xDuoo MT-604 Balanced Tube

I do love me some tube amps! It’s been a few years since I’ve had a tube amp but the one I had was handmade by my father when I was a little kid and I had that for a long time till it got lost in a move a while back. I haven’t had the chance to really sit down to hunt for a tube amp. I’ve always liked xDuoo’s tube amp designs and I was quite interested in their TA-26 as it used the same big and little tube that my old tube amp used. I will have that on the way for review but I was asked if I wanted to check out the balanced MT-604 tube amp first and since I thought a fully balanced tube amp seemed interesting I said yes. The xDuoo is an entry level tube amp coming in at $169 and uses four 6J1 tubes in a “hybrid” tube/solid state design.

Thanks to Apos Audio for hooking me up with a loaner unit to review. While I always appreciate stuff being sent in to test and review, It never affects the rating of my review.

The xDuoo MT-604 can be picked up from Apos Audio below
https://apos.audio/products/xduoo-mt604-balanced-tube-headphone-amplifier

Gear used​

S.M.S.L. SU/SH-6, Moondrop Aria, Moondrop Variations, Sennheiser HD560S.

Looks and Feel​

The MT-604 is fairly weighty for a smaller desktop device. It’s shorter and wider with a nice matte gray finish. The tube amp is well ventilated with holes throughout the housing and tubes never get so hot that they will burn one's fingers or arm if they accidentally touch the device when left on for a long time. The bright red metallic volume knobs give a modern look and it reminds me of Big Boss’s arm from the Metal Gear Solid 5 game. I like the looks of the 604 tube amp overall.

Accessories and unboxing​

The box is a simple affair but inside it's well padded since the tube amp was at least shipped to me with the tubes pre installed with fitted foam to keep the tubes in place. Inside we get a basic user manual and the external power brick. For desktop amps this is perfectly fine in terms of included accessories. I would maybe like to see a super cheap set of 3 pin XLR cables though most people will probably be fine buying an external set of braided cables elsewhere.

Power switch and volume knobs​

Power switch is on the back. I never liked power switches on the back and I especially don’t like it on a tube amp either. The tubes never get toasty so I’m not as worried about getting burnt turning this unit off. The left and right volume knobs are a thing that I don’t really like. I have experienced channel imbalance from bad sets of tubes before so this is actually a kinda nice feature but I would really like a way to “lock” both volume pots so I don’t have to fiddle around with them individually. Luckily I just set them to a proper channel balance and use my desktop amp to adjust overall volume.

Sound(overall)​

These final impressions were done off the MT-604 attached to the SMSL SP400 amp via XLR. This will be what the MT-604 sounded like with all the headphones I used. Unfortunately I’m just getting back into tubes so I wasn’t able to tube roll. Things like headphone pairings, tube rolling and other source gear will produce different results and impressions vs what my ears hear on my specific gear.

Well minus the hybrid design and fake tube glow I’ll talk about later, this very much has a tubey sound. It still comes through really clear overall but there is a wider sound and the upper bass to lower mid have a boost that gives it that airy and lush sound that I remember from my older tube amp. I find the subbass is a little on the neutral side so thumps come through well when called for but the rumbles feel held back. The mid bass is where the tubes start to kick in and there is definitely air between there to about the middle of the midrange that sounds airy but sweet. Without using all the “Audiophile” terms, it is good and very different when I use it next to the cheaper sub $200 SMSL solid state stack. The treble is actually decently clear. I could tell this is where the solid state amp side started to show. The treble isn’t very crisp and even though this was fed from my main desktop stack, It did alright. Not bad for a hybrid tube amp at all.

Imaging/Soundstage​

There is a sense of space in the stage and while I normally rely on headphones for soundstage and imaging, I think this a decent exception since the tubes do add a little to the sound. The depth and width were still about average compared to similarly priced amps but it just had those tube vibes that made it feel special. Imaging was fine as I expect from all amps.

Input and output​

We get XLR inputs on the back and a 4.4mm pentaconn balanced input on the front right side. We have a 4.4mm balanced headphone jack on the front left side and a 4 pin XLR on front in the middle.

Personal grips with the xDuoo MT-604​

So the tubes they include don’t really produce any “tube glow” naturally and their solution is to simply add orange LEDs at the base of the vacuum tube sockets to give off a “tube glow”. I think it’s cheap looking BUT I was actually convinced it was a real tube glow at first since I turned it on without looking and walked away to let it “warm up”. It wasn’t until I turned it off that I noticed the tube glow instantly disappeared and I was like “WHAT?”. The LEDs turn on at the same time as the unit so while I would prefer no fake glow, my roommate and fiance thought it was pretty looking and they had zero idea what was going on. I mentioned above my gripe with the separate left and right volume knobs and while it is fine for mismatched tubes, I would prefer a way to sync those knobs if possible. I also would have liked to see a pre out on the back. Even if it was RCA outs. For the price I’ll let the pre outs slide however.

Balanced power output​

The MT-604 will output 2 watts into a 32 ohm load which is pretty good. There isn’t much effort put into the sensitivity of the overall volume so be cautious when using IEMs and easier to drive headphones. This thing will show channel imbalance like most potentiometer pots in the first 30 percent so I recommend starting low and then adjust via the output source(if possible) to avoid the initial channel imbalance and to not blow your eardrums out.

IEM pairing opinions​

Moondrop Aria​

The Aria is amazing for its price and what better way to test the 604 than hooking up a wonderful single DD IEM to it. Well, except there's no single ended option on the MT-604 so I did have to use an aftermarket 4.4mm cable to pair them up. The Aria did have a slightly more lush sound but I felt the upper mids and treble weren’t affected nearly as much. The bass does get a little extra thump and the mids do have a little extra sweetness but the treble is still super energetic. This was one of the more solid state sounding combos when it came to headphones I tried with the MT-604. A good pairing for sure but it didn’t have as much of a noticeable tube sound to my ears. The soundstage was slightly wider and deeper however.

Moondrop Variations​

I did use my expensive(to me) Effect Audio Axiom for testing but I figured most people would probably be using something around or under $500 with the MT-604. The Variations are a warmer IEM in the lows and mids when used on my normal gear. The MT-604 definitely boosts those areas a bit but it's tasteful to my ears. Lows and mids get that hint of tube sound and the treble was a little more relaxed overall. It definitely has a thicker sound with a sweet overall experience. I don’t think anyone who wants a more neutral experience will like this pairing but it still makes for a super smooth combo.

Over ear pairings​

Sennheiser HD560S​

The MT-604 pairs pretty well with the HD560S. I was expecting a super smooth and warm tube experience but I got about halfway to what I was expecting off the MT-604. This was a right in the middle experience from a solid state amp to a full on tube amp with good tubes. I got a warmer overall sound but it was somewhat selective throughout the FR when I was A/B testing between amps. On the MT-604 I got a warm and slightly stronger bass which didn’t result in any better subbass but it did provide a little extra thickness which I did like. The mids are about the same as a solid state amp but the big difference was the treble brightness. Everything came through with about the same detail but a lot of the brightness is toned down just enough that I found the HD560S much more enjoyable overall. I think this is a good pairing and I would have liked to have experienced the HD600 series headphones on this amp with some rolled tubes thrown in.

Overall thoughts​

Just like part of my title says, I get tube vibes from the MT-604. While it’s not a traditional looking tube amp, it still gives a different flavor of sound vs a standard solid state amp. I wish I could have tube rolled to see how it handled different upgraded tubes but I was perfectly fine with the performance for $169. I would say give this a shot if you’re interested in feeding balanced headphones a taste of tube goodness. I look forward to checking out some of xDuoo’s other headphone amps soon. Thanks for reading!

xDuoo MT-604

Sub 200$
Fully balanced tube amplifier
Seperate left and right volume control ( I love this for channel matching )
2 watts power output
Can change sound signature with different tubes
Single XLR output in center
4.4 pentaconn output on left
4.4 pentaconn input on front panel
No single ended output
4 6J1 Tubes ( 2 per left 2 per right channel )

Cons
ONLY BALANCED INPUT ( not a con for me )
No pre amp out
No rca input
Seperate left and right volume control ( depends on your POV )
Stock tubes aren’t very good

Class a amp
Built in mute , no pop sound
Neutral sound signature but bass is full and good extension on low and top end

Specs
• Power supply: DC12V/2A
• Output power: 2000mW @32Ω 1kHz
• Inputs: XLR
• Outputs: XLR, 4.4mm balanced
• Dynamic range: 85dBA @32Ω
• Frequency response: 10Hz - 60,000Hz (±0.5dB)
• Suitable headphone impedance: 16Ω - 600Ω
• Gain: +15dB
• Size: 6.6” x 3.9” x 2.3” (17 x 10 x 6cm)
• Weight: 1.2lbs (0.55kg)
• S/N: 115dB
• THD+N: ≤0.01% (@32Ω, 1kHz)


xDuoo MT-604 Review

disclaimer: The MT-604 was provided for the purpose of this review and the tube rolling article by Xduoo Audio. I have no financial interest in Xduoo or any of its distributors, nor did I receive any compensation beyond the product itself for this review. If you have an interest in purchasing any of the Xduoo amplifiers I have recently written about, I suggest you check out Xduoo’s website directly as they now offer direct sales. You can also follow them on Facebook for more information.

Unboxing / Packaging:

The packaging of the MT-604 is the same style as the 601 and 602 packages with a very western retail look to the box. The front has the line drawing of the amp while the reverse has company info and some suggested uses but does not have even basic specs. Lifting the top of the box reveals the manual and then a couple of blocks of heavy foam. The top most block serves to protect the pre-installed tubes, while the lower block protects the 604 itself. Beneath the unit and foam protection is a box containing the power supply and power supply cable. Audio cables are not provided so you will need either a 4.4mm pentaconn interconnect cable or a pair of XLR inputs as the 604 does not have RCA inputs.





Build/Fit:

The case is gray anodized aluminum with removable end plates and rear plate in very much the same style as the 601 and 602. To remove the internals for cleaning or repair, only the rear plate needs to be removed as the internals no not attach to either side plate. The top of the case has two X logos with a pair of tubes centered in each and venting to the front and rear of the X. Additional venting is provided by the side plates that also sport the logo pattern. The unit has four 6J1 tubes that protrude roughly ½ an inch. above the top surface.

Inputs are a 4.4mm pentaconn jack on the right front and a pair of XLR jacks on the rear. Outputs are a single XLR on the center front and a 4.4mm pentaconn to the left of it. Unlike the 602, no pre-amp outputs are provided and the 604 has no single ended input or output options.

This is purely an amplifier so no usb, optical, or coax connectors are needed and the only other features not covered yet are a large red volume knob centered immediately in front of the tubes and a 12V DC input on the rear of the unit. Overall the unit stands a little over 1.5 inches tall (excluding the tube) , about 7 inches long and a little over 3 inches deep. The unit weighs slightly over a lbs as well so is quite compact.

This is where I got the first surprise, no two of the tubes were the same height when I unboxed the amp. I figured the amp had just been jostled in transit and needed the tubes reseated but to my surprise all four tubes were fully seated in the sockets and are different heights. I checked the dates on the tubes and all have the same date code so I can only guess either the codes are fake and applied sometime after the fact or the QC was non-existent that day at the factory.





Internals:

The MT-604 uses four 6Ak5 tubes as the pre-amp followed by eight D882M transistors that are used as a buffer stage. The D882M transistors are mounted to the bottom of the board and use the case as the heatsink rather than needing a dedicated sink like the 601 uses. Those interested in the D882M can find the datasheet here. Capacitors are either Elna Silmic or RubyCon branded throughout. The 604 also lists a built in mute circuit in the headphone output to prevent pops at power on. The 604 lists gain as +15dB exactly the same as the 602, but output power is much larger at 2000mW into a 32Ω load compared to the 1300mW of the 602. SNR is listed as 115dB with a THD+N of <.01 at 1kHz with a 32Ω load. The 604 lists as suitable for use with headphones between 16 and 600Ω impedance and indeed was capable of powering just about anything thrown at it. The 6AK5 family is quite large and provides lots of options for tube rolling as it has been in production worldwide since the 1940s nearly continuously. For more on that, see Tube Rolling the MT-604.




Sound:

The MT-604 is a hybrid amp so can have some warmth added by tubes but retains some of the same character regardless based on its output stage. If you’d rather avoid such coloration the Xa-10 or XD-05 balanced provide all solid-state options but both are considerably more costly than the 604. The 6J1 tube used offers a fairly wide range of options and the amount of warmth, extension, and detail can be impacted considerably. This family also includes tubes labeled 5654, 6J1 (Chinese and Russian designation), CV4010 (British), EF95 (Euro), 403a/b (Western Electric) as well as 6AK5W and 5654W designations. I used the Khadas Tone2Pro with a pair of balanced RCA to XLR cables to feed the MT-604, and worked my through no less than 12 different sets of tubes. Tube rolling the MT-604 proved a more complicated exercise as with each new tube, the volume had to be re-tested and matched before listening. While the MT-604 does have some tube characteristics, it is ultimately a fairly clean and detailed amp without the syrupy flavor of some tubes. I think this is due in part to the use of the 5654 tube which has less warmth than some other tubes and partially to the output buffer stage design. The end result is a detailed signature with a hint of warmth depending on the tube used. Overall, I found the 604 to have good output power as it was able to run the HD800 without issue. The He6 was not quite driven to full capability and that duty is better handed off to the Ta-30. For more on the sound with various tubes, see my tube rolling article.

Thoughts / Conclusion:

The MT series are all budget models that make trade offs in order to get the desired result. The 601 is a pre-amp with an earphone output and has very limited output power for using full sized headphones. The 602 has more power but may be a bit over-powered for sensitive in-ears and best reserved for full sized headphones, and the 604 has no pre-amp outputs, no rca inputs, has separate volume controls per channel and is balanced only. I include the separate volume controls among the cost savings here as bringing the two independent signals into a single volume pot would require more hardware to accomplish and no doubt increase costs. All models in the MT line share the issue of lacking input switching so be careful not to have multiple sources connected. Effectively, Xduoo has built a series of single purpose budget amps and with price tags of all three being sub $200 USD and the 601 and 602 hovering right around the $100 USD mark, we should expect no different. The 604 offers a fully balanced tube amplifier with independent volume controls that allow the end user the ability to offset small channel imbalances elsewhere in the chain for a cost of less than half of the next least expensive balanced tube amp I was able to find on the market. It has very usable power with nearly 2 watts of output, and the signature can be tweaked with readily available and inexpensive tubes. Will it displace the Eddie Current Balancing Act in totl setups? No, but for the asking price you can buy 15 of the 604s and still have less than the cost of a used model. That gives newcomers to tubes a very cost effective way to stick a toe in the water and see what this tube stuff is all about. Kudos to Xduoo for making tube audio more affordable and accessible. Everyone should own a good tube headphone amp and the MT-604 makes a good place to start.

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