Decware ZMF Reference OTL - Review
: Lush, sweet, silky smooth sound.
Cathode bypass mode offers intoxicating soundstage and resonant harmonics.
Excellent synergy wtih ZMF dynamic headphones (and other headphones).
Excellent preamp. In specific, incredible synergy paired with the Decware Taboo.
Lots of tube rolling options.
4 headphone outs.
Beautiful and timeless retro design.
: Quirky and impractical retro design.
Did I say quirky design?
DECWARE ZMF REFERENCE OTL - REVIEW
The long anticipated ZMF and Decware amplifier collaboration was first confirmed by Steve Deckert last year. As he then posted on the Decware forums: “A CSP3 with full anniversary mods comes close, but this is noticeably better, and it should be... If you live in your headphones and want a reference OTL headphone amplifier/line stage, I seriously doubt you'll find anything that sounds even close for the money”.
I have had the rare pleasure of testing this output transformerless (OTL) amplifier over the last four months across several design iterations and, a year after this promise was first made in public, I can enthusiastically concur. The final result is absolutely superb.
Some discerning folks might have caught a glimpse of this amp at ZMF’s room at the recent CanJam in NYC, where it was doing a quiet trial run somewhat under the radar.
Dubbed “Decware ZMF Reference OTL” (catchy
), this exciting new amplifier is now, at long last, ready for launch! Per Zach, it will be available later this month, on or around the launch of the new Atrium Closed headphones. It will retail for $3,999 and will also be available for $3,799 bundled with ZMF headphones.
For simplicity, I will refer from here on to the Decware ZMF Reference OTL as the “Decware OTL”.
A noteworthy anecdote: the current waitlist for Decware amps is close to two years (not a typo). This may be both an opportunity to get assertedly the best OTL Decware has made, and an opportunity to get it directly from ZMF upon its upcoming release.
SPECS
Tubes
The Decware OTL uses one gain (input) tube, two driver (output) tubes, and a rectifier tube. It offers extensive tube rolling options:
Input Tube
A tube roller dream, this amp will accept most 6V dual triode tubes, including: 6DJ8 (ECC88), 6922 (E88CC), 6BQ7A (ECC180), 6N1P (6Н1П), and 6N5P (6Н5П). It will also work with 7V tubes, such as 7DJ8 (PCC88) and 7ES8 (PCC189). The stock tube is the 6N1P.
Output Tubes
Stocked with a pair of 6N6P (6Н6П) tubes, the Decware OTL’s circuit design is optimized around these tubes' electrical characteristics.
You can also roll in 6922 and similar tubes per above; however, note that these would increase impedance and cut down output power by about 3x which is not ideal.
Rectifier Tube
There are many options here. All of the following rectifiers are supported: 5AR4 (GZ34), 274B (CV684), 5Y3, and 5U4. You can also use the USAF 596 with an adapter.
Power
1800 mV in cathode resistor mode.
2000 mV in cathode bypass mode.
More on that, further below.
BUILD & FEATURES
Design
The Decware OTL has a classic Decware design. An old-fashioned design, it offers a heavy dose of authentic nostalgia, with a hint of retropunk or quirkiness, depending on your esthetics. Like drinking a glass of excellent single-malt scotch, it is served straight up.
Encased in a beautifully minimalist wood casing, it will sit harmoniously alongside your ZMF wooden headphones. The Decware OTL also has a very small footprint at 8” wide x 15.5” deep. It is also very light (you can easy pick it up in one hand, even if you’ve never done a bicep curl in your life).
What’s What
Volume Control
Up front, we have the black volume knob. It is smooth, and offers excellent and fine volume control, with perfect channel balance from the very lowest volume.
Headphone Outputs
You will note two XLR headphone outs to each side of the volume knob; and if you look further toward the back of the amp, you will notice two more 1/4" headphone outs (behind the blue capacitors, to each side of the black transformer). There are a total of 4 headphone outs which work concurrently. So, if you usually invite 3 friends over to listen to headphones together (with exactly the same headphone sensitivity and personal volume preference) then good news! You now have an amp that can do that.. Note that since the amp is single-ended, the choice of XLR vs. 1/4" offers a welcome convenience, but otherwise has no significance to your listening.
VU Meters
The Decware OTL has VU meters. They look beautiful and glow in soft orange, adding to the fun analog aura of the experience. Like all VU meters, they have no practical use, but that’s not the point, is it?
Cathode Bypass Switch
In between the VU meters you will see a small switch. It is one of the more notable features of this amplifier: a cathode bypass switch. When pulling the switch toward you, the amplifier will bypass the cathode resistor. I do not have the technical diagram of the amp (nor the technical expertise) but, in layman terms, in cathode bypass mode, the line-in signal would reach the gain tube’s cathode via a capacitor connected in parallel to the cathode resistor, which is being bypassed. Bypassing the cathode resistor should reduce some of the negative feedback caused by the resistor, resulting in both increased gain and improved dynamics and frequency response.
This switch has a real and meaningful impact on the sound signature. The Decware OTL sounds great in both modes. With the cathode resistor engaged, the sound is more focused and centered. With cathode bypass engaged, beyond a small audible increase in gain, the music is rendered more holographic and, at least in my mind’s eye, there is a feel of slightly more second-order harmonics and reverb, to taste. That is my preferred mode.
Tubes
Behind the cathode bypass switch you will see the input/gain tube, behind it follow the two output tubes, and further back is the rectifier (in between the two blue capacitors). The large black transformer follows. All line-in and line-out connections are further behind it.
Preamplifier
On each side of the transformer, behind the 1/4" headphone out ports, you will two RCA outs. These are the L and R channels of the variable line-out, which is controlled by the volume control contemporaneously with all headphone outs. The line-out and headphone outs are all “hot” (no switching in between headphone and preamp; if the wire is in, it gets the juice).
Line In
You can connect up to 2 different sources to RCA line-in connectors at the very back of the amplifier. Small switches, one to the left and one to the right, let you choose which of the connected two L channels and which of the connected two R channels are used (in short, you have to move two switches to change a source). There are 4 small black knobs, one next to each of those line-in channels. These let you control the gain of the line-in, before it reaches the gain tube. These are controlled separately for L and R channels.
The on/off switch is tucked in between the transformer and power outlet. Speaking of which..
Quirky Design
When turning the power on, you carefully insert your hand vertically from above, in between the various interconnect cables that jut upwards. When turning the power off, you will now also have the warm transformer to keep you company.
The cathode bypass switch is right next to the gain tube, so just make note not to touch it.
In practice, nothing is really too hot, and nothing is too sensitive. Retro design, retro Ux. A throwback to a more genteel time. It is really part of the charm of using this amplifier; and this design oozes with charm.
SOUND!
Chain Used
In the specific context of this writeup, I have used the following chain:
Roon Nucleus+ ➤ CAT 7a ➤ Audioquest Cinnamon RJ/E ➤ Bricasti M3 (power cable: Cardas Reflection) ➤ Moon Black Dragon (RCA) ➤ Zynsonix custom switchbox (silver cabled) ➤ Moon Black Dragon (RCA) ➤ Decware ZMF Reference OTL (power cable: Audio Sensibility Testament SE | power source: IsoTek EVO3 Aquarius) ➤ Decware Decware Taboo SE84TS+ (power cable: Audio Sensibility Testament SE | power source: Transparent PowerBank 6)
Overall Sound Signature
When one conjures up the sound of a great OTL, a few things come to mind. The Decware OTL offers these all in spades: it is holographic, natural, and smooth sounding. The sound will naturally be highly dependent on the headphones used. However, I have found the Decware OTL to impart the following consistently:
Lushness
This amplifier has a beautifully sweet sound, with a smooth and velvety voicing that washes over you like refreshingly cold water on a hot summer day. I don’t have the gear to measure harmonic resonances, but I imagine it would show a dose of second order harmonics in cathode bypass mode.
I find this essence of sweetness and lushness of sound most alluring with acoustic vocals. A great example for male vocals is Al Jarreau signing of “Breezin’” which sound as smooth as silk. For female vocals, Laufey voice sounds especially sweet on “Street by Street”. OTL saccharine without overdose. Really breathtakingly beautiful voicing.
Sense of Space
There is a real impact on the soundstage when switching to the Decware OTL, with a wonderful sense of diffusion of sound into a very large space coupled with a nuanced reverb. As if the music played within a large cathedral hall with the sound softly echoing across the faraway stone walls and vaulted tall ceiling arches.
On “The Wind That Shook The Barley” by Dead Can Dance, listening with the Atrium Open, I found the already incredible soundstage of this recording and that of the Atrium further expanded, echoing far and wide in the endless distance with a sense of beautiful resonance, while remaining organic and natural sounding.
Thumpy Bass
Not the most scientifically objective adjective, but if I would try to convey the feel of the bass with the Decware OTL, I would say that it is a lush and warm thump. It hugs you like a comforting flannel blanket. A wonderful example is the collaboration of Taj Mahal and Keb’ Mo’ on “Don’t Leave Me Here”. On this track, the bass is rich and thick. On Dominique Fils-Aimé’s wonderful song “Birds”, the Decware OTL offers a bass that is clear and transparent, while lending a sense of gentle decay to the low drum kicks and handclapping rhythm. At the moment, I am listening to “Good Morning Gorgeous” by Mary J. Blige on my Vérité Open headphones, and just caught myself bobbing my head with a full on stank face. Shut the front door!
To offer a sense of comparison, my beloved transformer coupled Auris Nirvana provides a more focused and crisp sound, with a blacker background and a more centered image. In contrast, the Decware OTL offers a wider sense of space, resonance, and organic timbre. If I borrowed an analogy from the ZMF headphone universe, the voicing of the Nirvana and Decware OTL offer a similar Yin and Yang contrast in the signal chain as would the Caldera and Atrium headphones, respectively.
Synergies
There is a common rule that suggests one should pair red wine with meat and white wine with fish. OTL amps are meant to drive high impedance dynamic headphones, and the Decware OTL does so wonderfully.
I practically love the pairing of the Decware OTL with the Vérité Open, Atrium Open, and Sennheiser HD800s. These combinations are magical. In all cases, the amp offered the value added sound attributes I described above, imparting its smooth, spacious, and harmonic sound signature. If I put one of those headphones on, they stay on for hours 
I have also found the Decware OTL an excellent match to the Atrium Closed and Vérité Closed.
There is also wisdom that suggests that one can drink any wine with any food, and that the aforementioned rule is meant to be broken. I’ve found that keeping an open mind can sometimes be rewarding, and that some unique cases that do not make sense on paper, work great against all logic. These included the AT L5000 (45 ohms) and MDR-R10 (40 ohms), both of which I found to be remarkably well controlled by the Decware OTL, including an unexpectedly tight bass. I can’t explain it. And, as expected, some other low impedance headphones were poorly controlled. So you should get this amp for your high impedance headphones but, if you do, don’t be afraid to experiment..
Tube Rolling
I have tried numerous tubes for the input gain and found that all worked great. A 6922 (Sylvania JAN 1970) and a 6BQ7A (RCA 1957 black plates o getter) both offered clarity, and a Telefunken PCC189 offered a tighter bass, possibly a good choice for heavy rock, metal, or other bass heavy genres. My current preference is the 6N1P tube, the very one which comes as the stock tube. I have found it offers the most holographic and smooth sound. An excellent choice for this amp imho.
For output tubes, I have stuck mostly with the 6N6P. I have yet to do more significant tube rolling here, but given these tubes are more powerful than the alternatives as I explained in the spec section above, I am not sure if I would end up changing these.
For rectifier, I have roll in a Philips Miniwatt GZ34 (Sittard brown base), GE 5Y3G, and a Raytheon 5Y3GT. I settled on the GZ34, having found its low voltage drop (lower sag) pairs best with the amp’s organically smooth OTL character, offering the best transient response and dynamic performance.
As a preamp
I tried the Decware OTL as a preamp to my solid state CFA-3 headphone amplifier, and to my TC Auris Nirvana headphone amplifier. In both cases, it provided a clear and neutral line stage with excellent and quiet volume control. The resulting sound characteristic was true to the amplifier used. I haven’t done extensive tube rolling in listening to these combinations.
However, there is one notable exception that was a true revelation.
In fact, this combination was so exceptional, it may be one of my all-time favorite amp configurations:
Decware OTL + Decware Taboo
A quick note: I have an older Taboo model (SE84TS+ from ~2011). However, @zach915m has the current Taboo (IV) and, as we’ve exchange notes on this, we have both found a very similar experience: when using the Decware OTL as a preamp into the transformer coupled Decware Taboo amplifier, the resulting sound is absolute magic. As best as I can describe it, it is like taking all the goodness of a tube amp, multiplying it by two and applying an exponent. I recall the first time I tried this combination. My jaw dropped to the floor!
On this combination, the Caldera headphones were as good as I have ever heard them. Insanely holographic and musical, it is the ultimate amplifier chain for these headphones. I also tried the Susvara, and it may have been the most melodic I have heard them, though the headroom with those would make it more of a specialist combo for specific genres only.
CONCLUSION
Our decisions in life are often guided by a blend of rationality and emotion. The Decware OTL is the embodiment of that duality. It offers great technical performance but, first and foremost, it offers a uniquely emotional experience. As your hand hovers above its beautiful casing to adjust the volume, you note the impractical VU meters facing upward and the warm tube an inch away from your fingers. You smile knowingly, just as the music washes over you and as you rediscover your headphones, as sweet and harmonious as you have ever heard them. The stirring notes tug at your heartstrings are the ones that amplify your life, transcending mere function to create a truly unforgettable experience. Yes, the Decware OTL will make even a poor writer want to wax poetic..
In a stack alongside several other amplifiers that I love, I have found that I can now not only select the headphones to match my mood or genre, but I can tailor the amplifier to the occasion.
Finally, after having had this amplifier on a very long and extended loan for review, I realized that I needed it to stay with me. I loved it so much that I bought it. I am not sure how many of these will be available, but I suspect they will be gobbled up as fast as they are offered. If you are considering getting an OTL, keep you eyes peeled for the upcoming launch of the Decware ZMF Reference OTL (and, don’t blink!)
