1101 Audio Blue Halo OTL Tube Headphone Amplifier

General Information

BLUE HALO is a dedicated amp for high impedance headphones such as the Sennheiser HD600/650/800, ZMF Verite/Verite C, Beyerdynamic, etc. (anything with 200+ ohm impedance). It's a design that eliminates the need for 6AS7/6080/5998 power tubes for output power. It has two 6J5 sockets and a 6SN7 socket. You can plug in a pair or 6J5, a single 6SN7, or both at the same time. With the use of adapters (purchased separately), hundreds (even thousands) of other types of tubes can be used within the sockets as well (including C3G, 12AU7, 12BH7, and many many others). A 12V switch is also included at the back of the unit in order to use 12V tubes. It produces ~1 watt into 300ohms, more than enough for all high impedance headphones.

BLUE HALO is designed to have a clean, neutral, detailed and smooth sound with great bass. Tube rolling alters the tone of the amp at your preference (e.g. warmer tubes, more detailed tubes, more mids, etc.). The tubes get "supercharged" so that they can produce as much power as 6AS7/6080 power tubes, while fully determining the tone. Not having the 6AS7/6080 tubes means one less gain stage between the input and output = More clarity. The high voltage power supply uses only high quality wax/oil filled polypropylene film caps. Output capacitor are also high quality wax/oil filled polypropylene film capacitors. This means there are zero electrolytics within the signal path - something that no other OTL tube amp has!

BLUE HALO does not use tube rectification, it uses my own custom designed circuit using critically damped and resistivily bounded HEXFRED rectification followed by a high voltage Maida style super regulator and utilizing a wax/oil polypropylene capacitor. This is a much better alternative to tube rectification as it leads to better overall tonal quality, better bass and cleaner overall tone. This also gives deeper blacks and bigger soundstage than tube rectification. It also uses regulated DC on the heaters for a silent black background and zero hum.

BLUE HALO has a base build that includes Neutrik RCA & XLR inputs, Neutrik TRS and 4pin XLR outputs, CNC Engraving, QQQ sockets, Alps Blue velvet pot, and high quality copper internal wiring.

Potential upgrade options include: Lundahl power transformer, Stepped Attenuator (e.g. Acoustic Dimension, Goldpoint, Khozmo, etc.), Kiwame power resistor upgrade, Vishay/Dale RN55/RN60 signal resistor upgrade, Supercharger capacitor upgrades, vibration isolation feet, tube socket upgrades, and RCA/XLR connector upgrades. VU Meters are also possible.





Latest reviews


1000+ Head-Fier
BLUE HALO - OTL Tube Headphone Amplifier
Pros: Top-notch overall sound, clear/ clean, consistent, quiet background, input tube optionality
Cons: No output tube rolling optionality (which does bring benefits too)
Let's start with a picture (Blue Halo close-up with 2x Hytron 6C5GTs)

You all, like me - love music. That is the main reason we do all of this playing about and obsessing! We also love gear of course. This post is about a great piece of gear that offers plenty of musical appreciation opportunity.

Whilst I'm science-minded, this will not be a science/measurements focussed review. The amp designer/builder certainly did build from an engineers perspective so "science has been applied", but I won't focus on it. I am aware of the various biases that we all have, myself included. I believe there is some objective reality, but our experiences are both path-dependent and subjective.

Therefore, some relevant aspects of my experiences/path here; I get to listen to music a lot. I may get 10h/day to listen between working and not working. The "best time" is whilst not working of course - just focussing on the music, but I still enjoy music a fair bit throughout the day. I like tubes and I have a modest collection of them. As with many reviews, there is a positive bias because 1) I chose to write the review, 2) I sought out a specific set of things and found them in this amp.

OK. I'm not the important one here, it is the amp and you, so I'll shut up now, having provided what I thought was minimal, relevant context to aide you in deciding if you should weight anything I say above zero.

What exactly is the Blue Halo?
The BlueHalo is an OTL tube headphone amplifier. This one has 4 sockets: 2x 6J5*, 2x C3G. This a minor variant on the "normal" Blue Halo design pictured above which comes with 3 sockets; 2x 6J5, 1x 6SN7. When considering tube amps, there are typically 3 roles a tube can play; rectifier, input/driver, output/power. Of note, the Blue Halo only has input/driver tubes. I believe this design is sometimes called a "spud".

I was specifically interested in an amp with a few characteristics; 1) low noise floor/ black background 2) great sound all around, 3) targeting higher impedance headphones like the ZMF dynamics 4) focussed around 6J5 and related tubes 5) I was willing trade off some optionality for maximum quality. There is a certain amount of "specialisation vs optionality" in these choices. FWIW - in the pros/cons for this review, I list "no rolling output tubes" as a "con", but I am not really clear that it is that. I think it is a design choice/ tradeoff made eyes wide open.

This particular Blue Halo has a few "extras"; Lundahl Power transformer, Goldpoint 47 step attenuator, capacitor upgrade: Jupiter Copper, power resistors upgraded to Kiwame, signal resistors upgraded to Vishay/Dale RN55/60. It also has a 6/12v switch so you can use 12v versions of any compatible tubes in the 6J5 sockets.

How did I get to the Blue Halo? #1) Glenn is one of the most influential and impactful amp builders here on head-fi. Thanks to Zach of ZMF (another one of the most appreciated craftsman here on head-fi), whilst at a CanJam, I was able to hear a GOTL (Glenn OTL) in person. I was also lucky enough to get one by a quirk of fate. #2) That led me to engaging and participating on the Glenn thread which is one of the most enjoyable and informative threads/ groups of people here on head-fi. Through tube rolling explorations with the GOTL, I came across the 6J5 and related tubes, probably thanks mostly to @L0rdGwyn. As a result, I collected many of these tubes. #3) Another Glenn amp owner, @leftside had an amp built by a builder named Mischa - 1101 Audio. It is a beauty of an amplifier. I decided I'd entertain myself by looking for another amp to "amp roll" with, one with some 6J5 sockets natively from a builder with a good reputation. Mischa came highly recommended and we began talking. That resulted in the Blue Halo we are talking about today.

Ending this introductory segment, it is important to mention that the engagement with Mischa was excellent. He was direct, responsive, professional, pleasant, informed, open and good by any-and-every measure. He listened to what I was after, engaged appropriately, responded quickly, answered many (probably ridiculous) questions, and managed my expectations well. Thanks Mischa.

Getting into it
I've had the Blue Halo for 53 days now. I spent the first 3-4 weeks just listening to the Blue Halo, and since then I've been rotating between it and my GOTL, and to a lesser extent a Bottlehead Crack + Speedball. I've rolled some amount of tubes in the Blue Halo; RCA 6N7Gs, Brimar 6C5Gs, Fivre 6C5Gs, Fivre 6J5GTs, Visseaux 6N7GTs, Seimens C3Gs, AnM 6J7Gs, Hytron 6C5GTs, GEC 6J5Gs, and Sylvania 6J5GT, Silvertone 6P5Gs, and National Union 6P5GTs. More details on "rolling" are/ will be posted on either the 6J5 & friends thread , or the 1101 Audio experiences thread . This list is provided to say; there has been some tube exploration to feel out the amplifier.

I'll lead the reader from here; I really like the Blue Halo, probably because it achieves what I was looking for - and then some.

For comprehensiveness sake, the listening setup was Roon (flac files, some Tidal) > SOtM SMS Ultra > SW1X DAC II > Blue Halo > ZMF Verite Closed.

Noise floor
One quote I've heard a few times "If it glows, it hums" (at least from you @mordy). The Blue Halo is quiet. In terms of OTL amplifiers, it is the quietest I've heard. Of course, this depends heavily on the tubes. If you put in noisy tubes; sorry - you hear noise! As long as you don't the Blue Halo is quiet.

Frequency response/ neutrality
The Blue Halo is a neutral sounding amp; it is not emphasised/de-emphasised anywhere in the frequency spectrum. You can, of course, influence this with your choice of tubes to at least some degree. It is not a "bloomy" or intrinsically warm "tubey" amp. Whilst that can be nice sometimes, for me, it fatigues eventually and you want to hear - not that. The amp does really well across the spectrum; if there is big bass, it is there. If there is airy treble or pleasant mids, it is there. Neutral and good.

Transient Response
This is a strength of the Blue Halo. You can hear the attack/decay of sounds, especially when they get busy/faster. This is not easy to pull off. The whole chain must enable good transient response, else you get a blur and lack of clarity/ separation.

Micro-dynamics and micro-detail
The Blue Halo is strong in the micro-detail, especially for a tube amplifier. There is a certain, detailed, revealing clarity without the "etched/bright sound" that comes from many solid-state amps. You can hear the small detailed, layered sounds.

The micro-dynamics have to do with the transient response to some degree. The localised attack/decay loudness vs quietness with the separation/ imaging must come together to create good micro-dynamics. Now, I don't believe that anyone sets out to "create micro-dynamics" per se but that it is the effect created by a well designed and implemented piece of audio kit.

The Blue Halo renders "sincere" sound images, not thin inauthentic or insubstantial tones. For me, tube amplifiers are often better at tone/timbre than solid state amplifiers which have detail but sound somehow flat or disappointing. Getting instruments to sound authentic and tonally rich seems an elusive goal for many amplifiers, but not for the Blue Halo. If you've seen the word "plankton" used in audio forums. I think the Blue Halo has good plankton, which to me - is this "authenticity" in the timbre of tones recreated by the amplifier. This is another strength.

Here is a good link to explain (one guys take on) macro/micro dynamics. To me macro-dynamics often is about "bringing the awe" when a song transitions. It is sort of like the overall dynamic range of a piece, but the other guy says it better. I think the Blue Halo has solid macro-dynamics. This, however, is one area where the GOTL may win.

Headstage/ Soundstage
The Blue Halo has an amply spacious and nuanced headstage. If you imagine 3d; "up-to-down", "front-to-back" and "left-to-right" - these are all proportional and spacious. This allows you to hear sound image placement throughout the 3d space. I think the GOTL has a slightly wider stage, however Blue Halo is no slouch. I think the Blue Halo may have a relatively taller headstage due to a bit more air upstairs in general. I have, on occasion (through rolling, changing headphone pads, etc) tried to narrow down the head-stage on my GOTL so I could get the image more "in front of me". The bottom line is that Blue Halo provides a solid spatial representation.

The Blue Halo has truly excellent bass and sub-bass. I listen to a lot of electronic music, which often uses the lower end of the sound spectrum amply. The Blue Halo manages to bring the bass, but not cloud the rest of the music. I have listened to a ton of headphone amplifiers and I think this is not universally achieved. With the Blue Halo the neutrality and solid imaging come together with the excellent transient response which together brings a tight control of substantial (when it is there) bass.

Tube Optionality
Natively this amplifier can support many tubes; 6J5, L63, CV1932, 6C5, 6P5. The 6J5 sockets also support 6N7*, ECC31 tubes. The C3G sockets support, of course, C3Gs. If you want to involve adapters (certainly contact @Deyan for some nice ones) other tubes come into play such as; 6C4, CV133, 76, EL3N, EL6, EL8, EL32, VT52, CV1052, EL11, EL12N, EL13, 6J7. As mentioned above, in this amp the 6J5 sockets also support 12v versions of those where available. Now, you have to admit, that is a fair bit of optionality. You can also run "either or both" sets of sockets to offer even more mixing options. Whilst I've done a fair bit of experimenting, there is still a lot more to go.

Worth noting, the amp is designed with a CCS (constant current source) in it. This does have the effect of making tubes perhaps sound more similar within a given tube family. There is still variety between different tubes, but more variety across tube types (i.e.; more difference between 6J5 and 6P5, than between 2 different kinds of 6J5). This aides consistency and clarity whilst bounding variety somewhat (but probably for the good).

FWIW, I think my top tubes so far in the Blue Halo are the; Brimar 6C5G, RCA 6C5, which contrary to my last comment - do sound different😉

Also worth noting; you don't need a boatload o' tubes. You can tube roll - or not - your choice.

The amp is beautifully made. The external parts, casing, etching, etc is all top notch. It is quite professionally presented. You'd be hard pressed to find a more well finished headphone amp from any manufacturer/ builder. It is built internally and externally with top notch parts, some of which contribute to the sound, some of which contribute to the durability or the aesthetic, all of which work well as a whole.

Compared to other amps

Compared to the GOTL. I've had many questions on this, so it would be unhelpful to exclude some commentary. I feel almost like I am trying to compare my two children (never do that!). I set out to find a compliment to the GOTL so it is not surprising that they each do certain things differently.

The GOTL is a chameleon and a companion to the intrepid tube explorer/experimenter. It can be rolled into virtually infinite arrangements. Part of this is output tube rolling (and in some GOTLs rectifier tube rolling, but not mine). This optionality is amazing, at the cost of consistency or clarity sometimes. The Blue Halo sounds very good on most things. The GOTL can sound amazing and "bring the awe" depending on your roll, the music content, etc. The difference between 2 kinds of the same tube may be bigger on the GOTL (which has no CCS, which has something to do with this I believe). The GOTL is somewhat more noisy (for me at least) and you trade off some clarity for some of the pleasant distortions. These pleasant distortions can sometimes be fatiguing for very long listening. GOTL may win on macro-dynamics. With most of my common rolls, the GOTL has sweeter mids and some overall pleasing smoothness (I bet with tube rectification, there is even more). This sweetness/ smoothness is beautiful and can mask some detail. It is perhaps sometimes more veiled, with Blue Halo winning on clarity and airy upper frequency representation. On the Blue Halo perhaps more music sounds good, but the GOTL, some music sounds really distinctive and amazing.

The GOTL is a prominent part of my listening life and will occupy probably ~1/2 of my listening time as it is a master of variety. People love the GOTL, because it can be that tube romantic amp, or a much more clear and steady champ, depending on your roll, your mood and the like. I feel thankful that I can "amp roll" and have both to compliment one another. People have said "you must have a favourite" or "you are being polite" or other such things. My feeling is that I really like them both for their own reasons. The Blue Halo is now an important part of my listening life and guess what - it will probably occupy ~1/2 of my listening time as it is a master of consistency. The GOTL is one of the most respected tube amps here on head-fi and I think it is very impressive that the Blue Halo favourably compares on many dimensions.

Compared to the Crack + Speedball (here is mine), there is not much comparison. I really like the Crack + Speedball (CCS), but it is a bit more classic tubey than the GOTL or Blue Halo. Whilst I somewhat liked the Crack without speedball, I prefer it with.

I am so torn, rotate Blue Halo/ Glenn from home to office, or keep the Crack at the office so I can amp roll at home? #firstworldproblems

Why write this? #1) Mischa/ 1101 Audio and The Blue Halo deserve positive attention. #2) taking notes, listening and writing this gave me something fun and positive to focus on today, and #3) I'm feeling thankful in general.

The Blue Halo is a master of high quality consistency. Furthermore, it is an excellent balance of consistency and variety whilst maintaining top flight sound quality. It is amazingly quiet, clear and non-fatiguing for long listening. It doesn't "give up" tube beauty but allows the best of tube attributes to shine through an overall neutral and clear presentation. As someone who listens a lot, variety is important. Being able to roll quite a substantial variety of tubes provides that variety, without compromising. So - if you are seeking a beautiful OTL tube headphone amp; I can wholeheartedly recommend the Blue Halo.


N.B. Here are some pictures

At Mischa's before it was shipped to me:
pasted image 0 (8).png
pasted image 0 (7).png
pasted image 0 (6).png
pasted image 0 (5).png
pasted image 0 (4).png

Also at Mischa's - some measurement pics:

Obligatory sinwave.
pasted image 0 (3).png

Here is a shot comparing an almost silent tube (Marconi 6J5 - in yellow) vs a slightly noisy RCA 6J5 (in blue):
pasted image 0 (2).png

Here is the amount of noise on the output of the amp with volume all the way down. Noise is not measurable by my oscilloscope (under 300uV noise):
pasted image 0.png

Here is the same with the volume turned all the way to max and nothing playing. Noise is still below what my oscilloscope can measure:
pasted image 0 (1).png

Unboxing at home:



Settling into the environment:

A few tube rolls:

Brimar 6C5G up close

Fivre 6J5GTs

Fivre 6C5Gs

Hytrons again:

And finally, ending where we started - MUSIC. Here is some music used whilst forming impressions; a Tidal playlist.
Last edited:
@chrisdrop It's well earned. I think I told you. Christmas is coming so I got my order in early with Santa Mischa for a end game Headphone cable using Neotech OCC Silver, Furutech connectors and a Eidolic Splitter along with another array of adapters.
Full disclosure I prefer Santa Mischa because he delivers more then once a year!!
Excellent writeup! I'm a little late to the party, but in line to have a Blue Halo built. This has been extremely helpful. Thank you, sir!
Mischa is great. I'm sure you'll love it.



1000+ Head-Fier
Wow, it looks hard core
Mischa is a great builder. There are some great tube amp builder options IMO. They are ALL busy with reasonable waits I bet.

(in no particular order).
  • @A2029 of 1101 Audio makes several nice amps, like the blue halo
  • @L0rdGwyn has a new amp called the Airmid. I've not heard it, but I bet it is great.
  • I think possibly Craig will do just one more round of Studio Bs. He was supposed to be done, but rumours of a parts remain for a few few more.
  • Or UltraSonicStudios who also makes several nice amps. I've not heard one, but they are popular custom amps here on headfi.
There are surely more out there in the wide world, but all of these builders are solid.

Good luck.