = 9.7Ω
I measured the Mini3's output impedance to be 0.3Ω, but it's probably smaller since my DMM does not have infinite precision. (the difference showed up as 1mV, which equates to 0.3Ω, but it could be less than that in reality)
The MRB is $112 excluding shipping.
For the Mini3 the price varies. A kit from Glass Jar Audio is $95.53 with an extra $41.75 for the front plates. I personally can't recommend Glass Jar Audio enough for Jeff's sublime service.
I personally ordered the parts myself. Including shipping I paid $41.88 for the parts from AMB, and ~$40 from the rest of the parts from Farnell, but that came with free shipping since I bough other stuff from there as well. I also paid about $10 for my battery, and $25 for my charger. If you order from Glass Jar Audio these two things are included, so it might actually be cheaper to get a kit.
If you want to use the Li-po battery upgrade for the Mini3 then you'd need to spend about $25 extra for the parts, and I have no idea how much the batteries cost. You can't get the parts for this directly from Glass Jar Audio, but he may include them for a small extra cost to save you double shipping costs. I'm not sure.
Both amps are nice DIY projects to try out. I think I'll have to give the edge to the Mini3, despite it being a little more expensive than the MRB. Reading my review you may come to a different conclusion, since both options are very viable.
I'm really dissapointed by the high output impedance of the MRB, as well as the fact that they made a very serious error in their diagram. Those two are the main reasons why I'd prefer the Mini3.
Both these amps are rather large, and are not what I'd call 'ultra portable'. While I have no personal experience with them, if you're looking for something ultra portable look at FiiO's or Ray Samuels' products. If you're looking for something semi-portable, but still DIY, the O2 might be fun to build too.
I'd only advice you to build the Mini3 or MRB if you want to do a simple DIY project, and want an amp that still fits inside your pocket. If this description does not fit you, then both the MRB and Mini3 will not be for you.
And that concludes my first amp review. Please leave a comment with suggestions if you have any. Don't be scared to criticize my work either, I can take it.
Edited by Tilpo - 8/28/12 at 4:16pm