The xDuoo MT-604 is an affordable Class A balanced hybrid tube headphone amplifier with independent channel control. It is priced at $169.
Disclaimer: The xDuoo MT-604 sent to us is a sample in exchange for our honest opinion in this review. Thank you to xDuoo and Shenzhen Audio for giving us this opportunity.
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Note, this 2-page review follows our new scoring guidelines for 2021 which you can read up on here.
The xDuoo MT-604 undoubtedly offers a great value proposition by introducing a warm overall tonal balance and a holographic soundstage presentation. All of which can be achieved as long as you can accurately adjust the volume knobs to achieve a good balance between the left and right channels.
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When I hear the brand xDuoo, I immediately think of amplifiers and DACs. And true enough, xDuoo has consistently been releasing high-quality amplifiers and DACs at affordable prices. Just a few months ago, they sent over the XA-10 DAC/amp, which I thoroughly enjoyed.
So when I heard that xDuoo was sending over their pure balanced tube amplifier, the MT-604, I thought it would be interesting to give it a thorough listen.
The MT-604 is a balanced hybrid tube amplifier. The pre-amplifier section is made with 6J1 tubes configured in a fully balanced circuit, which means that 4 tubes are running the pre-amp section of the MT-604. The amplifier section on of the MT-604 on the other hand is made with a discrete solid-state Class A capable of 2W into 32Ω.
The MT-604 is also a balanced-only amplifier. Even the volume control is balanced, and each channel can be controlled independently.
Internally, care has been put into the selection of components, where capacitors are either from Rubicon or Nichicon. Both capacitor brands are known in the audiophile world to have a sweet and transparent sound.
Most amplifiers, particularly more powerful ones, tend to have a pop when the circuit goes on. This might get annoying, and might cause some damage to more sensitive headphones or IEMs and the tube section of the MT-604.
To ensure the reliability of the MT-604 in these situations, xDuoo integrated a built-in mute circuit to mitigate the effects of the switching pop.
The front fascia of the MT-604 is highly symmetrical, where there is a 4-pin XLR headphone output in the middle, and it’s flanked by 2 volume knobs on the sides, one for the left and the other for the right channel. Also on either side of the 4-pin XLR output are the 4.4mm balanced input and outputs.
On the rear, the MT-604 is very simple, since it only has a pair of 3-pin balanced XLR inputs, power input, and a power switch.
The casing of the MT-604 is sturdy, where the majority of the case is made of sandblasted aluminum with a gray finish. The inputs and outputs are clearly marked, and the volume knobs are graduated, to make it easier to volume match the left and right channel by counting the graduations along with the volume dial if needed.
Sitting on top of the MT-604 are 4 6J1 tubes that serve as the balanced pre-amp section. When the amplifier is on, these tubes take on an orange glow, partly due to the tubes sitting atop some orange LEDs.
The MT-604 has 2 input options and 2 output options. The 2 input options are the 2 3-pin XLR inputs at the rear and a 4.4mm balanced pentaconn input to the fore. On the other hand, there are also 2 possible output options in front, which include a balanced 4-pin XLR and the 4.4mm pentaconn connection.
Although there are 2 pairs of inputs and outputs, both inputs are electrically connected, and so are the outputs. So the user should be careful to not use either input at the same time to avoid damaging your source gear.
While I initially found it cumbersome to have to manually adjust each channel separately, and find the correct channel balance every time I change the volume. The separate volume knobs come in handy in 2 instances.
The first instance is to get around the problem of having channel balance particularly at the lower range of the volume knob. These 2 volume knobs ensure that there will be channel balance for the entire volume range.
The other instance that it could be useful is for getting around using matched quad tubes, where channel balance can still be achieved even when the tubes are not a perfectly matched quad set.
Furthermore, the MT-604 can be used as a pure headphone amplifier with just balance controls, while the upstream DAC or a separate pre-amp can be used for actual volume adjustment.
This would then ensure that the MT-604 will always have a balanced volume range, while having the convenience of just turning 1 volume knob, provided that the separate volume knob is accurate.
Packaging & Accessories
The package that the MT-604 arrives in is standard and comes in a simple low design white cardboard box. Out of the box, the MT-604 has the stock tubes already inserted in the sockets.
Despite the tubes being pre-installed, there is nothing to worry about since it’s packaged with foam inserts, so nothing will be moving around while the package is in storage or transit.
Underneath the foam inserts and the MT-604 itself, there is a box underneath that contains the power brick and a standard IEC power adaptor. Aside from these, the box doesn’t have much else inside, but it’s a fairly standard package for an amplifier, especially at this price point.
Since the MT-604 has a tube pre-amp section, it is necessary for the tubes to warm up substantially before listening to them. For good measure, I let the tubes warm up for about 30 minutes before writing down my listening impressions.
When talking about the tonal balance of the MT-604, it stays on the warmer side overall. Where the mid-bass is bumped up and is allowed to spill over some of the warmth into the midrange response. This makes the vocal response more euphonic and broader in tone.
Being on the warmer side of neutral, the MT-604 stays safely away from being too bright, whilst retaining an overall sweet treble response. This makes me think that the MT-604 will make for an easy listen, allowing for hours of listening pleasure without experiencing fatigue because of its smooth and soft presentation.
While the sub-bass is not particularly forward, the warmth injected by the MT-604 mainly comes from a more forward mid-bass region.
The mid-bass presentation is impactful, but with a hollowed texture, allowing the size of the drum cavity to be easily visualized. Bass notes take on a natural sense of attack and decay, where the flow of bass guitars comes to life easily.
On the flip side, the MT-604’s treble response stays on the safer side of being too bright. This allows the MT-604 to convey a sense of girth in the treble region while retaining the sweetness in treble instruments such as cymbal hits and higher-order harmonics.
Due to staying safe though, the treble on the MT-604 ends up being more glossed over, rounding off the edginess and blunting the bite typically found in the treble region.
Since tubes are sought after for their more euphonic midrange quality, that is what I was expecting to hear with the MT-604. The vocal reproduction with the MT-604 takes on a sweeter character where the vocalists have an overall richer quality and are front and center in the mix.
Instrument timbre on the other hand leans towards being on the warmer side of neutral. However, the sweetness of the treble retains an amount of harmonic overtone that will still make violins and cellos take on a flowy character making them sound natural overall.
Staging & Dynamics
Another strength of tube-based amplifiers is staging, and the MT-604 doesn’t disappoint in this front either. When it comes to the imaging that are formed, they are holographic in their sense of scale, allowing images to take up a realistic amount of space. However, the images aren’t particularly defined within the space that they occupy.
Despite the images occupying a realistic amount of space directionally, the images tend to smash together, overlapping particularly with the positional depth of the images.
While the MT-604 is pretty good at creating larger emotional crescendos and creating a sense of scale with weight behind the bass, it lacks a sense of gentle finesse when presenting softer piano passages. They end up sounding a bit louder than I expected them to be.
Overall, while the MT-604 can create a dispersed soundstage, the elements within the soundstage staying relatively near my head despite having a soundstage that feels expansive and wide.
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