[ClieOS]

The BL-2 is a little tube amp from a fairly new Chinese amp manufacturer called ‘Tube Amp’. Perhaps not the most ingenious of naming, they do however only focus on portable tube amp. The DIY’er behind the company began his own quest of building a portable tube amp years ago, and has successfully launched a commercial model named BL-1 (with subsequent reversions) even before starting up the company. BL-2 is his latest update and replacement of BL-1. It has a MSRP of RMB2600 in China, roughly equal to US$410. As far as I know, no official international dealer besides a few in Hong Kong and Taiwan for the moment. The discontinued BL-1 on the other hand can still be found on eBay for around $290 with shipping included.

Spec
Frequency Response: 10Hz ~ 100Hz
Input Sensitivity: 200mV
Output Power: 2 x 400mW
THD: ≤0.001% (1kHz, 2.5V)
SNR: ≥106dB (input shorted)
Recommended Headphone Impedance: 16ohm ~ 300ohm
Continuous Operational Battery Life: 3.5hours
Li-ion battery with over-charged protection circuit
Dimension: 110mm x 70mm x 25mm
Weight: 245g (battery included)

Packaging, Accessories and Build Quality
The amp comes with an excellent PU leather box - remind me of HifiMan’s packaging, but a little better. Accessories wise, there are the AC charger and a short 3.5mm interconnecting cable that looks rather good in quality.

The design of the amp itself really isn’t something ground breaking. It has a relatively big grill for better air circulation, but otherwise it is similar to other portable tube amp that comes before it. However, the previous model, BL-1, has been in the market for a while now (just not the international market), and BL-2 is based on the rarer 6N16B-Q miniature tubes and not the much more popular 6111 (on portable tube amp application). For those who don’t know, 6N16B-Q is the same tube Little Dot used on its desktop bound, dual tube hybrid ‘Micro Tube’ amp about 6 years ago. While I don’t have a lot of knowledge into electronics, I can tell the BL-2 uses a tube + MOSFET topology rather than the Micro Tube’s tube+opamp design.

The amp runs fairly hot in operation, somewhere around 30°C to 40°C (on the housing) and lukewarm to the touch. Infrared thermometer shows that the tube itself is closer to 75°C, which is definitely enough to burn your skin if you touch it directly (if you removed the top cover). That being said, it definitely isn’t a good idea to use this as an actual portable unit attached directly to a DAP or stuff it in the bag with other gears when operating. It will probably serve better in a transportable or home-away rig when traveling around (or even a miniature tube setup around the house). Plus, the battery life isn’t great as well, maxing out around 3.5hours (might go a little longer depends on how you use it). But given how hot the tube goes and the general inefficient of tube amp, that’s to be expected. Recharging is done via the included 13.8V world adapter, generally takes around 1~2 hours in total. The amp’s LED will turn red when charging, but you will have to look for the LED indicator on the charger to know whether the amp is fully charged or not. The Li-ion battery inside has overcharge protection, so you can leave it over night without any problem.

Hiss can be noticeable when volume knob is turned over 12 o'clock if you are using very sensitive IEM such as SE530. But as I will later explain in the SQ section, IEM of such sensitivity is not recommended with BL-2. EMI is also noticeable when BL-2 is placed very close a cellphone during call, but it isn't too terrible.

Size and weight wise, BL-2 is almost on par with O2, nearing 300g in actual measurement. However, unlike O2 and its heavy housing, half of the BL-2’s weight seems to come from its over-sized Li-ion battery back. Even without the heat issue, I would call this kind of size and weight as semi-portable or transportable.

Size comparison: O2, UHA-4, BL-2 and C421.

Sound Quality
I almost always start my amp review with some basic testing using RMAA, and BL-2 is no exception. Like a lot of tube amp, BL-2 doesn’t have a stellar RMAA result. The overall noise is much more significant (twice as worst, if not more) than any solid state amp I have measured using RMAA under the same 16ohm load. It also shows coloration to the frequency response curve, indicating noticeable impedance interaction. However, the result gets a lot better once I re-measured it again under a 150ohm load. Bass still has some roll off (-3dB @ 20Hz, probably due to too small of output caps) but no more coloration. Maximum gain, as estimated via RMAA, is around 19dB. On separate test, output impedance is calculated to be around 20ohm – much more than your typical solid state amp but somewhat expected in this kind of design. Current output to low impedance load isn’t great as well, which explains why noise is high on the 16ohm RMAA test but gets a lot better with higher impedance load, as the higher the load impedance goes, the less current is needed. All and all, BL-2 is a not for IEM. Well, not for most IEM. It is better suited for 150ohm and beyond – which is more in the realm of full sized headphone.

Unfortunately I don’t really have any full sized with that high of an impedance, so my listening is mostly done with ER4S (~100ohm), RE262 (~150ohm) and Lear LCM-5 with the Studio Monitor Sound Tuned Adapter (~180ohm @ 1kHz). The first thing I noticed is the overall cleanness. Having own a desktop hybrid amp myself (the Little Country 2 by FireStone Audio), I am kind of expecting a warmer, richer sound from BL-2 – not that BL-2 doesn’t sound richer than O2 or C421-AD8620, it is still very neutral and clean in the overall presentation. If I wasn’t A/Bing these amps carefully, I would think I could have missed the subtle difference between them. The second thing I noticed is a softer bass impact from BL-2, probably because of the slight bass roll-off. It isn’t bass light per se, but definitely doesn’t go as deep or impactful as it can be.

As said, BL-2 does sound just a tad richer in tone, but it isn’t an obvious warmness or forwardness. It just sounds like all the background detail is pulled slightly to the front, filling out the ‘space’ between instruments and vocal. If you played with Photoshop before, it will be the equivalent of making color saturation a little higher, thus making detail more visible to the eye yet doesn’t look unnatural. One aspect of BL-2 I really like is how it renders guitar. Several Kaki King’s guitar tracks that I am very familiar with plays absolutely beautifully over BL-2. Perhaps that’s the reason why many guitarists like tube amp? I don’t know. But I do prefer guitar tracks on BL-2 over O2 or C421-AD8620, that’s for sure.

As far as subjective listening is concerned, I’ll place BL-2 among some of the best portable headphone amp I have listened, namely O2, StepDance, C421-AD8620 and UHA-4. I won’t say BL-2 surpasses any of them, but I think it is safe for me to say that they belong to the same category.

Glow in the dark.

Conclusion
BL-2 is one of those cases where you get mediocre-to-decent measurement with a good sounding amp. However, as far as I can tell, driving high impedance load seems to avoid most of the issue regarding high output impedance and low current output – and the result can be very rewarding for sure. The price tag of BL-2 is quite high, well above most solid state portable amps but still not the most expensive miniature tube based portable amp around. With the rather high price tag, the discussion of value seems very much an oxymoron, so I’ll leave you to decide that factor for yourself. I can only say, subjectively and with the right headphone, BL-2 can sound really good

A big thanks to Tube Amp for the review sample.

 

[Evshrug]

Thanks for the review! I had been looking forward to this review.

It sounds like this amp is a better match for mynext headphone, as my current flame (the AKG Q701) is only a 50 ohm load, and half the price of the amp. Still, it is very interesting to read the review in the company of your other SS amps.

 

[TUBE AMP]

Thanks for your review.

 

[Swimsonny]

Just recieved mine and i can not help but instantly wondering that while sound impressions are nice so far even with out burn in and the size can still pass for portable, the amount the grill heats up surely stops it being portable, i almost burn my hand anywhere near it, probably not ideal in a pocket or the back of a DAP. 
 
EDIT: Just realised that is nicely in line with your impressions!

 

[Swimsonny]

 

[Swimsonny]

Re Reading your review and comparing the BL-2 directly with the O2 i have actually found it fair easy to distinguish some differences. I have been using the Heir 4.Ai and have found the high mids a lot more prominent and the prominence in fact continues down into the treble. it makes the amp sound like it is oozing a lot more clarity. The sound is as you said noticeable richer and this is generally across the range, more energy in everything, mainly the treble and the mids a little bit. Bass i have to agree with you in terms of the roll of is worse on the BL-2 by a little bit but would not call it bad, mid bass is more impactful with the BL-2 however. One thing is for sure though this does not have the warmth i usually hear with tube amps like the Little Dot series i have heard. I really wish i could take it on the go and the silly heat up of the tube prevents that!

 

[smial1966]

I'm struggling to understand why anyone would purchase a BL-2 when the ALO Continental V3 is a comparable price, has significantly better battery life and doesn't become nearly as hot in operation.
 
Andy.

 

[ClieOS]

Quote:

I'm struggling to understand why anyone would purchase a BL-2 when the ALO Continental V3 is a comparable price, has significantly better battery life and doesn't become nearly as hot in operation.
 
Andy.

 
True to some extent. As I have no heard or used Continental V3 before, I have no idea on how it actually performs. But the question, does that lower temperature affects the performance of the amp? As you may have know, tube must runs up to certain temperature (which takes a certain voltage) to perform at its best. It would be easy to just lower the voltage to the tube to get lower temperature, but it won't run nearly as good as what it is designed to be. Using lower voltage will also extend battery life as well, but they all come with a price.

 

[smial1966]

ClieOS
 
My ALO Continental Mark 2 runs fairly warm but can still be partnered with an iDevice/MP3 player without temperature issues, sounds astonishingly good and the battery longevity is at least 8 hours. So I really can't see the rationale for purchasing a BL- 2.
 
Andy.
 
Quote:

 
True to some extent. As I have no heard or used Continental V3 before, I have no idea on how it actually performs. But the question, does that lower temperature affects the performance of the amp? As you may have know, tube must runs up to certain temperature (which takes a certain voltage) to perform at its best. It would be easy to just lower the voltage to the tube to get lower temperature, but it won't run nearly as good as what it is designed to be. Using lower voltage will also extend battery life as well, but they all come with a price.

 

[Swimsonny]

I do struggle too see the use of the BL-2. While it clearly has a high performance, it is not really a desktop amplifier and not a portable one either. I guess transportable is what it is but then you have the likes of the Pan Am which include a DAC for just a lil more.

 

[ClieOS]

Quote:

ClieOS
 
My ALO Continental Mark 2 runs fairly warm but can still be partnered with an iDevice/MP3 player without temperature issues, sounds astonishingly good and the battery longevity is at least 8 hours. So I really can't see the rationale for purchasing a BL- 2.
 
Andy.
 

 
Yes, but the key point is whether the tube itself is natural running colder then most tube or is it deliberately designed to run underpowered so it can run on lower temperature, as running on under-voltage condition (which result in lower temperature) will cause the tube to fail much early than it should, or at least greatly increase the chance of serious performance issue down the road. This could be a problem, though might not affect immediately, on an amp that isn't designed to roll tube easily as the tube is soldered onto the PCB. That's good reason why all tube are rated for some minimum working voltage and shouldn't go under it. I am not saying Continental has a under-volatged design, but it would be potential problem if it does and worth finding out if you are an user / potential buyer.
 
 
 
Quote:

I do struggle too see the use of the BL-2. While it clearly has a high performance, it is not really a desktop amplifier and not a portable one either. I guess transportable is what it is but then you have the likes of the Pan Am which include a DAC for just a lil more.

 
With those two tubes sticking out on the top, I really don't see Pan Am as much of a transportable as you will need a big box to put it in order to protect those tubes - if you choose to do that, might as well bring a larger tube amp with you since you are blurring the line between being transportable and being movable.

 

[CJG888]

I have heard a strong rumour here in China that C&C are about to launch a successor to the TU. Does anyone know anything about this? With any luck, it should be somewhat more reasonably priced than the BL-2, and (if the old one is anything to go by) easier to use on the move. If only it didn't look like a hipflask....

 

[smial1966]

ClieOS
 
The Continental is now in it's Mark 3 derivation and has been extensively reviewed on Head-Fi and elsewhere. In all of the threads that I've read tube failure has not been discussed and longevity doesn't appear to be an issue. 
 
The BL-2 is just too darned expensive, runs far too hot to be considered portable and has a lousy battery life. If these three factors are addressed I'd consider purchasing it, as I'm a huge thermionic fan, but it just doesn't make sense in it's current form.
 
Andy. 
 
Quote:

 
Yes, but the key point is whether the tube itself is natural running colder then most tube or is it deliberately designed to run underpowered so it can run on lower temperature, as running on under-voltage condition (which result in lower temperature) will cause the tube to fail much early than it should, or at least greatly increase the chance of serious performance issue down the road. This could be a problem, though might not affect immediately, on an amp that isn't designed to roll tube easily as the tube is soldered onto the PCB. That's good reason why all tube are rated for some minimum working voltage and shouldn't go under it. I am not saying Continental has a under-volatged design, but it would be potential problem if it does and worth finding out if you are an user / potential buyer.
 
 
 
 
With those two tubes sticking out on the top, I really don't see Pan Am as much of a transportable as you will need a big box to put it in order to protect those tubes - if you choose to do that, might as well bring a larger tube amp with you since you are blurring the line between being transportable and being movable.

 

[ClieOS]

Quote:

ClieOS
 
The Continental is now in it's Mark 3 derivation and has been extensively reviewed on Head-Fi and elsewhere. In all of the threads that I've read tube failure has not been discussed and longevity doesn't appear to be an issue. 
 
The BL-2 is just too darned expensive, runs far too hot to be considered portable and has a lousy battery life. If these three factors are addressed I'd consider purchasing it, as I'm a huge thermionic fan, but it just doesn't make sense in it's current form.
 
Andy. 
 

 
Most review I have read on HF and else where hardly touch the actual hardware itself. As said, problem might reveal itself in time if it is there. If there isn't any problem at all, all the merrier. Again, as I have said before I have not heard any of the Continental variation (all 6111 based, IIRC) and will not assume its performance unless I have actual tested it myself.

 

[smial1966]

ClieOS
 
The first Continental was released over 3 years ago now, so I'm sure that any valve longevity issues would have surfaced by now. Incidentally, have you noticed how aesthetically the BL-2 is very similar to the Continental? The volume knob markings are identical. 
 
Andy. 
 
Quote:

 
Most review I have read on HF and else where hardly touch the actual hardware itself. As said, problem might reveal itself in time if it is there. If there isn't any problem at all, all the merrier. Again, as I have said before I have not heard any of the Continental variation (all 6111 based, IIRC) and will not assume its performance unless I have actual tested it myself.