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Otomatsu BDR-HP01E DIY Balanced Headphone Amp Review

post #1 of 5
Thread Starter 
Victor Kung of VKMusic contacted me to ask if I was interested in reviewing a kit amp from a new Japanese manufacturer, Aurorasound. After positive experiences with Elekit and because I have a bit of a DIY bug to scratch, I agreed to build their balanced kit amp, the BDR-HP01E.
The president of Aurorasound, Shinobu Karaki worked for Texas Instruments for 28 years, including being in charge of their Digital Signal Processor division and a senior manager of their Digital Audio/Video LSI development and Marketing Group. He describes himself as a "serious audiophile" interested in everything from 78 RPM vinyl through to high-resolution digital in addition to playing and teaching guitar locally.
Aurorasound is his company for high-end audio amplifiers, with Otomatsu being the DIY division of that.
The basic product of the Otomatsu division is a portable headphone amp, called the "Otomatsu Headphone Amp Kit" which he sells for 9,580 yen (including the case) or about US$125 at today's exchange rate. The design, which will no doubt have those people more understanding of audio circuits mumbling appropriate comments in acknowledgement, consists of a very straight-forward OPAMP-based circuit using 2 LME49600 (fixed to the boards) and 2 OPAMPs of your choice -- LME49710 by default, but OPA604, OP134,  LME49990,  NE5534A,  LT1115 or  OPA627A are given as options, with other common single-channel OPAMPs likely to work as well.
The reason for my mentioning this basic amp is that it forms the basis for the HP01E I built. Two of them are used, bridged, to build a balanced amp in much the same manner as some people have build balanced versions of AMB's Mini3. In addition to two amp board kits, the HP01E comes with a very straight-forward power supply. The unique aspect to this amp is the input, which is single-ended and fed into a phase splitter to get the balanced outputs. This is where the design increases a little in complexity, requiring another board to stuff and somewhat more wiring. It would be easily possible if one wanted balanced input to use a 4-gang potentiometer instead, but it would require both a larger box (something I'll discuss later) and there wouldn't be any SE input.
From a DIY'ers point of view, this kit is good for someone with a couple of builds under their belt already who is familiar with the parts and basic amp circuit layouts, as the increased complexity of bridging two amps as well as a couple of tricky points I found a little challenging.
The first challenge is the last: The included case makes for a very tight fit for components, especially with all the wiring involved. It took me relatively little time to stuff the boards, but getting everything wired up and in the case took far longer.

The Build.


What's in the box.


The kit itself was kindly delivered with a usefully bookmarked copy of the local MJ audio magazine which covers both regular gear as well as DIY, in both cases slanted more towards the technical aspects of the designs of both.  I was pleasantly surprised to find each board in its own little kit box with neatly packaged components, making it easy to get straight into building.
Comprehensive step-by-step instructions are included as well, though here in Japanese. Victor Kung, as he did with the Elekit amps, has translated the instructions into English for North-American buyers. Suffice to say, the excellent pictures and diagrams are pretty much all that is necessary if you understand how amps work already.  The only thing I forgot to do was set a lower gain to start with, which I'd definitely recommend as the default is too high.


Dual amp boards and parts.


The amp board kits are those from the portable amp, so reading the instructions beforehand is necessary, as the boards aren't built quite the same as they are in portable. For the balanced amp, some components have been upgraded. The coupling capacitors have been upgraded to Nichicon MUSE (though if you don't mind risking a bit of DC on the output, they can be bypassed), and the OPAMPs to a set of LME49710s. Since both boards are the identical, they were very fast to assemble, as I didn't have to hunt for part locations more than once. However, the MUSE coupling caps were a tight fit and I needed to cut the wires to a suitable length beforehand and spend some time wiggling them into the holes. A nice touch was that the resistors came attached to a piece of card with double-sided sticky tape and labels for their values, which saved having to deal with numerous small snaplock bags.
Attenuator/splitter board and parts.
The attenuator/splitter board was the only place I had confusion, as a couple of resistor labels sit in locations that allow for confusion. Thankfully I tend to insert all the resistors before I start soldering so I spotted a mistake I had made almost straight away.
The transformer and power supply board were very straight-forward. The resistors used are all the same value and are un-labelled an in the bag with the rest of the small components. The only fiddly thing was soldering in jumpers from the transformer board to the PSU board. A good reason why experience is helpful here is making sure the transformer is the right way around with the primaries towards the fuse. A slightly odd thing I encountered was that the power board came with two 2-connector screw terminals instead of 4-connector screw terminals. In another bag along with other components for the case, I found two more of these, but by that time I'd soldered the screw terminals into the middle of the slots in the power board and couldn't be bothered to make changes. However I managed to get everything wired up without issue despite this.
The case provided is from the Japanese manufacturer Takachi. They are decent quality cases which, usefully, slot together, making it easy during assembly to remove and return the front and back panels as necessary when the top is off. This was a lifesaver as the space inside the case is very tight with all the wiring. One thing I do dislike about the case is that the power socket, being in the middle, causes the back panel to bend appreciably when a plug is inserted.

Signal wiring diagram.

At first, seeing two whole pages on how to wire up the amp, one for the power wiring and one for the signal wiring was very daunting (I'm not an experienced DIY'er). The kit comes with a length of black and blue wire and 2 lengths of red. However, using one length of red for the mains wiring and braiding up the other lengths of red, blue and black, then cutting braided lengths as required made it very straight-forward to do the wiring, as almost all of it, except for small lengths used for jumpering various parts, uses all 3 colours.
In the midst of wiring everything up.
Importantly, make sure to get the boards positioned well using the stick-on feet, then unscrew them for the wiring. I made the mistake of sticking the boards down last, being worried about their position and it turned out worse in the end.
Once everything is assembled, the next stop, before installing the OPAMPs, was to power it up, calibrate the PSU voltage and measure the voltages at various points on the boards, to which a whole page of the manual is dedicated. In my case, after adjusting the power output on the power boards, everything else was spot-on.
The kit I received included two Neutrik 3-pin XLR combo jacks to allow both single-ended and balanced output. I've suggested, since it has become more common nowadays, that offering a 4-pin XLR + TRS socket would be more useful, especially for someone who wants to re-terminate their own headphones or use a pair of orthodynamic headphones, and also as both Audeze and Hifiman offer their headphones with 4-pin termination by default. It is also much simpler to wire than the combo jacks. In the future, if I built a amp like this, I'd buy some pre-cut and terminated jumper wires from the local electronics store which would make it much easier, or a second pair of "helping hands" to hold everything in place for soldering.

The Sound.

For me, the interesting thing would be comparing how well a basic circuit would compare to my other balanced amp, the Audio-gd Phoenix which, at the time it was designed, was intended to be as good as the designer was capable of (though now has been superseded in that sense).
Suffice to say, I'm quite pleased with the results. With my LCD-3s it has a rather forward presentation that presents detail intensely. It doesn't have the spacious effortlessness of the non-feedback Phoenix, which tends to disappear, just leaving the music. Rather, the amp is more excited and wants to tell you everything about the music.  It seemed to want to tell me about what source I was using as well, as switching between more lowly kit as a source, such as an Audioquest Dragonfly and one of my high-end DACs the differences were significant.
I spent a few hours the evening after building it enjoying listening with it, not feeling like switching back to my amp for some time. I did find the intensity slightly fatiguing over long listening periods, but I still enjoy the amp when I want more intensity than I usually get from my system.  To that end, it is just the kind of amp that suits more "relaxed" open headphones such as the Sennheiser 600, 650 and 598, the latter of which is shown in the instructions. It reminds me a great deal of the mid-range amp I used to use with the HD-600s some years ago, the combination of which I enjoyed very much. 
Switching to the single-ended output with my Symphones Magnums, which are easy-to-drive headphones, the result was equally crisp and clear as balanced. The benefit of the extra power available was readily more apparent when I switched to using LCD-3s and played some complex and dramatic orchestral tracks, where the slight blur during dramatic movements using the single-ended output (and just one of the two amps) disappeared when the balanced output (both amps) were used. Unfortunately I don't have any suitable high-impedance headphones to repeat the test with, but when I do, I'll update this review.
IEMs: With my quite sensitive RE-ZEROs, even with the volume at maximum, there is no audible noise or hiss. Using my Sony XBA-3s, which have a very unusual impedance curve, which is a serious test of the linearity of an amp, there were no issues that I could detect, which is an excellent sign.
Interestingly, it mated very well with the Audioquest Dragonfly, which I have running out of my Apple USB keyboard. Out of my Calyx DAC the sound was a bit too intense. It would probably be a good pairing with a more mellow DAC, such as the Metrum Octave.


One of the most obvious benefits of an OPAMP-based design is the ability to roll different ones in to the circuit since they are, in themselves, amplifier circuits. Most of the popular single-channel OPAMPs can be rolled into the amp boards, though something like the OPA627 is going to be a costly option as you need 4 of them, as well as compensation caps. The attenuator board, on the other hand, takes dual OPAMPs.
I rolled both OPA627s and AD797BRZs into the amp with predictable results, the former giving a slightly warmer presentation and the latter a touch gentler clarity compared to the LME49710s that come with the amp, though without the changes being as great as I had remembered from doing so in other devices. I felt a better improvement was had switching the NE5532s from the phase splitter/attenuator board with LM4562s, though being unable to quickly A/B the differences, I'm not entirely sure they were significant.
The LME49600, by default, has one pin not connected to put it in low-bandwidth mode. Connecting it puts it in high-bandwidth mode, which, I felt, improved the sound, removing that slight aggressiveness in the presentation. The risk here is, of course, that the opamps will oscillate, but even though I didn't add compensation capacitors, the AD797BRZs seemed to have no audible issues. 




The question that lingers in my mind after building the amp is the price, which is arguably quite high. It sits in a region between the basic amps and the high-end balanced amps such as the Beta 22. Around half of the cost, however, is the case (the last item in the list above). If a cheaper, locally sourced case were used and the kit correspondingly much cheaper, the price would be good value, so a good idea might be to just buy the kit without the case if sorting out a case yourself is possible.
Still, I enjoyed building the amp and the sound from it is very good and it would be a good amp for someone with the DIY bug who likes the idea of a lively-sounding, balanced, and very organised kit amp for a variety of headphones.
post #2 of 5

The kit is composed of the following modules. You can buy the kit without the case kit..


  • Headphone Amplifier  x2  (ヘッドフォンアンプキット)
  • X1 + balance volume conversion circuit board kit (ボリューム+バランス変換回路基板)
  • Power supply PCB (安定化電源キット )             
  • Transformer kit ( トランスキット )
  • Case kit  ( including accessories ie. RCA and XLR jack , knob, AC inlet...) (ケースキット)
post #3 of 5
That looks so difficult to make..
post #4 of 5
Thread Starter 
Originally Posted by William007 View Post

That looks so difficult to make..


If you eliminate the phase splitter and just use a 4-gang pot and balanced inputs, it's not really that complex. The phase splitter adds more wiring, which makes it look complex. If I didn't understand, at least basically, how it is supposed to be wired up, it'd be daunting.


What just occurred to me is that, rather than using 3 braided lengths of wire, what might have been handy would have been to use mic wire (eg: Mogami) for the audio connections, which would take less space. 

post #5 of 5

I'mm borrowing another of Otomatsu (Aurora Sound)'s amps now. It is amazing. Truly truly fixatingly good.

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