The OTL Headphone Power Amplifier is unlike anything you’ve ever heard before. Designed to work with a wide range of headphone impedances all the way down to 80 Ohms! You’re no longer limited to cans that are 300 Ohms and up.
And that isn’t where the differences stop. This amp is a pure stereo POWER amplifier only. No gain stage, no preamp tubes. You might be asking why? Doesn’t that make it less convenient? And our answer is maybe, but the drawbacks are worth the results. The 6AS7 / 6080 is better suited to a lower voltage, higher current power supply. Something that isn’t as good for a preamp stage. So we decided it would be best to split the two! Pair the OTL Headphone Power amp with any low noise preamp and listen to the difference it makes when you have carefully designed discreet components in your system.
ProTip: Many DAC’s have built in preamps with a volume control, if your source is low noise and can produce at least 1Vrms, you should be good. Obviously more input voltage will produce more volume but for the average listener you should never need more than 1Vrms, even with less efficient headphones.
Cheers and Have Fun Building!
Jim & Charles
jahozafat (verified owner) –
Well, I’ve completed my build. Everything went smooth as silk. MTA kits are so enjoyable to build. Their videos make it so easy and informative. It makes me feel almost part of the design process the way they break down the circuit and explain why and what parts are in the kit. It is so satisfying to bring a piece of equipment to life and be able to hear the resulting sound of your labor.
While listening the first thing I picked up on was the incredible bass detail. Right away I heard room bass reverberations from the recording room. I can feel the bass of the stage amplifiers on the stage in my ears. At first I thought I heard a distant outside car bass then realized it was in my ears. I wasn’t expecting it, but it was very noticeable on older stage (live) recordings.
Another thing that really struck me was the stereo image of the vocal harmonies. The Beach Boys “In My Room” was musical bliss. It is erie how distinctly I can focus on individual voices during the harmonies all while not losing the detail of the instruments. While listening to Aaron Neville’s “It Feels like Rain” the background bass vocal made me audibly rejoice. When I hear that kind of thing it makes me want to replay that side of the record over and over. That feeling never gets old. It’s audio nirvana.
I continue to be impressed by the layers of detail. I can pick out little things like the sound from inside of acoustic guitars and pianos. It makes an incredible difference compared to just hearing the strings. It’s also a real treat to be able to focus in on the drums and hear the ghost notes. These are generally the details I can only discern during instrument solos. I particularly enjoy picking up creaking chairs, jewelry moving, and other room noises. It is amazing to be able to focus on those specifics through the layering of instruments and vocals.
I think I still prefer a well set up speaker setup most of the time but some things are better on some tracks with this OTL Headphone setup. It’s also much easier to set up a headphone system for great sound since I don’t have to address speaker position and room acoustics. I can also lay back and get comfortable and not worry what direction my head is facing.
For the majority of my listening I have been using a phono source. I am also using the MTA Universal phono preamp, Rocket Preamp with the OTL Headphone amp. Finally I have been listening with the Sennheiser HD 660 S2 300ohm headphones. If you are looking for truly amazing and affordable audio separates that do their job to perfection, look no further. Mellow Tone Amps has you covered. I really could not be happier. My review would have been complete much earlier but I just couldn’t get myself to stop listening to the music.
Thanks Jim and Charles
-Gary
aaloggers (verified owner) –
From Parts to Poetry!
ML OTL Headphone Power Amp Review:
———————————————————————————————-
-What gave me the idea for building this?
When I noticed that Jim and Charles were asking for Test Builders I immediately volunteered.
The plan was to use this for my new Sennheiser HD-820 (300Ohm) Headphones.
Since I had already built the ML Universal Phono Preamp from MTA I knew what to expect to
the build quality and the attention to detail. I had no doubts this was going to work perfectly!
Since this is a headphone power amp and has the need for a pre-amp I will use the pre-out of
my Denon PMA-1510AE for the time being.
-The wait and arrival!
After its journey across a lot of water and after having paid some import tax it arrived in a sturdy box.
Everything was very well packed and identified.
And the unpacking could begin which also meant it was time to start the video assembly guides.
Let the fun begin!
-Start building.
First thing was to check if everything was there and then place the parts in a box with compartments.
Also made sure my workspace was clean and I had all the tools at hand.
-Solder station….check!
-Lights….check!
-Small tools….check!
-Coffee….check!
Now we’re off 🙂
-Following instructions!
Yes this is an important step as they also mentioned in their videos.
First watch an episode then start soldering/assembling.
Very important with some parts that must be lifted from the PCB a bit because of heat.
If you hadn’t watched the whole episode and already soldered those parts and cut the leads there is no way you can redo that mistake.
And there will be more of those moments….so keep paying attention!
Also you may use the suggested soldering temperatures.
-Any problems while building?
No actually not.
I was already familiar with putting together the chassis so that went smooth.
The layout of the PCB is very spacey and it is very easy to have access to the components when soldering.
Although my hands have become more shaky through the years it was no problem soldering and assembling all parts.
That would have been impossible for me to do if there were any SMD parts involved.
When you’re well on your way building it, there will be the part of connecting the parts on the back to the PCB connectors.
That can be a bit tricky but with a bit of patience success is guaranteed.
-Almost done building.
When everything has been put together (except the bottom) it is time to measure the voltages.
Although all voltages seemed within spec I did email Jim with my results and he saw no problems.
The switch mode power supply that I ordered with the kit when the amp was powered up gave a wee lower voltage (just a tad below 6V) but according to Jim that was no issue
I have to mention that my contacts with Jim were a joy and answers to my questions always arrived soon.
You can tell Jim and Charles are very passionate about they do!
Time to put on the bottom and make it final.
Wow… it sure does look good.
Really love the Modern Line chassis with the Toshiba 6080 tubes!
-Time to listen and judge!
From many parts to a beautiful looking and great sounding Headphone Power Amp that was the plan and that was also the result after many listening sessions.
I split my sessions in two.
One for CD’s and one for LP’s (vinyl)
The CD setup:
-Philips CD-202 (vintage)
-Denon PMA 1510AE used for pre-amp purposes
-Sennheiser HD-600 and a HD-820
I listened to various CD’s from Sade, Phil Collins, Mylene Farmer, Madeleine Peyroux, Dave Weck, Melody Gardot, Al Jarreau, Dire Straits, Andreas Vollenweider and Billy Cobham to name a few.
Listening to music using tubes and headphones is a different experience I would say.
Music sounded very detailed and you are really drawn into the sound.
Instruments have a very good seperation (depending on the recording of course).
With Melody Gardot and Madeleine Peyroux you could almost feel the instruments. Of course that type of music lends itself the best for that kind of experiences but nevertheless it was a very good listening experience.
Andreas Vollenweider’s recording already sound great but hearing them now, the music seemed all around you….. love it!
All in all the CD listening session was a success and the fact that it was hard to stop listening to music probably says it the best!
The Vinyl setup:
-Denon DP-47F (1982)
-Cartridge Denon DL-103R (MC)
-Denon AU-340 SUT (step up transformer)
-ML Universal Phono Preamp (built from a Kit)
-Denon PMA 1510AE used for pre-amp purposes
-Sennheiser HD-600 and a HD-820
For vinyl listening I picked some other albums.
Amongs others I played: The Alan Parsons Project, Marcos Valle, Benjamin Herman, Sting, Fleetwood Mac, Stanley Clarke, Miles Davis, Art Blakey, John Coltrane, Al Dimeole-John MC Laughlin-Paco Lucia, Herbie Hancock, Chip Wickham, Stan Getz, Joao Gilberto, Steve R. Vaughan, Chris Rea, Rodriguez, Pink Floyd, Boz Scaggs and Agustin Pereyra Lucena amongst others.
Using the ML Universal Phono Preamp in conjunction with the Headphone power amp was something new for me.
The whole system was dead silent.
The only thing I could very faintly hear was the sound of the cooling fan coming from the headphone power amp when no music was playing…. and I was sitting 2 feet (60cm) away from the amp.
After listening to all that music I can only conclude that this is an extremely great sounding amp that can handle all sorts of music with ease.
When listening to more detailed music like jazz, guitar and more music for later in the evening it really opens up and the music draws you in. You can hear all nuances and bits and pieces that normally would escape your attention.
When playing music like rock and pop it also sounds fantastic. But I learned that those recordings are not always that detailed.
Modern recordings can be a whole other discussion but let’s not go there 🙂
-My conclusion:
If you like building electronic kits and you love music then this ML Headphone Power Amp Kit is for you.
The building is not difficult and the video guides are excellent.
I do have to warn you though …. once you start listening in the evening don’t be surprised when your hear the morning birds chirping! Yes the sound is very addictive!
Well I guess it’s time for another round of listening and some more coffee.
Happy building and listening to you all!
Cheers,
Arno
Stefan (verified owner) –
Phenomenal! Once again, Jim and Charles have proven their skills with this device!
After a two-week delay due to our holidays, I was finally able to start my test build, which I had been patiently waiting for years… And it was worth the wait!
Assembly was really very easy thanks to the flawless and very comprehensive videos provided. For experienced builders, it may seem almost too easy, but it clearly shows how well thought-out the design and construction of the single large circuit board is!
Compared to previous kits, the significantly shorter build videos are now even easier to follow.
I had no trouble whatsoever building the kit. Sure though, I have already built several others (Universal 6or12SN7 Preamp, GU50 Monoblocks, Universal 6or12SL7 Phono Preamp), but this one is even easier to build and perfectly suitable for beginners.
In fact, the instructions are so comprehensive and clear that I didn’t have a single question.
The result is Phenomenal Sound Quality – Though always difficult to describe, as it is subjective, but the words that come to mind are:
• Particularly dynamic (probably due to the OTL design)
• Powerful
• Crisp and detailed but warm and never sharp, harsh or edgy
My first couple of Test Tracks were the following:
• Steven Wilson – The Overview
Wonderfully hymnal, à la Mike Oldfield, fantastically expansive, dynamic and rich in detail
• The Young Gods – Play Terry Riley in C
Very dynamic and rich in detail, yet soft and pleasant, tends to become annoying on ‘too hard’ chains – Not with this OTL amp!
• Ozric Tentacles – Strangeitude
Dynamic and clear, not blurry
• Gerald Clayton – Bells on Sand
Fantastically melodious
• Nick Cave (solo) – Idiot Prayer / The Mercy Seat
Dynamic and emotional, makes you want to cry
Analogue Setup:
—————-
1. Turntable: Micro Seiki DDX-1000; Tonearm: MA505; Cartridge: Grado Reference 2
2. Phono Stage: MTA Universal 6or12SL7 Phono Pre; Tubes: Sylvania 7F7 (rebased)
3. Pre-Amp: MTK Universal 6or12SN7 Preamp; Tubes: Melz 6N8S (6SN7GT)
4. Power-Amp: OTL Headphone Power Amp; Tubes: Toshiba 6080
5. Headphones: Sennheiser HD660-S2
Verdict: Very detailed and balanced, not dry but heavenly warm.
Digital Setup:
————-
1. Source: PC/Digital
2. DAC: Chord Mojo 2
3. Power-Amp: OTL Headphone Power Amp; Tubes: Toshiba 6080
Verdict: Very dynamic and powerful but balanced, still about the right amount of warmth.
A further interesting test: What is the difference with and without the OTL headphone amp??
1. DAC Chord Mojo 2 —> OTL Headphone Amp —> HD660-S2
2. DAC Chord Mojo 2 (Using the internal amplifier stage) —> HD660-S2
Very slightly audible difference, not significantly, rather subtle! In other words, the OTL headphone amp adds a little warmth (presumably due to the second harmonic distortions) and is therefore certainly less tiring to listen to. At the same time, however, it seems to lose hardly any detail, or if it does, then only very subtly, as far as I could perceive with critical listening.
What I particularly like is the bass range – absolutely cleanly defined, not blurred in the slightest. You can hear it particularly well on a bass guitar when low notes do not only sound, but actually growl. I like that!
In general, the layering of the details are not only astonishing but truly stunning and Blew My Mind! I couldn’t detect any blurring across the entire frequency range. But ultimately, of course, that depends on the entire chain.
Conclusion:
———–
A truly fantastic device! At last, I can also enjoy vinyl records with headphones.
What’s more, tube rolling in the preamplifier stage is extremely enjoyable, as I can hear even the slightest differences much better with headphones, picking up the finest details that my Tannoy Devon speakers only reveal at higher volumes, if at all…
Ultimately, however, I have to say that I’m not really convinced by the Sennheiser HD660-S2. Compared to my Grado Hemp and especially my Hifiman Edition XS, the HD660-S2 sounds unexpectedly flat and lacking in spatiality, hence not particularly lively. I have heard similar opinions from others, who say that the HD660-S2 does not measure up to the earlier HD650 or HD490. That’s why I just ordered the latter and will probably sell the HD660…
(Recommendations in the same price range are very welcome.)
Finally, I would like to express my sincere gratitude to Jim & Charles for accepting me as a test builder and thank them for their trust! I am glad to have been able to contribute, even if only in a small way, to the further development and continued existence of MellowtoneAmps!
Would I buy this amp again?
100% YES! Moreover, I would recommend it to everyone for sure!
Best wishes from Switzerland!
Stefan
totemarro (verified owner) –
I got the OTL Headphone Amp Kit in July after its launch and managed to build it in 4 days, but then I was away in France on holiday, hence the delay in posting my review. Eager to get it built, having previously built the Universal Phono Preamp and being delighted with that, I must advise caution to anyone building the kit to take the utmost care in following the videos and thoroughly check everything. I had soldered the tube sockets onto the board and was sure I had done them correctly only to discover I hadn’t. Fortunately, I hadn’t gone much further and suddenly my heart sank. I laboriously desoldered the sockets knowing full well that it would be a chore, careful not to damage the board or other components. I was praying I hadn’t done any damage and thank goodness on testing all was well. When you don’t build kits all the time like Jim and Charles, lack of experience can bite you and dent your confidence. Jim and Charles are such encouraging teachers saying how they have learnt so much through their own mistakes in the past. Every day is a school day as we say in the UK. Jim and Charles had said how quiet this OTL was, and they weren’t lying.
I plugged my Sennheiser Momentum 4’s in and there was deathly silence even with the volume turned up. I first tried a cd from Syml (Brian Fennell) called ” Nobody lives here”. He makes a lot of atmospheric recordings with a lot of space around the vocals and instruments keeping things simple. I also tried a cd by Dutch group Haevn. Their background is in writing tv and film scores with lush orchestral backing. It really comes through in their songs which are very atmospheric and poignant in their delivery. I’ve seen them live in London and were a joy to see and hear. The OTL conveyed all the timbre and detail through my Momentum’s, but I would be intrigued to try some other makes or Sennheiser models. The sound was refined and smooth, so long term listening was possible without fatigue.
Finally, I plugged my Universal phono preamp in and played an album by Joe Jackson called “Body and Soul”. It’s an excellent LP, very well recorded and hardly any surface noise. I have used it as a test album several times because not only is it great sounding, but I love the music. The OTL was still whisper quiet with the Universal Phono Pre in circuit, a testament to Jim and Charle’s expertise in design and experience of electronics.
The OTL was easier to build than the Universal Phono Preamplifier in my opinion, due to its simpler concept and thoroughly thought-out design. Jim pointed this out in the build videos, and I know they aim to achieve this in all the kits. The new mono block solid state kits will be exciting, I’m sure, when they become available.
Another superb kit to add to MTA’s range. Converts, like myself don’t need any more encouragement, but for any new disciples to quality audio or returning after a hiatus, should seriously consider the kits or purchase a ready made unit. You can spend an awful lot of money on quality hifi, but it doesn’t guarantee satisfaction. Valves may be old technology, but in today’s ultra complex solid state digital world analogue still has a valid place. Count me in.
Ian
United Kingdom