Product page: https://www.massdrop.com/buy/massdrop-x-eddie-current-zdt-jr
Disclaimer: @CEE TEE provided me with a review sample last month (identical to production unit, not prototype). However, while my country uses 220V mains, because there were no 220V units in stock, I received a 120V unit which I'm currently running it with a step-down transformer. I've never had to use a step-down transformer with my audio gears, so I don't know how much this would impact sound quality. Some say there's no audible difference, but I've heard others say the quality of the transformer does make a difference. At any rate, I intend to return this review sample and get a 220V unit once it becomes available.
Sources: Apple iMac, Schiit Gumby [Gen 5 USB], Sony NW-ZX2
Amps: Schiit Mjolnir 2, Schiit Jotumheim
Headphones: LFF Code-X, HFM HEK V2, Massdrop x Sennheiser HD 6XX (modded), Campfire Andromeda
Pics of my previous gears mentioned in the review below, for the sake of reference:
https://imgur.com/1NWPssQ / https://imgur.com/F5ncQC5 / https://imgur.com/5kMbhEg / https://i.imgur.com/iIqgBPI.jpg / https://imgur.com/3O7TOuP
Unless I'm mistaken, the ZDT Jr. is identical to Laconic NBM but supposedly uses better parts and components, which results in lower noise, better volume pot and channel matching, and more controlled bass according to people who have A/B'ed them side-by-side. I've never heard the NBM so I can't say for sure. While the price of ZDT Jr. is similar to the NBM + shipping and export fees from Russia, it was unobtainium for most of us due to Russia's customs regulation (especially the 120V version) and virtually had to be smuggled outside the country Han Solo-style. Now it's become much more accessible to us all, to introduce high-quality OPT sound for an entry-level price of $399.
In my opinion, of all amp topologies out there, I feel a transformer-couple tube amp using high quality transformers (e.g. EC AF/Studio, DNA Stratus/Stellaris) have the highest potential in achieving audio nirvana when paired with the right headphones. Having owned and heard many of them, I was surprised at what this little amp could do for the money. No, don't expect it to rival $2K+ tube amps like ZDS or ZDT, but it sits comfortably in the league of $1K+ amps in performance. It might as well be the gateway drug for people to get into OPT tube amps without breaking the bank.
The ZDT Jr. unquestionably has the characteristic warmth and bloom of tube amps, but it is by no means overly done to crap like some tube amps do. I would say it is more liquid sounding than, say, Schiit Valhalla 2 or EC Studio, but not as much as EC ZDS. It pairs quite well with my modded HD 6XX, a pairing I prefer to Valhalla 2, Jotunheim, and even Torpedo 3 from my recent memory. Now it may sound counterintuitive since both HD 6XX and ZDT Jr. are warm sounding gears, but the ZDT Jr. surprisingly has very good treble extension which gives the HD 6XX more air and sparkle in the top end.
Schiit Jotunheim, my favorite amp in the current Schiit lineup for the HD 650/6XX, boasts excellent transparency and bass oomph, but its soundstage feels flatter and closed in compared to the ZDT Jr. After A/B'ing I surprisingly found myself leaning toward the ZDT Jr overall, but I could understand some prefering the Jotunheim especially if you're a basshead. For me though, the ZDT Jr. has become my personal favorite amp in the sub-$500 range. Regardless of price, I would take the ZDT Jr. over any Schiit amp (including Vali 2, Valhalla 2, Mjolnir 2) if my headphones were HD 6XX or HD 800.
Speaking of HD800, I currently don't have one but I've owned it long enough and have listened to it on a myriad of amps to know which amps I like or dislike it with. I wish I had borrowed one for the sake of review, but after listening to the amp, I have a strong feeling this would be an exceptional match with the HD800 as well, perhaps more so than the HD 6XX in terms of tonality.
So if HD 6XX and HD 800 are ideal pairings, what could be said about other headphones? I did try driving the Andromeda from the ZDT Jr, but as with my previous experiences, the Andromeda is a no-go with most desktop tube amps due to its low impedance and high sensitivity which results in distortion and a lot of noise. It does best with low noise, low output Z portable devices like AK Jr or Sony NW-ZX2.
What about less efficient planar magnetic headphones? I only had 2 while writing this review, the Code-X and HEK V2, but to my ears, the ZDT Jr. wasn't very good at driving either of them. The soundstage was okay, but it was lacking the tactility and viscerality these headphones are known for, with the bass quantity and quality being the weakest department. This was something expected, reflecting on my previous experience with OPT tube amps like EC Studio or DNA Stratus. The HE-5/Code-X, like HE-6, is one of the more inefficient planars, even the more sensitive HEK V2 gave my Stratus a hard time to make it come out alive. And ZDT Jr. outputs even less power than either Studio or Stratus. I don't listen at ear-piercing levels, but high-power SS or hybrid amps like my Mjolnir 2, or the high-end but discontinued Cavalli amps like LC or LG serve these inefficient HFM headphones better. Or use 2-channel speaker amps if you're feeling adventurous.
Of course, there are more efficient planars with sensitivity ranging from 91-100 dB out there, so I wouldn't completely rule out ZDT Jr. for all planars. Many people use Audezes or Oppo planars (which are more sensitive than HFMs in general) with OPT tube amps made by EC or DNA. Still, the ZDT Jr. is best suited for the high-impedance Sennheisers IMO.
If you are an owner of HD650/6XX and are looking for a decent sub-$500 amp, amps like Valhalla 2, Jotunheim or ZDT Jr. should be seriously considered. The latter two are more versatile in that Jotunheim drives most inefficient planars as well, while the ZDT Jr. would excel with HD800 too.