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[REVIEW] OTOMATSU DIY Portable Headphone.. AMP-X

Discussion in 'Portable Headphone Amps' started by goodvibes, Oct 10, 2012.
  1. goodvibes
    Member Vkung of vkmusic in Canada was kind enough to send me this product for review. http://www.vkmusic.ca/
    It's a portable kit intended for an outboard supply or battery. The supply is optional and does not charge a battery. I haven.'t really read the instructions other than parts position and wiring diagrams so I'll fill in more about it in the review portion. I wanted to post pictures and describe the build which was quite easy and quick for me but I'm used to a soldering iron.
    Contents and carton:
    Example of instructions with description of different gain options:
     I built at standard 11 times gain.
    Bottom of board after components installed:
    Top of board after components installed. Op Amps shown come preinstalled:
    After supply leads secured:
    This is about 1 hr in from the first shot.
    How it sits in the anodized aluminium case:
    Front and back are nicely done plastic. If it weren't for review, I'd lose the front sticker because it detracts from a otherwise nice looking piece. Kit did not come with nuts for the jacks so the only thing securing the board to the face is friction and the positioning of the volume knob. Haven't mentioned it to vkung yet. Easy fix and believe it or not, it feels secure as is. I would still like more. I may throw a couple screws in the track at the back edge of the board. It would be even more secure than the common nuts at the front method.
    Finished amp, front,  with tools needed. Allen was given and no soldering iron shown:
    Back showing PS jack and thumb screws for quick battery access:
    So from start to finish, it's about 1:15 if you can solder and are handy. Took about 1:30 with pics and some conversations. Probably up to 3 hrs for normal DIY taking this on as an early project. For those trying this sort of thing for the first time, learn how to flow solder and practice a bit.
    The board and parts are of better quality than others I've done and the board is 2 sided so flowing properly helps. Parts position markings on the board and the way the parts are presented in the packaging makes it a breeze to populate the board. I didn't have a small tip for my iron handy so I made do. It's not rocket science. Leave the leads long and cut after soldering. [​IMG]
    I'll do a listening and possible tweak review within a week. There's an output coupling cap and resistor at the output so since the amp chip has some momentary short tolerance, I may negate the resistor to see if it holds up. I may also bypass the coupling caps (adding a small value cap in parallel) to see if it improves things. Haven't even turned it on yet so fingers crossed. LOL
    raoultrifan likes this.
  2. goodvibes
    It works!
    Duff led but so what? Needless drain in my book anyway and yes, it's the right way around.
    What I don't like about the design:
    * Simple splitting circuit probably has some bounce or lacks stiffness into certain loads at certain levels, Could be worse as most low impedance things are also high efficiency and most low efficiency things are high impedance so the issue is mitigated in headphone amps. I'd still like it stiffer. Nice thing about this circuit is that you could easily replace the voltage splitter with 2 9V batteries and run with it.
    * Unneeded output resistor. Perhaps it's a bit safer if the amp is trying to play into a dead short for a while but the chip has some protection and there's a DC blocking cap at the output. With my 20 ohm IEMs, removing it was a VG thing and I see no issues as I've tested plugging in and out at power.
    * The need for output DC blocking caps. This is not entirely bad as they can help in more ways than one but the value needed with the size constraints limits you to electrolytic. They are however nice to have when your voltage rails drop unevenly which can cause more offset in some circuits and you can always voice it with bypass caps. The amp measured only 4mv of offset in front of the cap so the design has inherently little offset. Possibly usable without but better with. I don't need efficient IEMs 'biased' with 4mv even if it's not dangerous. They are actually quite common in $100 amps and these like the rest of the components supplied are of VG quality.
  3. ClieOS Contributor
    Seem like an easy enough build even for a first time project. Looking forward to your sound impression.
  4. goodvibes
    Thanks, I'm waiting for my other amp back from loan for some sort of reference. I'll fire it up before hand and just see what I think. The chip set and case are actually quite good and I think fair for the $135 price. The output and input caps are upgraded compared to most for their more critical needs etc. An assembled O2 looks like a better deal but I don't consider it portable in the true sense.
  5. goodvibes
    First Impressions:
    So a listen and as described earlier I removed the output resistor. Probably fine for 100ohm cans but mushed up the bass in low impedance IEMs. Fine once removed without any negative side effects. All good. At this point it's pretty good and worth putting on the back of a line out. I've bypassed the output caps with some 1.5mf bennic polyprops I had about and it's continuing to improve. They're big but found a spot for them. I'll try a few more things and button it up with a final tally.
    Basically, this amp comes fine as is for high impedance cans and works well for IEMs with a simple jumping of the output resistor. I'll do some burn in and comparisons when I can.
  6. ClieOS Contributor

    That give me an idea - the PCB seems to be big enough for them to have some jumper put in and let the user select whether they want to bypass the output resistor or not.Even a selection of two different value would be fine IMO. Now that I haven't seen on any portable amp.
  7. goodvibes
    Wouldn't hurt but it's there for safety so if the designer feels it's needed, it's staying but I don't think it does need to stay. I've taken up some of that space.[​IMG] Tried bypassing the input caps as well and it's better as is. Good bits included that don't need help.[​IMG] Next will be using 2 9v batteries sans splitter.  Won't fit in the included case but I'm curious at this point. It's a decent amp with good parts included. On the warmer side as is.
  8. goodvibes
    I've decided to leave the PS be and just review the unit as is since this is a kit and not supposed to be a modders wet dream. I will leave the 1.5 mf caps in parallel with the output caps and the output resistor removed as simple and useful upgrades for anyone capable of building a kit. The output resistor (r7, r8) would not need to be removed for high impedance cans but I feel it should certainly be gone for IEMs. All seems fine without the resistor so good to go. The amp is fast neutral, controlled and informative. It doesn't reach in as far or get as black as true high end amp but most wouldn't notice. You certainly don't hear the floor. It's not a world beater but I'd take it over something like an E11 or most cmoys sonically as it is now. It's neither grungey nor colored.
    Other than the simple PS the basic circuit and parts quality is excellent. The rounded case is easy to carry and rear cover screws are convenient. It also has plenty of apparent drive. The PS is probably as simple as it is because the circuit was obviously more designed for high impedance cans where a simple splitter is not an issue and 2k resistors are used so performance was chosen over battery life which should still be a full day of usability into difficult loads. Fortunately, with the output coupling resistor removed, low impedance IEMs are a viable match since they pull so little current. I'd still like to see more done at the supply side so you could be sure that it can't get caught out but there's only so much you can do for this price and it's performed plenty well enough as is. I think if this amp came stock with a stiffer PS section no output resistor and a well voiced bypassed output coupling cap (paralleling smaller value preferred caps with C11 and C12), it could be a real contender for cost effective excellence. As it is, it's still a VG value for the right customer.
  9. goodvibes
    One thing I forgot to mention. It is RF susceptible. If you check the circuit board, you can see the edge where it slides into the case groove is exposed ground plane. This is to ground the case. The anodizing of the case is working as an insulator and prevents the case from gronding as shipped. A simple scraping of the grove suporting the circuit board should easily fix this. The front and back are still plastic but that's minimal exposure if the case gets grounded, escpecially with the battery on end and gronded bits against the face on the other.
  10. Armaegis
    The Practical Devices XM6 has a 75 ohm switch, and I think the upcoming Centrance M8 has an impedance switch as well. 
  11. goodvibes
    The output coupling caps are generally also better suited to a higher impedance phones. The 330uF supplied should be flat to 30hz into a 16 ohm load with a gentle roll below.  24 ohms would have it flat to 20hz so even the vast majority of IEMs are covered and even being down a db at 20hz is no big deal. I'll be sending this to ClieOS in the near future for a review but I may try a couple things first. I planned to just send it but it has more potential so I may want to try better coupling caps in and out. I may also try a dual 9v supply an bypass the V splitter.
  12. ClieOS Contributor

    Right, the built-in P-to-S adapter - totally forgot those. XM6 isn't the first though, I now remember seeing it in the early day MiniBox amp. But they are kind of different in purpose compared to OTOMATSU.
  13. Swimsonny
    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:
    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).
    Driving Power:
    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.
    Build Quality:
    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!
    Overall Sound:
    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!
  14. goodvibes
    The amp will voice up nicely with a bit of care. The output resistor is a big negative with IEMs but fortunately isn't needed with the output buffer used. I currently have the output caps bypassed with some polyprops and stopped there as they seem nice. The input cap is a Wima so expected to be VG but it has a lean, dry character. That's now a (God forbid,[​IMG]) 10mF tant that fits the sig. Other things I've tried haven't helped. I still need to get to a radioshack as I want to hear a dual 9v arrangement even though that wouldn't fit in the case. The amp circuit is solid depending on how you feel about output caps. The PS portion is a bit simple but like in the O2 can be fitted with dual 9v rails. Problem is then, like and 02, it's not that protable.
    Swim, if you still have one of these to play with. Short across the large output resistor and see what you think. It will lean out and speed up, Too lean actually but cap massaging sorts that.
    One other thing you may want to do if you plan on using it or sending it to try is a zip tie at the back of the board by the battery connections. If you pull the wires towards the outside and put the tie through the hole next to the opamp and around the back edge, it will hold down the wires and stop them from folding back and forth at their solder connections when replacing batteries. That is always prone to failure if not stress relieved.
  15. Swimsonny
    Yeh i still have this one. Can you explain what you want me to do in noob terms please :wink:

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