Are those vacuum-tube loving chaps of xDuoo ever sleeping at night? Are they dreaming tube gear and make them happen by the day? We’ll never know that, but I know that xDuoo are one of those guys that aren’t walking the same path two years in a row. Last year, matte-black everything was their driving force, but this year they splashed their children with shades of red and grey, becoming their new motif for the rest of the year and look how many units were released in such a short period! They’ve delivered nine products in nine months, that’s exactly one product per month. That’s crazy, since some of them were quite unique, including the unit that I’ll be testing today. Their lady in red (TA-10R) charmed me with its beautiful looks and sweet tonality, their XA-10 combo won my Golden Buzz of the year and the little MT-604 seems like the most affordable vacuum tube input, transistor output true-balanced headphone amplifier I’ve seen of late.
It sings tunes for about two weeks now and it’s time putting it into the spotlights and tell you more. It fights in the lightweight category, but I assure you…it punches way above its weight class. With 2 Watts of power under its belt, 4 vacuum tubes at the input and 8 transistors at the output stage, this tiny unit could drive pretty much everything, inducing a sweeter tonality without breaking the bank. It goes for $169, but does it perform well? Let’s find that out together.
The unusual shaped amplifier came double boxed with plenty of foam surrounding the unit as an extra protection measure. There is an accessory box below that holds its external power brick, a power cord, a warranty card and a detailed user manual in English. I recommend checking out its safety instructions, since a tube amp biased into Class-A, will consume considerably more power and will dissipate more heat in the process. xDuoo recommends using it in a well-ventilated area at all times. Since MT-604 is only a balanced headphone amplifier and doesn’t have additional features, you won’t find the usual stuff their DAC/Amp combos are bundled with.
Design & Build Quality
MT-604 might look decently sized in those photos, but in reality, it’s a little longer to a modern smartphone and slightly bigger than my palm. At just 17 cm long, 7.8 cm deep and 4 cm tall, its small and lightweight enough, so you can easily carry it around. xDuoo moved its power supply outside, leaving room only for its analog amplifier circuit. It uses an external switching mode (SMPS) universal power supply, so you can use it anywhere on the globe. It outputs 12V and 2A, meaning that it consumes around 24 Watts at maximum volume. Its body uses bended aluminum sheets, perforated on all sides so that hot air could move outside its case, prolonging the life of its components. The holes on top are taking the shape of xDuoo logo, a small, but interesting detail nonetheless. Beneath it, they’ve added four circular silicone pads as tiny shocks absorbers.
In the end, we are looking at a dual-mono, true balanced amplifier and xDuoo decided to completely separate both channels, so you’ll control its volume with independent volume wheels for its right and left channels. You’ll need at least a few seconds for a perfect channel matching and that could become a little annoying as time goes by. It’s not the most elegant solution, but they lowered channel crosstalk as much as possible with a design like this, it’s not only for the looks, but also for a better performance. xDuoo removed all the unnecessary LED lights from its front plate, so nothing will be showing its working status. There is less light pollution in late night listening sessions and only the tube glow will suggest if it’s powered or not. After a few hours of use I don’t find it hot to the touch, as it usually happens with Class-A tube amplifiers, it’s mildly warm and you can easily handle your gear around it, just make sure you don’t touch its vacuum-tubes.
Controls & Layout
Since I’m describing a balanced only amplifier, xDuoo removed all the unnecessary features like preamplifier outputs and single-ended inputs, MT-604 offers only two balanced inputs and two balanced outputs. A pair of XLR inputs, a 12V DC input and an On/Off switch can be found on its back, a 4.4mm and a 4-pin XLR headphone output can be found on its front, plus an additional analog input in the shape of a 4.4mm Pentaconn connector. Two volume wheels are controlling its right and left channels and that’s basically it.
Under its Hood
After a successful launch of their entry level MT-601 and MT-602 single-ended only headphone amplifiers, xDuoo decided to up the ante and release a balanced-only unit that would be considerably more powerful and just a little pricier. xDuoo went nuts with the power output, increasing it by tenfold to their MT-601 and by 1.5 times compared to MT-602. Outputting exactly 2 Watts at an added gain of 15 dB, I’m pretty sure, MT-604 would drive everything you’ll throw at it.
With four 6J1 pentodes that are usually used in single-ended amplifiers (for tube rolling, you can also try Russian 6J1P tubes) they were unable to pull-out as much power and this is where, xDuoo pulled an ace, adding 8 additional transistors at its output stage, seriously boosting its current output. At its diminutive size, 2 Watts per channel in a hybrid configuration was unheard of until now, but there’s always a first for everything. Its transistors are buffered for Class-A operation, hence consuming 24 Watts and dissipating a decent amount of heat in the process.
Apart from this, MT-604 is a DC coupled amplifier, using some of the best capacitors as coupling caps, more exactly ELNA Silmic II and at power filtering stages – Nichicon audio grade caps have been used and those are always nice to see in analog amplifiers of all sorts. Generally speaking, vacuum tubes weren’t made to be used in headphone amplifiers due to their high voltage swings, but xDuoo developed a slow start and slow shutdown circuit, so it would to save your precious headphones from pops at crackles. Several Takamisawa analog relays are doing that – and those are some of the finest relays that you’ll usually find in high-end amplifiers of today, like Flux Labs Acoustics Volot ($2550) for example. Alright guys, my ears started to itch, so let’s hit some ear-drums!
I. Preliminary Sound Impressions
There are few audio manufacturers that don’t pride themselves with a house-sound, but xDuoo always had one, wanting to impress music addicts first and not the measurement crowd that are putting SINAD above everything else. I’m pretty sure it won’t get outstanding results when it comes to measurements, but my ears are telling that there are a lot of good things to say about this tiny amp.
Even after the first song, it was clear to me that it wasn’t shooting for linearity, for absolute transparency, for an outstanding detail-retrieval and it wouldn’t pull the last drop of information from your tunes. MT-604 was injecting smoothness where there was none, it was boosting midrange presence, making those guitars sweeter and the voices gentler. It provided longer decay of the notes so I could easier trace a musical note from its inception, sustain and decay. MT-604 was elevating bass and midrange, making those regions fuller and thicker sounding, while gently rolling-off the upper treble. Connect any digital or analog source to it and prepare for some magic tricks a minute later. I’ve tried a bunch of recordings and I could never make it bright or offensive sounding in a span of two weeks. It was going smooth like an aged single-malt whisky, leaving a long-lasting aroma after the music stopped.
MT-604 is not about being correct, super linear or honest sounding, it’s about feeling every second of your blues or jazz, it’s about disconnecting from the outside world, it makes you stop from what you’re doing, highlighting the beaty of the music itself. With it, you longer listen to an empty shell, there aren’t only leading edges flying around your head, there’s weight, fullness and a higher density compared to your usual solid-state amplifiers. It reminds a lot about their lady in red (TA-10R DAC/Amp combo), but this one is even meaner sounding, even more powerful, slightly more colored and charismatic sounding. I’ve already imagined that its gain of +15 dB would be more than enough or too much with a lot of my headphones, but driving the notorious Hifiman Susvara at two o’clock, leaving plenty of headroom on tap…was something that I didn’t imagine even in my wildest dreams. It drove with authority everything I was throwing at it, desktop dynamics or planar-magnetics didn’t scare it at all. It felt so incredibly powerful, that I wouldn’t use the most sensitive headphones with it, it could be too much with Focal or Kennerton headphones, even at the 9 o’clock volume position.
Another thing that impressed me right away, is that the tiny xDuoo was sounding bigger to several kilo-buck amplifiers that were sitting on my table. If you think your Topping or SMSL amps are big or airy sounding, wait until you try the little one. It’s shocking hearing your music flying farther and going into the abyss. It was decompressing my music, improving the scale, making it grand so I would always look at a bigger picture. Even busy and crowded passages felt airier, adding empty spaces in-between them and dragging me a few meters away. No matter the music that was playing, I could never make it up-front and claustrophobic and not a lot of amplifiers have such extraordinary skills. No matter the headphone, I could never choke its dynamics or change its engaging sound signature. It kept on going and going, adding flavor and saturation, without giving a damn about linearity.
With all that said, it wasn’t the cleanest sounding unit, it wasn’t pulling the smallest details from my tracks and it wasn’t that impressive as far as noise-floor goes.
II. Power Output
How on earth, a desktop amplifier that weighs only 360 grams – that’s less than a pound (0.8 lbs) was able to drive all my headphones, without a single exception and with so much authority down low and control in the bass, it’s mind boggling! When my ears started flapping in the air like butterflies with the Hifiman Susvara at around 2 o’clock position (maximum volume being 5 o’clock), I’ve checked again if a Benchmark AHB2 (100W in 8 Ohms) wasn’t connected to them, it felt that there is unlimited power on tap. There is no other way in saying that it drove everything at my disposal like it was a walk in the park. Dynamics were going wild, always keeping up with my music. Those 8 transistors seem to provide enough current even for the most demanding loads as low-sensitivity planar-magnetics. The same happened with others as Audeze LCD-4, Hifiman HE1000SE, Erzetich Phobos 2021, Kennerton Wodan and Sendy Peacock – it was pushing and pulling those membranes like it was pumping iron on steroids. With most of my headphones, I couldn’t go higher that 10 O’clock and it was mostly unusable with portable over-ears or IEMs, since there was little to no play on those wheels, it became too loud and way too fast. A few days later, I’ve tried again the Hifiman Susvara just to be sure that I was not tripping and immediately I started tapping my feet – confirming my initial findings that MT-604 looks like the most affordable amplifier that is capable of driving the mighty Susvara to very loud levels. It sounded engaging and fun, changing the personality of several headphones, by rolling the upper treble and infusing more energy down-low. From a linearity behemoth, Susvara went smooth and relaxing, gently rounding the transients, delivering some hard punches down low, but never pressing the gas pedal to the floor when it comes to speed.
I’ve tried some of the heaviest bass riffs that choked and shut-down a lot of headphone amps with the Susvara and MT-604 was still going strong and smooth as butter. I wasn’t surprised anymore; I was stunned and couldn’t believe my ears! It felt like the most powerful amplifier xDuoo has ever released, but not only that, as I didn’t experience as much power before at less than 200 bucks…
III. Transient Response
MT-604 is not your regular tube-amplifier, it’s a hybrid amplifier – a very different beast altogether. Those vacuum tubes on top are doing only half of the job, working mostly as input buffers, as the final current amplification stage is done by the help of 8 transistors biased into Class-A. If this feels like unknown territory to you, in simpler words…the usual softness that calms and rounds the frequency response at both ends cannot be found on MT-604. This is a different specimen, it’s both a tube and a solid-state amplifier, borrowing skills from both designs. In the end we are getting faster transients and a nicer kick into the eardrums compared to regular all-tube amplifiers, but we aren’t getting the same slam and impact as a cold-blooded solid-state amplifier would deliver. xDuoo didn’t go with op-amps at the output stage and I can only congratulate them for that, that would be a big disappointment from my part. In all honesty, the boldest, the hardest kicking amplifiers (of all sorts) that I’ve ever tried at my place were transistor-based Class-A amplifiers and I’m happy that some of that tech made its way into the little MT-604.
Without too much fanfare, I will tell you right away that MT-604 was engaging, hard slamming and toe tapping all the time. This is a fun sounding unit that declared war to honest and linear sounding amplifiers. It doesn’t only amplify the weaker signal coming from your source, it adds weight and flavor, ultimately altering the final outcome. I find it more impressive when it comes to delivering powerful bass notes, then in terms of speed. This is a fast-sounding unit, but it can’t keep up with the latest generation solid-state amplifiers. Hifiman Susvara wasn’t as tactile and impressive as far as pace, rhythm and timing are being concerned. All my headphones slowed time a little and stretched wide those musical notes, there are definitely longer decays to be found. MT-604 isn’t your best buddy for fast-paced electronica tunes, but it would be just fine with modern pop, hip-hop, rock and metal. If I’m overlooking its slower tempo, then I find it considerably more impressive when landing hard punches in the lowest octaves. If you love your bass deep and rumbling, just press play and watch those earlobes fly in the air.
IV. Soundstage & Depth
Probably one of the most important thing to say about tube amplifiers is how easily they portray a bigger picture to chip-based solid-state amplifiers. While MT-604 won’t challenge the biggest and the meanest tube amplifiers as far as soundstage goes, it would easily outperform SS amps below the $500 price mark. I wasn’t surprised hearing my music flying at a higher altitude around my head, going past my shoulder level, disentangling every note and adding a bit more air in between. It sounded expansive and bigger even to (much) pricier amplifiers, my $3000 Benchmark HPA4 wasn’t adding so much air in between the notes and don’t get me started with Topping and SMSL units. No matter the music, it would open and stretch it nicely, adding a few layers of air and disentangling it nicely. It scattered around my music, putting all the notes on individual shelves, improving not only the scale of the music, but the pin-point location or the so-called imaging. Even regular music flew farther away, like I was no longer listening to studio albums but to live recordings. A pricier solid-state amplifier will follow on the test bench, yet at only 1/4 of its price, MT-604 pushed those notes father, as if they were no longer hitting an imaginary wall at some point. If you like your music stretched far and wide, then you can never go wrong with a tube amplifier at your side. Four pentodes and eight transistors improved its sound-staging capabilities, leaving most of the solid-state amplifiers in the dust.
V. Noise Floor & IEM Pairings
Did you seriously think that I would go roses are red and violets are blue for the rest of my review? A $169 amp can’t possibly do everything right…right? If you would check its dedicated webpage, you’ll observe that those clever guys of xDuoo didn’t mention its noise floor anywhere…how curious. For some of their best solid-state amplifiers, noise floor is being mentioned, but not here and that already tells me a few things about the MT-604. Seeing that external switching mode power supply, very few voltage regulators and only a handful of capacitors for power filtering, it was clear to me that I’m not looking at an outstanding power supply design that would fiercely reject noise coming from AC lines or downstream equipment. I won’t hold my breath anymore and will just mention that all my IEMs, but especially sensitive ones weren’t working that well with the tiny xDuoo, as a few nasty gremlins were adding noise and distortion into my music, even at lowest volume levels. I had exactly one click before it started being way too loud, so again IEMs aren’t working well this unit. Analog potentiometers as the one planted inside the MT-604 aren’t working well at low-volumes, their channel crosstalk is bad and at low SPL and I needed to constantly re-adjust its volume. In all seriousness, this little big amp wasn’t made for IEMs, it was plain bad with them. There’s plenty of noise and hum, call it as you want, I couldn’t enjoy my music with sensitive IEMs, it wasn’t a good match, IEM users please skip this one.
The bad news is that even super-sensitive desktop headphones as Apos Caspian and Kennerton Gjallarhorn weren’t super-clean sounding as I know them to be on desktop solid-state amplifiers. Some noise crawled inside those drivers, never to go away even at lower volumes. It seems that xDuoo created the little one only for big and heavy-duty headphones, such a paradox, it’s almost funny. It worked best with difficult loads, but especially with high-impedance ones that would burry noise-floor to inaudible levels. Nonetheless, MT-604 is far from being noise-less and sadly you can’t have them all at less than two hundred bucks.
VI. Detail Retrieval & Transparency
Here’s another area when the little fellow wasn’t shinning that bright. When noise floor rises, signal-to-noise ratio lowers, meaning that the lowest intensity notes might be buried deep in that noise, never to be heard again. Obviously, MT-604 is far from being muddy or really bad in here, but it isn’t exceptional either, as you won’t hear the tiniest nuances. I’ve tried a bunch of albums, including old ones that have a noise-floor of their own, MT-604 completely wiped that grain from existence, including some micro-details that I know for a lifetime. It’s clear to me that it was made to enjoy music and not to analyze it to the smallest details, that it cannot do. There is a decent transparency and that can be felt with regular and Hi-Res music alike. You’ll be experiencing music on a macro scale, the higher intensity notes would be highlighted, while the lowest ones would hide in the shadows. This can also work to your advantage in certain cases. There are plenty of underground bands that I’m listening for their lyrics and for their mood inducing atmosphere, but those are adding grain and brightness, making them impossible to listen to on a detail-oriented setup. MT-604 wiped clean all the grain and listening fatigue, smoothed out the rough edges and left alone the most important part: the music itself. With such records, the tiny xDuoo might work as a cure and vice versa, with reference recordings a layer of information would be wiped clean. Sadly, there aren’t that many 6J1 tube replacements that would improve its performance. Western countries never made a replacement for 6J1 pentodes, so you can rely only on Chinese and Russian variants of this tube, it is either 6J1 or 6J1P that wouldn’t improve noise rejection and SNR numbers that much.
VII. Frequency Response
MT-604 stood out immediately with its impressive low-end and midrange delivery. Both felt elevated by a few notches, adding warmth, sweetness and lots of giggles while listening to music. There is plenty of bass traveling around in silent passages and I feel it above any other FR range by one or two notches. It starts strong even from 20 Hz, always trying to impress with its impact and body slams. It’s not that often when a hybrid amp impresses much in the sub-bass territory, but MT-604 is a different animal altogether. A few dBs were added in here for an engaging bass performance and you know what? It worked as a magic wand with linear or bright tilted headphones. Mid-bass is even more present, standing by a shoulder to the rest of the spectrum, sounding even weightier and much fuller. This is not the cleanest type of bass, nor the fastest sounding, but it surely kicks and pounds with an impressive force when it’s called for. On the plus side, its whole bass region feels spread out, moving slowly, adding a few layers of air, increasing my perception that I’m experiencing a breathing type of bass, instead of a lower-quality single-note bass. MT-604 might be small and lightweight, but I can’t deny its strong bass delivery and ballsy attitude.
xDuoo – that’s midrange presence in Chinese, go check that out. On a more serious note, all xDuoo devices that were tested around here, including solid-state amplifiers, were outstanding as far as midrange goes. Sure enough, it felt elevated, guitars wept nicely, violins carried emotions with them, voices were making goose bumps all over my body and, in all honesty, this is what music is all about. It is about sending shivers through your spine, transmitting sad or positive emotions, about putting you into a state of mind, plain and simple it is about sending a signal and I do believe MT-604 does that extremely well. Hybrid amplifiers of all sorts were particularly impressive in the midrange department and the little fellow is no exception to that rule. With it, expect longer natural decays, feeling the wooden bodies of musical instruments and an added weight they carry with every single note.
MT-604 is just okay in this department. You can feel some of that shimmering, but lots of treble information would be skipping the action. The little xDuoo is rolled-off in here and that is precisely why I didn’t find it good enough when it comes to detail retrieval. It calms down the upper treble, it polishes the rough edges, delivering a polite and inoffensive treble at all times. If you can’t stand the brightness and listening fatigue, then you can listen to music for hours at no end with the MT-604 at your side. It could literally transform many headphones from bright to linear and from linear to warm, making them manageable and a lot more pleasing in late night listening sessions. In the end, I would describe it as organic sounding, infusing plenty of bass and midrange into the mix, while gently rolling-off the treble. MT-604 feels like the biggest enemy of linearity and it doesn’t care a single bit, it will be making music and not just an amalgam of sounds that were thrown into a metallic box with tubes on top.
Would you believe an entry-level hybrid amp could drive the hardest loads with so much headroom remaining on tap? If I wouldn’t try it for myself, I wouldn’t believe it. The little MT-604 won me over with its alter ego to everything I’ve tested around here. It sounded warmer to their TA-10R and even punchier to their XA-10 and I don’t recall trying such a smooth, easy rider that added so much color and nuance where there was none, always sounding refreshing from a sea of instrument grade amplifiers. It was like tasting a fine rose-wine after a hot sunny day. There were a few misses here in there, transients weren’t particularly fast, detail retrieval was just decent, noise-floor was too high to be used with sensitive IEMs, but God damn! It was pure fun, always leaving a mark no matter the song playing in the background.
Once again, xDuoo proved their different thinking, cherishing different values, caring for the end result and not for some numbers appearing on a screen. As an entry-level hybrid amp costing as little as $169, it’s my pleasure recommending the xDuoo MT-604 as a powerful and unique sounding unit. If you ever wondered what’s the fuss about tube amplifiers, but never wanted to drill holes in your wallet, now’s your best time.
MT-604 was kindly provided HiFiGo and it can be purchased from their web-store by following this link. They are selling it world-wide and it will be sent from the nearest warehouse to your location to avoid additional fees. In case you get one, leave some feedback below, I’m curious to know how it performs in your setup.
- Small, lightweight and transportable
- An above decent build-quality
- A fully-balanced input to output design
- Hybrid architecture marries the best of solid-state and tube amplifiers
- Over-powerful with everything I had on my hands
- Slams and pounds like a mad man, very impressive in the bass
- Warm and organic sounding midrange, adds emotions and meaning to your music
- Open sounding, layered and quite spacious
- Decent pin-point imaging
- There is no listening fatigue whatsoever
- An outstanding value!
- Not very fast sounding
- Lacks detail retrieval and transparency
- A higher noise-floor to usual hybrid amplifiers
- Forget about IEMs with this particular unit
- Rolled-off upper treble
- You’ll need some time balancing both channels for the same SPL
- DACs: Musician Aquarius, Audiobyte HydraVox & HydraZap, Matrix Audio Element X, Gold Note DS-10 Plus & PSU-10 Evo, Gustard X26 PRO, X16, Topping D90SE, Topping EX5, Burson Timekeeper 3i
- DAPs: FiiO M11 Plus LTD, M15, Shanling M8, M6, M3X
- Headphone Amps: xDuoo MT-604, Flux Lab Acoustics Volot, Benchmark HPA4, Singxer SA-1, Burson Soloist 3X, Musician Andromeda, SMSL SP400, Topping A90, Gustard H16
- Preamps: Musician Monoceros, Benchmark HPA4, Topping PRE90
- Power Amps: Benchmark AHB2 (x2), KECES S300, SMSL SA400, Burson Timekeeper 3i
- Loudspeakers: KEF Reference 3, Natural Sound NS-17, Sound Of Eden Crescendo UNO
- IEMs: FiiO FA9, FH7, FH5S, FD5, Meze Rai Penta, Rai Solo, LittleDot Cu KIS, Hiby Crystal 6 & others
- Portable headphones: Sennheiser Momentum 2, Meze 99 Classics, Sony WH1000-XM4
- Full-sized headphones: Hifiman Susvara, HE1000SE, Arya, HE400SE, Audeze LCD-4, Erzetich Phobos 2021, Erzetich Mania, Kennerton Wodan, Magni, Gjallarhorn, Vali, M12S, Ollo S4X Reference, Apos Caspian, Sendy Peacock & Aiva
- Interconnects: QED Reference (x2), Topping TCX1 (x2)
- Speaker cables: Kimber PR8, Audioquest Type4
- Power Cables: Isotek EVO3 Premier (x3)
- Balanced Isolation Power Conditioners: PLiXiR Elite BAC1500 (stereo setup), Elite BAC400 (headphone setup)