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iBasso PB2 and Headroom Ultra Micro Amp - first impressions.

post #1 of 11
Thread Starter 

I promised to do a review of my two portable headphone amplifiers, the IBasso PB2 and the Headroom ultra-micro amplifier. To do this review I employed 2 headphones, the Sennheiser HD 650, and the Grado SR225i. All the equipment was run in for two weeks before commencing any evaluation. On order, and yet to arrive is a balanced silver cable for the HD 650, to take full advantage of the iBasso balanced output. Since this is going to be get very complicated very quickly, it makes sense to do this evaluation in several stages. My reference system is my Sugden Bijou CD and preamp. The headphone output of the preamp is such that the preamp is often used as a headphone amplifier in its own right. I have been playing about with hifi since the days of John Linsley Hood and I have got to the stage that an ‘upgrade’ is not really meaningful unless you get the same buzz when you return to a recording weeks later. A-B-A comparisons are all very well but unless you can identify A or B in isolation, then the benefits are a diminishing return. Understanding this philosophy explains why my reviews will not be in questionable detail.





I chose a number of diverse recordings, not the usual “simple" acoustic recordings, like Chris Jones, Krall and Melua which tend to sound good on almost all systems but the more complicated recordings with both classical and electronic instruments, that often sound muddled on poor systems. These included a Bach violin Concerto, Vivaldi's Gloria and a dynamic track from Rush’s “Counterparts" album. The two headphones are very different beasts, with the well-known diverse sound signatures. The Sennheiser's opulent output versus the revealing nature of the Grado.


In the iBasso amplifier, I selected the AD797 Op-amp and the normal buffer. On order I have 4x OPA627, which I hope do not turn out to be fakes. These are the same up-amps that are used in the now out of production headroom Ultra Micro amplifier. When it becomes available I will order the iBasso DB2 DAC.


It has to be said that neither amplifier, when fed by an iPod 5.5g is close to the Sugden system, even with lossless files. With either set of headphones the Sugden system is more involving, dynamic and natural. One is not aware, in an analytical way, of the frequency extremes but when you listen out for them they are clearly apparent. I think I'm trying to say that the sound is cohesive and unforced with either set of headphones. Let's move on to the portable amplifiers. I fitted a Kimber dock connect cable to iPod 5.5g and fed the single ended inputs of both amplifiers. In the case of the Headroom amplifier, with either set of headphones it is fair to say that the sound output is warm and polite. Bass notes go very low, are quite ‘obvious’ and are accurately rendered, treble is smooth and the mid-band is quite tame. Playing the Rush track, as the music escalated the sound didn't enthrall in the same way that the preamp gave out and the tunefulness of plucked guitar strings wasn't there. It seemed to be holding back. This amplifier did curb the sibilant treble of the Bach violin concerto with the Grados. The combination of the Headroom amplifier and the Sennheiser HD 650 was too warm for my liking but offset the more forward balance of the Grado headphones very well. Both amplifiers reproduced the ambience and separated the voices of the massed choirs in the Vivaldi work, a little more in the case of the iBasso.


Moving on to the iBasso with the supplied AD797 amplifiers installed, it is fair to say that this amplifier has a lighter touch. In combination with SR225i headphones it is getting close to sibilant with the close-miked violin solos. With the Sennheiser cans, it clearly has less bass extension and general warmth than the headroom amplifier but is able to convey the dynamics of the Rush album and the tunefulness of the bass-line that is more pleasing. It will be interesting to see how this amplifier works with a balanced output into the Sennheiser headphones. I have a feeling that the silver cable will be more revealing and the better dynamics will work well with this little amplifier. It is becoming obvious that there are obvious pairings here – Sennheiser/iBasso and Headroom/Grado for an even balanced sound both tonally and in dynamics.


In both cases these little amplifiers give more authority and ambience to my music than the direct headphone output of my iPod 5.5 G. Bass was clearly better defined, deeper with more weight and treble quality was more refined with better resolution. In the next phase I will take the digital output of the CD player and feed it into the DB2 DAC using its balanced output directly into PB2. It will be very interesting to compare that with the ‘Sugden’ sound. That is something for next month, with a little luck. I hope this is of interest. regards Chris

Edited by iVinyl - 3/20/11 at 11:21am
post #2 of 11

Chris,  thanks for the nice write up.  Yes, your impression is consistent with my own on the Ultra Micro amp and with Headphonia on the PD2.  Headroom's amp does have a warm sound signature, I had the Ultra mico amp and dac for two years before I sold it for the BUDA.  I can see why it is too warm with the hd650/Ultra combo.  The HD650 should and will match much better with the DB2/PB2 with  balance connection but do keep us posted on the silver.  With the slightly lighter/thinner body and low range on the PB2, I am curious if the difference will be big between silver and copper.  

post #3 of 11

So are both headphones balanced? 

post #4 of 11
Thread Starter 

Not at the moment. The dragon cables are on their way for the HD650's and we'll see how it goes. The Grado's slip into a shoulder bag at a pinch and were designed to be a semi portable solution. The HD650's I plan to feed direct from my computer, via SPDIF into the DB2 / PB2 when it becomes available.


I'll also try the iPad/camera USB connector idea with the DB2 and see if that works and get around the iPod limitation.


Sorry for the big text by the way. I just did a cut and paste from Word and didn't realise the forum would take any notice of the Font settings!


It's fun this stuff, isn't it smile.gif

post #5 of 11

Thanks for the impressions. Look forward to your experience with the OPA627 in your PB2.

post #6 of 11

Well the primary design of the PB-2 is that of balanced. Some balanced amps don't offer SE as an option. So while it is valid to test out the SE mode, I would think using it in its optimal mode, balanced, would better render what it is fully capable of. The D6 and P4 are excellent SE portable amps. 


To my ear, the HD650 sound better balanced than SE. 

post #7 of 11
Thread Starter 

I have just seen on the other thread that the DB2 is on the way - fun fun.  I haven't had so much fun since I blew up £100's worth of Mosfets when testing a 100W RMS into a fully capacitive load!

post #8 of 11

I know what you mean. This almost equals when I got hit with 720 volts from a supply cap on my preamp after a "friend" had incorrectly installed a transistor. Oh, for the smell of burning flesh, though made all the more unpleasant as it was my own. 

post #9 of 11
Thread Starter 

Silver Balanced HD650 cables and OPA627's should be with me for the weekend and I pre-ordered the iBasso DB2 today - proceeds to Japan. Do I need to run-in IC's?  I thought it was capacitors which were the main reason for burn-in?

regards Chris

post #10 of 11
Thread Starter 

After about 50 hours burn-in I evaluated the Silver Dragon balanced cable on the HD650's against the stock single-ended cable on the PB2.

Gosh what a difference! - and not in the way I expected.  With AD797 and BUF in place, I evaluated the two configurations back to back. In balanced mode, I was bowled over by the improvement in dynamics and bass definition. I hadn't expected that. The top end was more refined and less extended, I certainly didn't anticipate that either. It managed to convey the dynamics of the music better at the expense of treble extension. That is a little unfortunate as the HD650's are in need of a little more high frequency energy. However you get carried away with the music in a very natural ambience and forget this on good recordings. When I returned to the stock cable, I realised it was a little lifeless and unrefined in comparison.  


I then checked the web and found a couple of reviews of the Silver Dragon cable. These confirmed a very similar sound signature. Taking that into account, I am thinking that the PB2 'balanced' part of the overall improvement is down to the dynamics and the stealth-like improvement in the midrange. I do like the balanced connectors by the way. Really nice action and positive lock.


There is quite a gap opening up between my SR225i's and the HD650's.  Each improvement widens this gap, especially in tonal balance, to the point that I might have to choose another portable headphone. The SR225i's, even with extensive burn-in, can be quite uncouth at times especially when the brain has no chance of coping with a back to back with the HD650's. I would really like something smack in the middle tonally - which probably means I'm heading towards rockbox again on the iPod and alternative Mac OS music players with decent equalisers.


Next step is to try the OPA627's when they arrive, with and without buffers and then when the DB2 arrives, with an iPad with the camera connector....:)


have a nice weekend

post #11 of 11
Thread Starter 

Part III with the iBasso PB2 - comparing AD797 / BUF 634 with OPA627BP / BUF 634 in balanced mode with Silver Dragon cables and Sennheisser 650's. The source was an iPod 5.5g with a Kimber dock cable.  The date code on the OPA627BP was 9820WM5016 and measured 50-60Kohms between pin 1 and 5 across the four chips. There was a semi circle cutout between pins 1 and 8 but no dimple next to pin 1. Items were bought from eBay seller 'tube_buyer', $72 for four including shipping. I believe these to be genuine chips.  (BTW, one reason I reckon these chips appear cheap in the Asian markets is that many are not ROHS compliant - you can tell from the official datasheets - my guess is that the old production was dumped and the more enterprising communities bought them up)


I'm not clear about burn-in requirements for IC's but in a straight comparison, I compared the above combination. I don't really want to keep taking chips in and out all the time, so this was not an ABA but an AB comparison. I listened to a number of blues, rock, classical and choral works several times, taking notes of certain subtleties. Swapping to the OPA627BP, the sound was more rounded. Lower mids were more pronounced and the treble didn't feel like a dusting on top. In all, the presentation was less forced and more natural. Vocals were more prominent and the bass was marginally less obvious. I would venture that the overall sound was less focussed but at the same time less fatiguing. Is it worth it?  marginally - I think the sound will grow on me in time. I was finding the bass a little overwhelming before.


Does that gel with what others have experienced?  I'll burn in the assembly and also compare with the more revealing SR225i's later on in the week.


The DB2 will soon be on its way - which is when the fun starts.


kind regards



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