NIce review, i have put together a little something though from a consumer take (not technical as i do not have the knowledge). It was only a small write up so i will not give it its own thread or anything and though i would share my experiences here! PICTURES TO COME!
Otomatsu AMP-X Headphone Amplifier Review
I caught the DIY bug when I made the Miu Audio MRB amplifier and when I had the chance to review this from VKMusic I jumped at it. Victor Kung over at VKMusic is as he says a DIY promoter who loves good DIY kits and he imports kits like this into the US and sells them. This is a kit similar to MRB as it comes with all the necessary parts as well as housing to fit the PCB and battery in snugly. However this does actually offer some variables to you before you start the build because the gain is not set at any particular level until you solder the resistor in and depending on which one you choose to do, you could get either 2x, 5.5x or 11x. This really depends on your plan of use for this amp and if you want to driver high sensitivity IEMs then you will want 2x, closed portable cans and then you may want 5.5x and high impedance headphones then you will want 11x. Looking at the form factor of the amp I though that it is more transportable and because of this is thought I would want it for my full sized headphone so I made it have a gain of 11x.
The build is quite a straightforward process but of course you will need the tools. It is case of first up soldering everything to a PCB in the right place and this takes the simplest of skills and I finished up in under an hour for this without any special skills here and just having done a few of these before. You just need a small tipped soldering iron and some solder. You then have to put together the housing and sort out the battery which is all a bit fiddly but all in all does not take all that long. Overall building it is enjoyable and relatively easy.
Design and Features:
The design of this is very nice in some areas and down right tacky in others. It has a slim form factor that looks nice along with the all black metal casing that has a smooth finish. However you get included stick on face plates which say the name of the amp on and what each switch is but it looks badly cut out, is in an awful white colour and just clashes with the amps casing and makes it look cheap.
As this can use rechargeable batteries if you supply them your selves it has a socket for charging the battery on the back. If you decide to use batteries that are not reliable however you will have to undo the housing to change the battery and this could be a tedious job. However the screws on the back are made to be used with your thumbs and you can quickly and efficiently unscrew them and access the battery, this is an awesome feature
As for the amp itself it is nothing out of the ordinary. It has a power button which lights up a purple LED when turned on, it has a 3.5mm input and output and then a volume knob, nothing more fancy than that, it is an amp and to the point at that.
Size and Portability:
Putting the size and the weight of this together, I think we have a transportable amplifier. The metal body is quite heavy, more so then any of the portable amps that I use and even the extremely well built ones like my Fiio E17. Its overall size could be taken around as it is not as big as the Objective 2 but I would never really take it out the house, maybe have it on me in the car but that is furthest extent of its portability.
Its dimensions are:
Width – 8cm
Height – 2cm
Length – 11cm
As you can see it is far from huge but pushing the boundaries for portability and in my opinion into transportability.
To be honest I suspect it is as big or even smaller than some of the real powerhouse amps but they are extremely good and warrant being carrying around, this is a cheaper amp so I am not sure its sound warrants its portability.
On a side not the amp is also completely knocked out by the presence of a phone or something with a radio signal which makes it hard to use and could not be near a phone in a bag or pocket (if you have one big enough).
I set these to 11x gain and with the hopes that it would amp my Sennheiser HD580 which are 300 Ohms with no stress and they do a fine job, they drive this power hungry cans efficiently and without to much stress, they say they are made for the job and they do it!
Nothing even has to be playing with some fairly sensitive IEMs and you can hear the hiss and power surge. This is not the purpose of the amp on 11x gain so I did not really enjoy doing it as I could see that it was not that healthy. I am sure on 2x gain it would not have a problem though.
The build is of course in your hands as you can do a poor job of the PCB and the housing and I guess the thing would be a mess but I doubt you will and as long as you tighten the screws for the housing well etc. you will have a well built amp. In face the metal housing is very strong and nicely constructed and the only part of concern is the silly white stick on faceplate.
However the housing does not seem to align perfectly with the PCB even though I soldered it completely flat and the input and output need a little twist when inserted to get both channels to play!
For the sound comparison I am using my HD580 and my Epiphany acoustic EHP-O2 as a reference. The AMP-X has more warmth and mid-bass presence that is looser and muddier and this does have an impact on the mids. The mids are far from being as clear as the O2 and sound slightly veiled and a lot warmer, especially in the low mids. Highs are fairly smooth and not as extended or as detailed as the O2. Transparency is also far from the same level and this is mainly due to the coloration that the warmth brings.
The biggest problem I have found is does distort in the sub-bass regions which is not great at all, I found this on all the headphones I tried with different impedance and it was a common fault with everything I tried.
This does not sound terrible but is not a patch on my O2.
I think this is a good place to go if you fancy building your own amp, it is not the most ideal amp in the world, as its size is not really in any category specifically but it is easy to make, fun to make, fairly cheap and sound not bad once you have done it! However I would try and DIY a O2 instead as it sounds so much better!
Edited by Swimsonny - 11/9/12 at 12:14pm