"If it measures good and sounds bad, it is bad.
If it sounds good and measures bad, you've measured the wrong thing."


Building a reference system - Part 2: Glenn's headphone amplifier

Posted by Amine Slimani on October 21, 2014 at 2:25 PM


The story of Glenn’s OTL headphone amplifier started as that of a relentless endeavor to make the ALO recabled Beyer T1s sound as good as it was possible, and ended up being that of finding the center piece of my system that will probably outlast many other major components.

At the time of the purchase of Glenn’s amplifier, I have had the Beyer T1s for around 4 years and had mixed results with those headphones. While the Beyer T1s granted me, at times, with the best music I had experienced in my system, it also made me, at other times, want to sell them.

After various listening sessions with the amplifiers I had on hand, I came to a surprising result: the amplifier that sounded the most coherent with the Beyer T1s was the entry level and very affordable tubed OTL Little Dot MKIII. Although the LD MKIII amplifier did not have the resolution nor the extension at the frequency extremes of my solid state Audio-gd C2 amplifier for instance, it just opened up the sound of the Beyer T1s and sounded more natural and dynamic. As a result of those comparisons, I knew that I did not want a better version of the Audio-gd C2 amplifier but that I should look for a (preferably much) better version of the LD MKIII.

Given that I did not have a lot of experience tube amplifiers, I started doing some research around to see what type of tube amplifiers would work best with the Beyer T1s (which were my reference headphones at the time). It was one of those rare occasions where everyone seemed to agree that the Beyer T1s paired very well with OTL headphones. I was not much surprised with this finding as OTL headphone amplifiers tend to have higher than usual output impedance and Beyer’s own A1 amplifier also happens to have an output impedance of 100 ohms.

Once I narrowed down my field of research to the OTL configuration, I was still faced with a lot of choices from well respected manufacturers: Woo Audio, Decware, etc. The choice was not easy to make especially that I could not listen to any of those alternatives before buying. It was at that point when I was advised by rosgr63 to get in touch with Glenn R. who makes custom made amplifiers of his own design.

Rosgr63, a respected member of both tweak-fi and head-fi, has a very large collection of headphone amplifiers and headphones. Some of his equipment includes the reference class Eddie Current Balancing Act, amongst other high-end headphone amplifiers. So when rosgr63 told me than Glenn’s amplifier was on the same level as some of his best amplifiers, I was very intrigued. After doing a little bit of research, mainly on head-fi, I quickly realized that Glenn made some very impressive products. So I took the plunge and started discussions with Glenn.


The amplifier:

After discussing a little bit with Glenn, we settled on a flexible configuration. While the amplifier uses a 6SN7 tube as driver and 3DG4 (or similar tubes such as the 5AW4s) as a rectifier, there is much more possibilities in the output side.

His amplifier can use, as output tubes, any of the following configurations:

- Six 6BL7 tubes

- Six 6SN7 tubes

- Two or four 6AS7 tubes

- Two 6336 tubes

The fact that an amplifier is capable of using 2, 4 or 6 output tubes is not something very common. But it was a very interesting feature that I was excited about.


Internals (while being built) of the amplifier :

Review system:

System 1 – Headphone Based (optimized)

Computer source: HP Pavillon DV6 (Core i7, 8GB, SSD), Windows 8.1, Fidelizer, JRiver Media 17

Transports: Audiophilleo 2, Kingrex UD384, Jkeny’s modified Hiface MK1, Audio-gd Digital Interface (w/ Tentlabs upgrade clock), Teralink X2, Musiland Monitor 01 USD, EMU 0404 USB

DACs: Metrum Octave MKII, Audio-gd DAC-19 DSP with DSP1 V5 upgrade, Kingrex UD384

Amplifiers: Glenn’s OTL headphone amplifier, Audio-gd C2, Little Dot MKIII

Headphones: Sennheiser HD800 with Wiplash Audio TWau Reference cable, Beyerdynamic T1 with ALO Chain Mail cable, Sennheiser HD650 with Artisan Silver Dream upgrade cable

Digital interconnects: Hifi Cables Sobek BNC, OyaideDB-510 BNC, Wireworld Ultraviolet USB, Artisan Twinline Pure Silver USB cables

Analog Interconnects: Artisan Ultimate Silver Dream RCA, Norse Audio custom 8 conductor UP-OCC ACSS, Deep Sounds SPS ACSS, Kimber PBJ RCA


Power filtration: Bada LB-5600 Filter, Essential Audio Tools Noise Eater, Essential Audio Tools Pulse Protector, Supra Mains distributor

Power cords: Hifi Cables & Cie PowertransPlus (x2), Hifi Cable & Cie SimpleTrans, Olflex power cords

Vibration Control: Aktyna ARIS decoupling feet, Maple and Acrylic Platforms, E&T rack, Stabren Damping pads, Sandbox, Brass cones, Vibrapod, Yamamoto Ebony footers and Various Herbie’s Audio Labs tweaks


System 2 – Speaker Based (not fully optimized)

Yamaha CD 600/ Audio-gd DAC19DSP --> Kimber PBJ --> Yamaha AS500 / Populse T150 --> Hifi Cables & Cie Maxitrans II --> Triangle Antal EX floorstanding speakers


Reference tracks used for the review

My (usual) reference tracks:

CD Quality:

Mahler - Symphony n 5- Decca - 16/44

Sol Gabetta - Schostakowitsch Cellokonzert Nr. 2/ Cello - 16/44

Vivaldi – Concerto for 2 violins - Carmignola/ Mullova - 16/44

Natalie Dessay - Italian Opera Arias - Emi Classics -16/44

Puccini - La Boheme - Decca - 16/44

Glenn Gould - The Goldberg Variations (1981) - 16/44

The Essential James Bond - City of Prague Philharmonic orchestra - 16/44

The Dave Brubeck Quartet - Take Five - HDCD - 16/44

Diana Krall - Live in Paris - 16/44

Norah Jones - Come Away With Me - 16/44

Patricia Barber -Companion - 16/44

Johnny Cash – The Essential - 16/44

Soundrama - "The Pulse" Test CD - 16/44


High Resolution quality:

Rachmaninoff Dances -HD Tracks - 24/96

Mozart Violin Concertos - Marianne Thorsen - 2L - 24/96

Dunedin Consort -Messiah - Linn Records - 24/88

Glenn Gould – The Goldberg Variations (1955) – HDTracks - 24/96

Keith Jarrett - Paris/ London - Testament - 24/96

Jazz at the Pawnshop- HD Tracks - 24/88

Alison Krauss and Union Station - Paper Airplane - 24/96

Ella Fitzgerald/ Louis Armstrong - Ella & Louis - 24/96

Diana Krall – From this Moment on - 24/88

Diana Krall – The Girl in the Other Room - 24/96

The World's Greatest Audiophile Vocal Recordings - Chesky - 24/96

The Kinks - One for the Road Live - 24/96

The Eagles - Hotel California - HD Tracks - 24/96

Head-fi/Chesky Sampler - Open Your Ears - 24/96



The set-up:

Burn-in and warm-up:

While Glenn’s amplifier sounded fantastic from the beginning, it took a little more than two hundred hours (I did not clock the exact number) of burn-in before its sound finally stabilized. I also believe that Glenn burned-in the amplifier at his place for around 100 hours. Part of that change might be attributed to the burn-in of NOS tubes. For instance, after around 100 hours of burn-in, the noise floor of the Sylvania 6SN7W was reduced so much as to become inaudible with the Beyer T1s and go easily unnoticed with the slightly more sensitive HD800s. The good thing though is that it can be enjoyed straight of the box and any change that comes after that can be considered as a bonus.


Vibration control:

I have used Glenn’s amplifier, with the Aktyna ARIS footers, on both a marble platform and a 4 inch thick mahogany platform (my previous maple platforms were too small for Glenn’s amplifier). While I did prefer subjectively the mahogany platform, it also had a few (objective) advantages in comparison with the marble platform than no one could argue about.

First, it reduced microphonics in all tubes being used in Glenn’s amplifier. While even a gentle tap on the audio rack would be heard loudly on the headphones when using the marble tile, the thick mahogany platform seemed to isolate the amplifier much more effectively from outside vibrations.

Second, and most surprisingly, switching to the mahogany platform also reduced the very audible hum on the right channel of my Sylvania VT-231 6SN7 driver tube into the barely audible realm. Why and how that was possible was beyond my understanding, especially that the mahogany platform was more effective at reducing the hum of the VT-231 tube than Herbie’s Audio Lab RX tube dampers.

Third, I think that it also reduced the noise floor somehow of the Sylvania 6SN7W. However, I cannot be totally sure about this as the noise floor of my 6SN7W seems to vary depending on how long I have left them warm up and how often I have used them (in my experience, the more often and the longer it is used, the less their noise floor is audible).

Finally, I subjectively preferred the sound of the amplifier sitting on the mahogany platform. It was not a night and day difference with the marble platform, but the sound was a little bit more relaxed and a little bit richer sounding. In the subjective realm, the effect of the mahogany platform was smaller in amplitude than that of Herbie’s tube dampers.

In any case, one should take into account the platform support used beneath the amplifier and probably avoid very hard materials (such as marble, granite, etc.).


Tube rolling:

Output tubes:

Before getting into the heart of the review, I should probably first share my experience with tube rolling; indeed, as Glenn’s amplifier is an OTL amplifier, its sound is highly dependent on the output tubes as there is no transformer or buffer of any sort between the output tubes and the headphones.

In this case, moving from the two-6AS7 to the six-6BL7 configuration as output tubes is not a simple change in flavor. The move is rather similar to a complete change of headphone amplifier as the gain, the output impedance, the voltage swing, the drive capability as well as the flavor of the sound all change from one output configuration to the other.

With the two-6AS7 configuration, the sound of the amplifier reminds of a bigger and more complete version of the Little Dot MKIII. It has a very big soundstage with tons of dynamics capabilities. It is also very smooth and warm sounding. If I were to be listening blind and were told that it was a very upscale version of the LD MKIII, I would not be surprised. It is a very appealing sound, simple to like and very much how I imagined a big OTL tube amplifier would be.

Moving to the six-6BL7s, the sound transforms into completely different, as if it were coming from a different amplifier altogether. In comparison with the 6AS7s, the sound becomes more focused, the soundstage feels at first smaller and tighter (while it is, in reality, deeper and better defined than that of the 6AS7s), the resolution goes up a few notches, and the overall presentation becomes much more extended at the frequency extremes yet it remains refined and delicate at the same time. As a result, I have to admit that it took me quite some time to “get” this sound. With the six-6BL7s, the sound was unlike anything I could have imagined, leaning more towards the solid state gear type of sound but showing qualities I had never heard from solid state gear.

While I was very happy to have 2 different amplifiers in one package, I have to admit that I ended up mainly using the six-6BL7 option as it is the much superior option with the Beyer T1s as well as with the HD800s I bought after getting the amplifier. I was told by Glenn that rolling different brands of 6BL7 tubes will not change dramatically the sound of the amplifier. I have had no way to verify or deny that claim but I should be receiving soon a dozen of 6BL7s (6 GEs and a mix of other brands). In any case, the RCA 6BL7s configuration is so transparent and nice sounding that I doubt I will be finding a ground shaking difference with different brands. I will update this review if there is anything significant to report.

It can be noted that, for the remaining of the review, I will describing the sound of the amplifier with the six-6BL7s, unless stated otherwise.


Driver tubes:

While switching from one output configuration to the other totally transformed the sound of the amplifier, changing the driver tubes had a much smaller effect, in comparison, at least with the three 6SN7 tubes I had on hand: a grey glass RCA 6SN7, a Sylvania VT231 6SN7, and a Sylvania 6SN7W.

With the “stock” grey glass RCA 6SN7 that came with the amplifier, the sound is on the neutral side, with perhaps a slight emphasis on the bass, in comparison with the other two. The highs are perhaps a little less extended and have less “sparkle” than what is the case with the other two tubes. However, one can live happily with it.

With the Sylvania VT231 6SN7, the sound is very good: great timber of voices and instruments, great soundstage and excellent resolution but, in the tube I bought, there was an annoying buzzing (or hum) on the right channel that lessened with time but never totally went away. It is the closest sounding to solid state flavor, with perhaps the driest bass register. I would have been using it more often if it were not for that hum on the right channel.

With the short bottle JAN Sylvania 6SN7W, the sound is simply magical: it is much more refined, richer sounding and delicate than the other two. It also has a much higher resolution (and low level details rendition) than the other two despite having an audible noise floor. I first thought that it was a little bit lighter sounding than the other two until I realized that it was much more linear and quicker in the bass than the other two. With the 6SN7W, the sound is both faster and warmer than the other two tubes. But here, the warmth does not come from fuzziness but rather from the richness of the timber and the infinite variations between similar close sounding frequencies.

It can also be noted that, for the remaining of the review, I will be describing the sound of the amplifier with Sylvania 6SN7W, unless stated otherwise.

As an additional note, I have also ordered a (grade A) Sophia 6SN7 tube but I will probably not receive it before finishing this review. I am not expecting it to necessarily best the Sylvania 6SN7W but I was curious to listen to a (well made) new production tube. I will write a short paragraph once I receive if there is anything interesting to tell about it.


Rectifier tubes:

I tried 3 rectifier tubes on Glenn’s amplifier: a Westinghouse 3DG4, a Zenith 3DG4 and an RCA 5AW4.

I personally didn't like much the sound of the 3DG4 tube (both the Westinghouse and the Zenith tubes). The sound was very upfront and powerful but that sensation came at the expense of the soundstage layering, depth and spaciousness. It was a little bit too forward for my taste and the headphones I tried it with.

The RCA 5AW4 worked better in my configuration, especially in the soundstage department. It seems to my ears that the RCA 5AW4 tube offers the most refined presentation of the three tubes. It is also the tube of reference I will be using for the remaining of the review, unless stated otherwise.

Now that I have shared my experience with tube rolling on Glenn’s OTL amplifier, let us move on to the description of the sound, using the most synergetic combination in my system: the Sylvania 6SN7W as a driver, the RCA 5AW4 as a rectifier and the six RCA 6BL7s as output tubes.



The Sound:

Timber & tonal balance:

The amplifier is very well balanced, from top to bottom, with no particular weakness in any part of the spectrum. While that kind of balance is a basic quality one should expect from any decent sounding amplifier, it is not always the case as some amplifiers tend to highlight or favor one or more the spectrum over the rest of it. Glenn’s OTL amplifier is no such device as it treats every part of the frequency spectrum with the same level of attention.

While I am usually no fan of the bass, mids and high segmentation of sound, as it is too restrictive for high end devices and usually there is no really no real change in the measured frequency response, I will do an exception for Glenn’s OTL amplifier as It will better help understanding the tonal shift in comparison with solid state amplifiers.

When I reviewed the ALO recabled Beyer T1s a few years ago, I found it to be overall finely balanced but felt at the same time that it lacked dynamic heft in the bass, especially in some extreme dynamic passages. Given that I did not have any high-end amplifier at the time to further challenge that observation, I could not point out the culprit. However, now that I have tried the Beyer T1s with Glenn’s OTL amplifier, I believe that the feeling I described could be mainly attributed to the lack of power in the amplifiers I was using at the time.

Switching to Glenn’s OTL amplifier, the Beyer T1s seemed to gain in bass foundation, especially at higher volumes.

I suspected for a while that the relative lightness in the bass (in very specific conditions) was simply a matter of impedance mismatch, and that solid state amplifiers exercised too much grip over the Beyer T1s making the bass sound over-damped. That is why I ordered a 100 ohm adapter from ZX Amateur cables for my Beyer T1s. Using that 100 ohm adapter to artificially increase the single digit output impedance of the Audio-gd C2 amplifier made the balance of the Beyer T1s more natural (in those particular extreme passages), with a more generous bass section, but degraded the overall transparency and did not come close to the overall sound of Glenn’s OTL amplifier.

The bass of Glenn’s amplifier is surprisingly both more generous are more controlled than any solid state or tube amplifier I had tried before it. It is rare when you can get a bass section that gains in strength, depth, definition and richness in comparison to all previous references. But that is what Glenn’s OTL amplifier has granted me with.

While the increase in bass power and control was a welcome addition to the Beyer T1s, it was a necessary addition to appreciate the true capabilities of the Sennheiser HD800s.

When I last listened to the Sennheiser HD800s, just prior to ordering the ALO recabled Beyer T1s, it was at a high end store in Paris. I listened to the HD800s then with a 3D Lab high end CD player and my Audio-gd C2 amplifier (and also with one of their solid state headphone amplifiers). I remember that I did not like the HD800S at all, as I found the bass to be very light and unnatural, as if I were listening to some mini-monitors – I very much preferred, in that configuration, the silver cabled HD650s as well as the various Grados that were available at their store.

Fast forward a few years to the Beyer T1s and Glenn’s OTL amplifier combination. Once I knew that Glenn developed his amplifier with the Sennheiser HD800s, I could not help wondering how they would sound in my system. My reasoning was simple: anyone that could build such a wonderfully balanced amplifier could not have done so on a sub-par headphone. I had to check (again) his reference headphone.

Thus, when I first tried the HD800s with Glenn’s amplifier, I was very pleasantly surprised with the outcome. The bass light headphones I had discarded a few years ago morphed into some almost bass heavy headphones. Between the Beyer T1s, the HD650s and the HD800s, it was the latter that were surprisingly the “bassiest” of the lot with Glenn’s OTL amplifier. That bass boost did not come from uncontrolled or overemphasized mid bass, but from a very powerful, controlled and refined low bass register.

Moreover, Glenn’s amplifier has a very rich, open, pure and transparent mids section, as expected from a tube headphone amplifier. This helps instruments and voices to register as “realistic” sounding in our brains. I believe that while our brains have been trained (or forced) to tune out unwanted distortions in the frequency extremities, our ears and brains are very sensitive to the middle frequencies. Here, Glenn’s amplifier does not disappoint at all.

Finally, Glenn’s amplifier cleared out another misconception I had about tube amplifiers performance in the highest notes. Indeed, one of my fears was that a tube amplifier would not improve upon the already satisfying performance of the class A biased solid state audio-gd C2 amplifier. I had the unfounded fear that a tube amplifier would not have the extension, the sharpness nor speed of what I considered to be a good sounding solid state amplifier. It turned out that Glenn’s OTL amplifier was both more extended, illuminated, refined, as well as faster than the Audio-gd C2 in the top end. Switching from one amplifier to the other is similar to taking the dust off a TV or monitor set. While the lack of vividness might not be noticeable before taking the dust off, it can become very obvious just after taking it off.

In addition to its wonderful tonal balance (between the bass, mids and highs), the key strength of Glenn’s OTL amplifier, in my opinion, is its ability to make difficult to reproduce instruments and voices sound both real and beautiful. Regardless of the instrument or the volume it is being played at, Glenn’s amplifier is able to reproduce the harmonic structure of different sounds with great fidelity and little to no “editorialization”.

As I pointed out in my review of the Metrum Octave MKII, we have reached an age where many (decently built) consumer audio equipment surpasses the quality of most commercial recordings. With Glenn’s OTL amplifier, it is very easy to sense the shift in tonal balance from one recording to the other, from one track to the other. Here, the sound can easily shift from warm sounding to dry sounding, from smooth to edgy just by changing the album or the track.

As a result, it takes (a lot of) time to understand the full spectrum capabilities of Glenn’s amplifier. Quick A/ B comparisons are not the way to go as they only reveal a fraction of the sonic subtleties of the amplifier.

In that regard, the two-6AS7 configuration is easier to like in quick listening sessions, it is very pleasing tonally but ultimately colored. The six-6BL7 configuration is something totally different: it is controlled, precise, extended and powerful as you would expect from solid state amplification, and it is also delicate, refined, sophisticated and natural sounding as you would expect from tube amplification.


Soundstage & Imaging:

It seems that a common strength that many tube amplifiers share is the ability to portray a realistic soundstage, at least a superior and more pleasing soundstage than the one painted by solid state amplifiers. Glenn’s OTL amplifier did not disappoint me in this regard with both output configurations.

With the two-6AS7 tubes, Glenn’s amplifier throws a rather large soundstage. Whether listening through the ALO recabled Beyer T1s or the HD800s, the experience is very close to that of listening to speakers. Regardless of the recording, the two-6AS7 configuration is able to project (outside of the head) an holographic and pleasing soundstage. Once again, it makes me think of a bigger and a better version of the LD MKIII. However, the representation is not beyond reproach. While it is rather big in scale by headphone standards, it lacks absolute specificity in depth layering and imaging. When using the two-6AS7s as output tubes, the sound is very pleasing and easy to listen (soundstage-wise). Also, while the size and the construction changes from one recording to the other with the two-6AS7 configuration; it is nowhere near the chameleon act of the six-6BL7 configuration.


Indeed, with the six-6BL7 tubes, Glenn’s amplifier moves onto new territory and into something I had never experienced before with headphone based listening.

One thing that I regret, now, after listening to Glenn’s amplifier with superior headphones such as the Beyer T1s or the HD800s, is that I wrongfully described, in the past, the sound of previous (solid state) systems as 3D like or holographic sounding.

When an amplifier such the Audio-gd C2 is able to paint to some degree a map of the soundstage where you can locate different performers, a tube amplifier of the caliber of Glenn’s amplifier is able to not only paint a soundstage where you can locate performers but also to portray a soundstage where you can feel different performers playing. One has to make considerably less efforts to visualize the soundstage of the recording.

It is not that there is no soundstaging and imaging information on lesser amplifiers, but it is rather that the listener can just sit, relax and enjoy a very holographic scene without requiring any specific effort when listening to Glenn’s amplifier. Once, one gets used to this effortless representation, it is difficult to come back to lesser equipment that require more (brain) effort to visualize the same thing. As a result, I have to assume that there are perhaps amplifiers out there that outperform Glenn’s OTL amplifier in a similar fashion in the soundstage department given that it is the kind of attributes that require superior equipment to understand the flaws and limitations of lesser equipment. For now, though, all I can report is that Glenn’s amplifier make my other amplifiers sound as if they were mere entry-level toys in the soundstaging department.

So, what makes the soundstaging of Glenn’s amplifier so special? For one, it has a “chameleon” like soundstaging characteristic. The size and the nature of is soundstage is not a fixed quantity but it is highly correlated to whatever is stored in the recording.

In fact, I would go as far as to say that, with the help of superior headphones, Glenn’s amplifier is able to literally transport the listener to the recording venue. Thanks to its superior resolution, the amplifier is able to project sounds in front of the listener, outside of the head, as if performers were just “there”. Moreover, the amplifier is able to retain that realistic placement of sound while retaining a sense of depth and pin-point location on stage. Meanwhile, although everything is painted in high resolution, the sounds located in the rear do not get projected into the foreground.

Hence, the imaging is very stable with regards to location on-stage and with regards to the volume level. Every performer seems to have his or her specific bubble on the soundstage.

To sum up this section, I would say that there is no blending of sounds (for closely located performers) and there is no change in location in loud passages as if various performers were grounded to their location. But, at the same time, when a performer moves across the soundstage (such as in operas), it is very easy to follow his or her movements on-stage, as if he or she were a real person moving in front of the listener.

Dynamics & PRAT:

Given the physical weight of the amplifier and the size of its power supply section, I had no doubt that it would be a very powerful sounding amplifier with sufficient drive and dynamics capability. And, after countless hours of listening, my experience with it proved that assumption right.

During my previous round with headphone amplifiers comparison, I was surprised with the dynamics capabilities of the Little Dot MKIII. After a little bit of research, I realized that OTL amplifiers could swing much higher voltages into higher impedance headphones, such as the 300 ohm-Sennheiser HD800s or the 600 ohm-Beyer T1s, than most solid amplifiers are able to do on the same loads. Thus, that could explain (at least in part) what I was hearing.

In this regard, it was apparent from the very few instants I listened to Glenn’s amplifier that it was the kind of amplifiers that is able to throw a big and dynamic wall of sound. However, and contrary to Little Dot MKIII, its dynamics prowess is not limited to macro-dynamics but extends to the micro-dynamics realm.

Indeed, Glenn’s amplifier, especially in the six-6BL7 configuration, is able to track every variation of sound (no matter how big or small it is) with finesse. I believe that it is thanks to the grip its power reserve exerts on connected headphones.

A surprising test with Glenn’s OTL amplifier is to turn off the amplifier while the music is still playing. The amplifier can play (on its reserves) for around 30 seconds before it mutes totally. For the first 10 to 15 seconds, there is barely any drop in sound level and no noticeable distortion. The power supply section is obviously “overbuilt” for headphone applications.

As a result, this huge power reserve translates into extremely fast (and controlled) transients that make different instruments and voices seem very realistic and alive.

Moreover, in the most demanding tracks, where other amplifiers would have already thrown the towel or have had a hard time keeping up, Glenn’s OTL amplifier can still hit the headphones with a clean and dynamic swing if such a variation in sound is recorded. If one were to make a car analogy, Glenn OTL amplifier would be most similar to a big V8 (the 6.2 naturally aspirated AMG engine is the one that comes to mind) that has a ton of power, responsiveness and control.

With that being said, it should be noted that while the six-6BL7 configuration has the edge in micro-dynamics, I felt as if the two-6AS7 configuration had the edge in terms of macro-dynamics slam and “freedom”.

It is something rather difficult to explain as, objectively, the six-6BL7 configuration seems to track every change in sound level in an impressive manner, unheard of in previous amplifiers. But, subjectively, and especially on short term listening, the two-6AS7 configuration can feel as “freer sounding”. Whether the two-6AS7 configuration is truer to the source or just adding some sort of distortion that makes it feel as being more powerful is a question I cannot answer.

The only definitive observation that I can report, after many months of listening, is that all the tracks that I had listened to prior to Glenn’s amplifier seemed to have gained in dynamics, as if a (dynamic) expander was applied. It is even more obvious on modern and highly (dynamically) compressed music that is very enjoyable to listen to through Glenn’s OTL amplifier.

Keep in mind that all my observations were done with the three headphones I own (HD650, T1 and HD800) that happen to be on the high impedance side. I have no idea how the amplifier would sound on lower impedance headphones be it dynamic ones (Grados, AKGs etc.) or Orthodynamic ones (Audeze, Hifiman, etc.). Though, according to Glenn, they are supposed to benefit from the high drive capability of the 6AS7s and 6336s tubes.

Finally, I would have to say that Glenn’s amplifier have excellent PRAT (Pace, Rhythm and Timing) capabilities. Indeed, besides pure dynamics, the amplifier has speed, pace and timing qualities that really make the music fun to listen to and less homogenized from one recording to the other. However, these qualities are more apparent with the HD800s and Beyer T1s than with the HD650s. In fact, this is perhaps the area where the HD650s trail in comparison with the HD800s and to the Beyer T1s. While the HD650 can scale up very well with better equipment in terms of resolution, details, soundstage, imaging and even dynamics, they do however trail in term of PRaT. The other two headphones just seem faster and able to accelerate or slow down much in a much more effortless and controlled manner.

In any case, the three headphones I tried with Glenn’s amplifier showed increase in dynamics and PRaT in comparison with other amplifiers on hand, and were more enjoyable through a wide range of styles of music.


Transparency & Resolution:

This last section is perhaps the area where I was most surprised with Glenn’s OTL amplifier. While I was expecting somehow that a big tube amplifier will have a correct voicing, a big soundstage and a lot of drive capability (that translate into high dynamics), I was not prepared for an amplifier that was as much transparent (to whatever is playing upstream) and as much detailed as Glenn’s OTL amplifier was. It is simply the highest resolution amplifier I have listened to.

So how much resolution one should expect from Glenn’s OTL amplifier?

First of all, Glenn OTL amplifier digs very deep in the recordings to uncover details that can be missed on other equipments. On some familiar Diana Krall tracks that I had listened to on various systems (including some expensive speaker systems), I heard very clearly new low level details that I have not heard previously in the same manner. Here, details are presented in a way that it is easy to understand what is happening and what instrument or background noise is playing. On a classical piece of music from a Chesky sampler disc, it was very easy to hear the traffic noise outside the recording venue. On La Boheme opera from Puccini, it is very easy to hear people moving on the scene as if they were just close by.

This high resolution rendition translates also into a better and greater separation between different recordings, or sometimes different tracks on the same recording. Listening with Glenn’s OTL amplifier, it is a real pleasure to switch from one recording to another, as the ambiance and feel of the recording totally changes from one album to the other.

While I did find the transparency to the quality of recordings very enjoyable and entertaining as it avoids boredom (or the need to swap equipment to get a “fresh feel”;), I found that its sensitivity to the quality of the upstream equipment was less simple to deal with. Every little thing matters in setting up the system upstream, especially when using the HD800s.

It starts with the software side of the equation: using Fidelizer is very audible, moving from JRiver Media Center to Foobar is very audible, and changing the BNC digital cable between the Audiophilleo 2 and the Metrum Octave MKII DAC is also very audible. These variations in jitter (that should not be audible according to some) are very audible when using Glenn’s OTL amplifier. This is not the first time I have heard differences in the parameters listed above, but it is the first time I have heard those differences with that much clarity. Glenn’s OTL amplifier shows clearly key strengths and weaknesses of every configuration.

So did this high resolution capability translate into an overly analytical sound? Not at all, by reading the previous sections, you will probably understand that Glenn’s amplifier is not some kind of forward sounding and fatiguing solid state amplifier for “monitoring” purposes. The astounding high level of detail rendition comes, in this case, from a high resolution device that can fit whatever sound is playing without adding any objectionable artifact or distortion.

As a result, and despite its exceptional transparency to the recordings and equipments upstream, Glenn OTL amplifier makes the listener focus more on the strengths than on the weaknesses of the recordings. In some ways, it reminds of how the Metrum Octave MKII compares with lesser DACs. But here, the difference is perhaps even larger once the upstream.



Glenn’s OTL amplifier was a wonderful revelation. It is an extremely fast, powerful and detailed sounding amplifier yet it is also a warm, delicate and very refined sounding amplifier. Glenn’s OTL, as far as I am concerned do not exhibit any of the limitations usually associated with tubes (such as limited bandwidth or resolution). If it were not for the ability to swap tubes or for the occasional driver tube that has an audible noise floor, one would be hard pressed to guess that it is actually a tube amplifier.

While there are endless possibilities of set-up, I have found that the six-6BL7 configuration with a Sylvania 6SN7W as a driver is the way to go for me. Nonetheless, I am sure that the endless choices in 6SN7 tubes will make the amplifier friendlier to insert in different systems.

The question that has been bothering me for the past few months is whether or not Glenn’s amplifier is cheating somehow? And while I looked hard for an answer to that question, I am still not able to provide the reader with a definitive one. However, the amplifier shows some much difference between recordings, DACs, etc. that if any “coloration” of sound is being done, it must be done in a very subtle and efficient way, making the listener focus more on the quality of the representation than on the quality of the recording. The Metrum Octave MKII DAC happens to share a similar take to presenting the music; however, Glenn’s OTL amplifier does it in a bigger scale.

To conclude, I would say that Glenn’s OTL amplifier biggest strength is to reproduce, with the highest possible fidelity, music and not sounds.


Categories: Headphones, Amplification

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Reply rosgr63
3:43 PM on October 21, 2014 
Great review!
I am pleased you like Glenn's work of art as much as I do.
Reply Glenn
5:00 PM on October 21, 2014 
Thanks for the nice review