Xduoo MT-604

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.