On Headphone Listening Pt. III The Finale! Jupiter Audio Research HD600 abbasaudio OTL Amp (also Schiit Valhalla 2, Eddie Current Dana Deux)

On Headphone Listening, Part III The Finale! JAR HD600 abbasaudio OTL Amp

By mrgoodsound 

Part I, Part II

This article will be part three of a series on headphone listening, and for now, a finale. I have received my heavily modified HD600 headphones, and output transformerless amplifier, both of which have been on order since late 2019 and will be the main subjects of this article. After living with both for several weeks, I am ready to share my thoughts and feelings on them.

In some ways, the findings in this article and my current headphone system have been a work-in-progress for 5 years. As I mentioned in part one, I began my journey in audio with headphones and have been chasing a sound that only existed as a concept in my head. Until now I had never quite gotten there, despite many hours and dollars spent. Like all good stories this one has a beginning, middle, and end.

In the beginning

Schiit Valhalla 2

The first headphone system I would consider myself satisfied with was a stock pair of Sennheiser HD650s and the Schiit Valhalla 2 amplifier. I would define satisfaction in this case as the ability to listen for a period of at least several hours without fatigue and with involvement in the musical program. The HD650 was chosen for its linearity in the critical middle frequencies and lack of distracting sonic aberrations. The Valhalla 2 was chosen (at the time) because of its optimized power current delivery into the Sennheiser's 300 ohm load. Unfortunately, I did not comprehend just how important its transformerless topology was until much later. The Valhalla 2 had several compromises in its design, chiefly the use of copious amounts of negative feedback to lower the output impedance and achieve sonically unimportant low distortion figures. I understand that Schiit as a mass market company did not want to manufacture a 'specialist' amplifier that was only good for headphones with high impedance voice coils, and thus feedback and a cathode follower were incorporated into the design. Their marketing images showed the Valhalla 2 connected to an Audeze LCD-2 for example, despite this being a pretty terrible match.

This amplifier was still a superior match to virtually all transistorized and hybrid options I had tried, but unfortunately, these compromises meant that the Valhalla 2 had a dryness and grain to its middle to high frequencies which caused fatigue after several albums worth of listening. This could be alleviated to a degree with a warmer or laid-back source, but given that I wanted to listen for more than 1-2 hours at a time this ear irritation became the reason I started to look for upgrades to the Valhalla in 2017.

In the middle

Original Zana Deux, credit 6moons.com

To cut a very long story short, 2 years of time and many thousands of dollars were wasted searching for the ideal solution. I began reading headphone forums and researching all the high-end valve-based amplifiers available on the market. I tried many headphone amplifiers ranging from $1000 to $3000 (!) but was ultimately disappointed with each. Most of these models were traditional transformer-coupled single-ended designs. It is widely agreed that in these designs, the quality of the output transformer is responsible for the integrity of sound. For reasons I still cannot fully explain, it seems that the sonic compromises of output transformers are more easily detected or even amplified when listening with headphones. You cannot get over the impression that you are 'monitoring' the music rather than indulging in its aesthetic qualities. Some of these amplifiers made attempts at revealing subtleties in the sound but usually at severe compromise to one or more parameters such as dynamics or transients.

At first I thought the matter was related to impedance, as most audio transformers are specified with secondaries of 4/8/16 ohms, but then a later experiment with an output winding specifically optimized for ideal damping factor into a 300 ohm load proved disappointing, so I ruled this out.

I decided to return to OTL amplifiers, and settled on trying one model in particular, the Eddie Current Zana Deux. This model has been around since 2005 and is well-known in Head-Fi circles. It had a certain mystique about it and a very flattering review on 6moons. Unfortunately this experiment would further detour me from the ideal path. The amplifier was extremely colored and musically unsatisfying. I again attribute this to its design compromises: a copious amount of negative feedback and giant capacitors to output couple the giant 6C33C triodes, ruining any chance of musical integrity. I did own the latest version, the 'Super', and as I understand the original would've better suited me.

Discouraged and apparently frustrated after these experiences, I took several months hiatus from headphone listening and tinkered with stereo equipment instead. It wasn't until I exchanged PMs with an audio acquaintance from Denmark that I learned of the manufacturer Abbas Audio, and saw that he had an OTL amplifier in his catalog of products. I took quite a large gamble to order this amplifier unheard, and it is the subject of today's review.

Jupiter Audio Research HD600 Mod 

My JAR HD600s

Having sold my HD650 before taking a break from headphone listening, I needed a new pair of high Jupiter Audio Research through forums. Jupiter is an independent manufacturer of modifications to the popular Sennheiser HD600 & HD650 headphones. Many community-sourced modifications exist already for these models, most of which involve applying mass-loading and dampening to the driver housing and spider. I never tried these for fear of irreversible damage to my favorite pair, but I was attracted to Jupiter's mods as they took things a step beyond the DIY solutions. He fabricated a 3D printed screen to take place of the headphones cup grills and provide the necessary rigidity and dampening to the headphone housing. Some unnecessary elements of the stock housing were completely removed, and some experimentation was had with the amount of damping foam present in the rear of the driver.

The mod is fantastic! All the positive elements of the stock sound are retained but a much higher technical level is achieved. The sound possesses more clarity and cleanliness at all frequencies, transients become sharper and less smeared, and a pervasive softness present in stock form is replaced with a sense of solidity and previously unheard composure. The character of the headphones does not change, and the limits of the driver can still be heard, but I highly recommend it to anyone who is already a serious fan of the HD600 or HD650.

In the end: abbasaudio OTL amplifier

This amplifier has transformed my headphone listening experience. It is no longer possible to listen to music via headphones as background noise. Unfortunately many of the technical design details are not available in English, so I will speak only to what I can. The 6080 triode was chosen for the output lamp based on its exceptionally low plate resistance, making it a natural for a design without transformer loading. The passive parts and wiring are largely vintage. Of note are the 'bumblebee' paper-mylar coupling capacitors typically found in American-made electronics of the 1960s.

I now understand that the properly designed output transformer-less valve amplifier is an ideal (read: less compromised) solution to drive high-impedance headphones such as the Sennheisers. The sound from this amplifier is incredibly immediate and pure; both in its tonal colors and its clarity of the emotion and message of music. Grip on the drivers without over-damping is outstanding, with an ability to maintain composure and drama under the most complex and demanding of musical passages to a degree that I previously thought the Sennheiser's were not even physically capable of.

My headphone listening station

Despite the measured roll-off of the HD600 driver, the depths of low frequency content through this setup is surprising, not unlike Stax, but without the electrostat's plastic bass timbre and lack of muscle. It has been said before, on this very blog, that bass is the foundation upon which musical energy is built. I was completely oblivious before to just how much was missing in the presentation with an inability to resolve low-frequency information. With this amplifier the space in and around my head is pressurized in such a way that a believable sound field is created and the absolute presence of music is felt, at all times, even during passages of silence. I have not experienced this before and previously resigned to believe that headphone listening despite its intimacy was not capable of sounding more or less detached to a degree, as if one was 'monitoring' the playback rather than being immersed in it.

Above all else, the emotion of music is transmitted intact, regardless of genre or source. I am nearly always subject to involuntary physical movements and reactions when listening to music through this system. I have been moved to tears, on multiple occasions, with both popular and symphonic music! This is absolutely unprecedented for me. If only I could achieve such emotional compulsion as consistently and thoroughly through my speaker system, though I am convinced I will get there eventually. I tried to listen through this system while writing parts of this article, and was unable to. The music takes attention, whether I am listening to FLAC, MP3, a streaming service, YouTube or 78 RPM shellac rips.

The guts of the Abbas OTL amplifier

To temper myself after writing the above paragraphs, I went back to listen multiple times and try to hone in on deficiencies in the sound. The input stage of the amplifier using the EC88 lamp is based on a SRPP scheme, which is known for having a heightened sense of dynamism or speed. Indeed, the amplifier is intense, and perhaps slightly more so than what peaks in recordings of large scale music call for. However, given that the aesthetics of the musical performance are not disturbed, and a vividness is given to headphone listening that otherwise would not be there, I hesitate to call this a shortcoming.

For the JAR600, the Sennheiser driver has its limitations. It cannot resolve high frequency (>7kHz) transient information very well. There is a ceiling to its ability to portray subtle contrasts and intonations in music. Its frequency linearity and presence may be too monitor-like for those preferring euphony, and there is nothing wrong with that. However, anything that I would consider a true upgrade is very far away from its price point.

I consider this to be an audiophile victory of the highest degree, and well deserved in terms of the investment made to get to this point. The abbas OTL amplifier cost just over $1,000. It is also capable of serving as a pre-amplifier or buffer with simple modification, which I will attempt later.

Unfortunately, it is not clear if he is still offering it as a serial product. I loathe to heap so much praise onto a product that may very well not be available for purchase anymore, but such is life, and it is not too different from raving about a vintage amp you can't reasonably find anymore. What is equally exciting and daunting is that for the first time since 2016 I feel the HD600s are the weakest link in my headphone chain. Possible candidates for upgrades are the ZMF Verite and MySphere 3.2, although considering both these models cost well over $2,500 I do not think I will be experiencing them anytime soon.

I will conclude with some practical recommendations. For lovers of music who want to put together a headphone system without breaking the bank, I recommend the Sennheiser HD600/HD650 models. Be mindful that Sennheiser has recently switched production from Ireland to Eastern Europe and compromised quality control. I would recommend sourcing a made in Ireland pair, which shouldn't be too hard. You may modify them yourself at a later date or send them to Jupiter Audio to be modified.

For amplifiers, either the Schiit Valhalla 2 or Bottlehead Crack will satisfy on a budget. The latter is sold as a kit but is so popular several well-built examples are always available on the second-hand market. I believe this will serve most peoples needs, unless you are married to synthetic electronic music with deep bass content, in which case you may consider any number of planar magnetic models and an inexpensive transistor amp such as the Schiit Magni. Some have also reported success with using a small subwoofer playing in parallel with headphones.