Pros –

Excellent build quality, Smooth notes delivery, structured and coherent midrange, very spacious soundstage, drives high-impedance cans with aplomb

Cons –

Background noise evident on more sensitive gear, High OI limits many pairings, Not the most detailed sound, struggles with low-impedance loads

Verdict –

Combined with a set of high-impedance cans, it makes for a reasonably priced, truly unique and romantic listening experience that you’d be hard pressed matching with other amp types.


Introduction –
Tube amplifiers are quite an alluring proposition. They simply conjure great imagery, and few can deny the appeal of a warm, syrupy analogue sound. With that said, solid state amplifiers have become rather ubiquitous for good reason, the latest crop of releases especially are so flexible and powerful, it has become difficult to make a case for tube amps in the modern day. For all intents, they are considered obsolete by most. And, for those that find their colouration and distortion appealing, hybrid solutions with solid state output stages are considered to offer the best of both worlds. Why then, has xDuoo released an OTL, all-tube driven amplifier in 2021? OTL tube amplifiers drive headphones in a very different manner to solid state ones. In certain circumstances, their weaknesses can become unique positives. The TA-26 represents xDuoo’s take on the base design that has been assumed by many including Darkvoice. The company has realised this with modern components such as ELNA SILMIC film capacitors and NICHICON filter capacitors that provide tighter tolerances than regular, non-audiophile units. The result is a respectful reimagining of a beloved, time-tested tube amp design.

The TA-26 retails for $319 USD at the time of writing. You can read all about it and treat yourself to one on Apos Audio!

Disclaimer –

I would like to thank John from Apos Audio and the team at xDuoo very much for reaching out and for providing me with the TA-26 for the purpose of review. All words are my own and there is no monetary incentive for a positive review. Despite receiving the amplifier free of charge, I will attempt to be as objective as possible in my evaluation.

Contents –

Unboxing –

The TA-26 has a barebones unboxing experience though I can’t complain as this is a simple device and on the affordable side for what it is. The box is large with thick foam padding that keeps the amp and delicate glass tubes safe during shipping. The amp and tubes are separately packaged with cables in a separate box. xDuoo includes an IEC power cable and 2.5mm to 6.3mm adaptor for headphones. No RCA cables are included, but otherwise, nothing else is required to operate the amp.

Design –

I do feel the images I’ve seen of this device online didn’t do the TA-26 justice; the mismatched face and end plates certainly didn’t prepare me for such a rigid and well put together design. The TA-26 emanates an industrial vibe; a fusion of sound and machinery with its stainless-steel rails and gritty gunmetal housing. It is tall due to the protruding tubes and transformer block and quite long but very slim, which makes it ideal for placing next to your monitor for instance, if not stacking. The first aspect that strikes out of the box is the amplifier’s weight which, alongside its thick all aluminium alloy shell, provides an impression of great quality. The aluminium itself has a more aggressive sand-blasted finish than most, and you can tell xDuoo invested in a few additional CNC passes here due to the smoothness of each edge, cut-out and abundant chamfering. Of course, we can’t forget the gentle glow of its dual pre-amp and buffer tubes that, as on most tube amps, form an alluring centrepiece.

The rails serve little purpose but do contribute handsomely to the aesthetic without being too loud. Speaking of which, undoubtedly, focus is drawn to the saturated red volume knob which also contributes to a somewhat old-school aesthetic. It too shares an aluminium construction with tactile knurling all around. The pot is smooth and was free of noise during testing with a nicely weighted action. There are abundant vents throughout the device which coincides with the company’s recommendation to give the amplifier some surrounding space to allow for heat dissipation. In terms of IO, the TA-26 is a simple device with only single-ended connectivity and pass-through for speakers. This is done via a pair of RCA connectors on the rear and a ¼’ headphone jack on the front. Also on the front is the power switch and a blue status LED and power is delivered by a generic 3-pin IEC cable meaning it is easy to swap this out to adjust the length.

Usability –

Setting up the TA-26 is as a simple affair given its straightforward IO and generally limited feature set. This is a pure headphone amp experience with no superfluous additions. As on any other amplifier, simply connect your DAC to the input via RCA and plug in your headphones. Should you want to run speakers, they can be connected via the pre-amp output adjacent to the input RCA plugs. The preamp section does run through one of the tubes which will provide a warm, sweet tone to your speakers too. Insert the tubes into their respective ports, the connectors are keyed so they can only be installed in their correct orientation.

Once power is connected, you can power on the amplifier, audio gently fades in as the tubes warm up and this process takes about 15 seconds. The same goes for powering off the amplifier. Of note, there are no added noises, pops, etc during either power up or down. In addition, the volume pot introduces no additional noises during adjustment. Another thing to note is that the entire amp does get quite warm to the touch during use so I would definitely adhere to xDuoo’s recommendation to provide ample surrounding space for heat dissipation. The tubes themselves do get warmest but never to the extent that you would burn yourself, even after a few hours of use.

The TA-26 is a dual tube design, the larger being the 6N5P power/buffer tube, the smaller in front being the 6N8P pre-amp tube. Both are swappable, allowing the user to sizably alter the sound characteristics of this amplifier either at additional cost of tubes or if you already have these components. This has always been a hugely appealing aspect of tube amps and, as this is a purely tube-drive amplifier, you will notice large changes with different tubes. These can be purchased easily online.

As an aside, I did note that the power supply isn’t especially well-isolated. While it is impressively clean most of the time, I noted that the TA-26 would pick up hum from the powerlines, and this was exacerbated once the device was warm and in sustained use during longer listening. This means that, if possible, you should give the TA-26 it’s own power point rather than running it from a power board, especially if other audio equipment is connected. Under such conditions, the unit being tested exhibited zero noise.

Next Page: Sound, Pairings Verdict

Tube Amp 101 –

I am far from the leading authority on this subject, but have dabbled in tube amp design in the past – with little success mind you. I do think tube amplifiers are often too quickly discounted based on the high output impedance inherent to the actual vacuum tubes themselves in addition to their high distortion. However, these very same reasons are also the reason they’ve remained relevant. Output impedance can be lowered through the use of an output transformer or hybrid design with tube pre-amp and solid state amp. However, the TA-26 is not such a creation, being a close derivative of the Darkvoice 336SE, both based upon a schematic that is now many decades old. This is an OTL (output transformer-less) tube amp meaning there are no transformers or solid-state components in the audio circuit, the output is solely tube. Due to the nature of tube amplifiers, the metrics I personally use to evaluate amplifiers go out the window.

Distortion is often cited as being a main downside of tube designs. As it sounds, this is defined by sounds in the audio output that were not present in the input signal. Thereby, minimizing distortion has been an ongoing pursuit for solid stage amp makers as this is ideal from a fidelity point of view. However, the nature of tube distortion is often seen as desirable; many enjoy the added warmth and fuzz, the lack of note definition and more gradual attack and decay provide a presentation that arguably better reflects sounds in reality. Of course, often such qualities are captured in the recording and, as a result, further elongation of notes is usually not required.

Another factor that plays into this impression is damping factor. A prime benefit of solid-state amplifiers is the lower output impedance. Damping factor can be determined by dividing the headphone impedance by the amplifier impedance. The higher the damping factor, the more controlled the headphone transducer will be – which will manifest as a more defined and detailed sound. Furthermore, not all headphones have a flat impedance curve. This means the impedance of a driver may be higher or lower at certain frequency ranges. It also means the frequency response of the headphone will change with an output impedance higher than 1/8th of that of the headphone. For this reason alone, tube amplifiers are not the most versatile amplifiers. Simply put, you won’t get the best detail retrieval nor are you guaranteed a sound signature faithful to the headphone manufacture’s design.

Next, we must consider damping/output impedance and power output. If you are interested in passive speakers, you’ll note that speaker amplifiers generally have an impedance limit. The lower the impedance, the more difficult the speaker will be to drive. With headphones this is also the case. The higher the impedance, the more voltage the headphone will require but the less demanding it will be in terms of current demand and vice versa. This explains why tube amplifiers are often cited as being a poor pairing for planar magnetic headphones, often lauded for their ease of drivability (on solid state amps) due to their generally low impedance nature. Conversely, on current-limited tube amplifiers, these headphones will be poorly driven, introducing additional distortion and clipping even at low listening volumes.

Why Everyone Needs a Tube Amp –

So, what we’re left with is an amplifier that struggles to drive low impedance loads, causes frequency response deviation on headphones with a non-linear impedance curve and also has high baseline distortion even before the aforementioned qualities are factored in. So where do tube amps make sense? When these traits are desirable! Specifically, if you have a high impedance dynamic driver headphone and frequency response colouration is desirable. My eye-opening experience was with the HD650. This headphone has a non-linear impedance curve, it peaks at 100 Hz and gradually builds from 1 kHz and up. Stock, this headphone is naturally voiced with a warm tone, but it’s also some veil and its top-end is rather closed in with minimal air.

For these reasons, I’ve never personally been a huge fan of these headphones; that is, on solid state amps. Switching over to a high-impedance OTL tube amp absolutely transforms them. With a 300-ohm impedance, they benefit from huge drive and instantly, they have better bass extension, punch and presence. The top-end opens up and the sound balances out. The soundstage even expands, and separation especially is hugely improved. The results aren’t just subjective but are very measurable. If you haven’t tried the HD6XX headphones on an OTL tube amp yet, this will surely transform your perspective as it did mine.

Conversely, headphones like the Focal Clear that aren’t quite as high impedance at 55ohms and have a flatter impedance curve. You still benefit from huge drive and great dynamics, the Clear also receives a moderate bass boost but minimal impact otherwise from the output impedance. While colouration is usually not a good thing, it also isn’t inherently bad – in specific circumstances, it can be corrective.

Subjective Breakdown & Pairings –

I felt it most useful to judge the subjective sonic performance of the TA-26 by comparing it with a regular solid-state amplifier in a similar price range, the THX789. This is because most users will be familiar with this kind of source and also as a result of a general lack of direct competitors. For instance, xDuoo’s similarly priced amps are hybrid designs, and the Darkvoice amps are highly similar both in terms of design and tube selection – albeit, the TA-26 is using newer components. The Bottlehead Crack is considerably more expensive, especially if you add the Speedball upgrade. You also have to assemble this amp yourself unless you are fortunate to find a used unit from a reputable seller meaning it won’t be a feasible purchase for many. With that said, I was surprised and impressed by the utility of the TA-26. It was far more versatile than I had expected and the THX amplifier wasn’t always the clear-cut winner. While I would generally advise going the solid-state route if you can only budget for one amp in your setup, there are some cases where you can make a case for both and specific circumstances where the TA-26 can even be considered the logical winner.

Sennheiser HD650: A 300-ohm dynamic driver known for its synergy with high-impedance tube amps. The HD650 represents a best-case scenario for an OTL tube amp. If ever the term “opens up” were pertinent, it would be comparing the HD650 on a SS vs OTL tube setup. Immediately, the HD650 assumes a higher contrast presentation with greater separation and clarity in addition to a more engaging character. The TA-26 was barely breaking a sweat, requiring minimal volume to drive the HD650 to ear splitting volume.

While my THX789 provides a hint more sub-bass depth and has an advantage regarding driver control and note definition, the TA-26 is warmer, considerably punchier and more textured. The midrange is immediately less veiled, as the warmth is counterbalanced by a top-end lift. In turn, the sound becomes more articulate, separated and simply put, romantic. Notes are presented in a smoother manner which gives the overall presentation a less focused but more free-flowing character. Treble has more energy and zing with greater air and sparkle. Again, the THX789 has a bit more bite to its leading edge, but the tonality is just that much better on the TA-26. The soundstage too benefits from marginally more width and a noticeable bump in depth. As far as musical enjoyment is concerned, there is little competition between the two.

Audeze LCD-X 2020: An efficient planar design, the LCD-X also has a flat impedance curve, thereby, acting as a good indication of how most planars will respond from the TA-26. Immediately, the LCD-X requires a lot more power from the TA-26 than the HD650 despite both its impedance and sensitivity suggesting it to be more efficient – do note, current demands will therefore be higher.

The THX789 immediately had a deeper extending and hard-hitting sub-bass. The TA-26 had a softer slam and less visceral rumble, albeit similar mid-bass fullness. The THX789 thereby has a sizable advantage on dynamics, and imbues a thicker, more weighted note structure. The TA-26 is smoother with less definition and texture. It has a hint more mid-bass warmth and may suit those wanting a less aggressive presentation. The TA-26 follows up with a euphonic and intriguing midrange performance. It pushes the midrange slightly forward and provides a smooth, structured voicing. Once again, not only the smoothness of its voicing, but also its note presentation is a great asset for its musical performance.

Though less articulate, defined and delivering lower resolution, I would struggle to say the TA-26 is less enjoyable to listen to here as it does have better layering and greater coherence. The top-end is slightly laid-back with a progressive roll-off. The THX789 delivers noticeably more bite and fine detail retrieval in the foreground alongside greater air and sparkle above. The TA-26 interestingly has a slightly grainier lower-treble that provides the impression of texture at the cost of raw resolution. While imaging isn’t as sharp on the TA-26, the two are impressively well-matched when it comes to soundstage expansion, the TA-26 having more intimate vocals.

Soft Ears RS10: A highly resolving IEM with a higher impedance and flat impedance curve. The RS10 is also incredibly linear in its tuning, therefore, providing a clear reflection of the raw tonality of the source. The RS10 required quite a bit of juice and picked up a clear noise floor despite the Soft Ears flagship being on the less sensitive side. In turn, I would not suggest the TA-26 is suitable for many, if any BA monitors as they tend to be more sensitive.

The TA-26 provides a softer, more diffuse sub-bass, the THX789 being more physical, tighter and cleaner. Bass note definition is considerably higher with the TA-26 being smoother and slightly fuller in the mid-bass but altogether less resolving and separated. The midrange is as above, quite impressive if not the most resolving. Resolution is reduced but the tonality is enjoyable. The TA-26 introduces a light warmth and slightly increased vocal size. It sounds powerful, full-bodied and coherent.

At the same time, the TA-26 is reasonably articulate with a nice layering. The THX789 is quite a bit cleaner, the background has better separation and blackness. In general, notes are more resolved. The TA-26 has a crisp lower-treble but less bite than the THX789, lacking the same fine detail retrieval in turn. However, some may enjoy the slightly crunchier sound of the TA-26 and its lack of edge and sharpness. The TA-26 lacks the same extension, a recurring theme. However, soundstage once again isn’t affected too deeply, matching, often outdoing the THX789 in terms of width especially.

Final A8000: One of the most resolving, if not the most resolving single-DD earphones on the market. The A8000 isn’t as sensitive as BA or most hybrid monitors and didn’t pick up much background noise on the TA-26. The Final’s unforgiving tuning and emphasis on technical ability suggest it may be a great match for a tube amp.

Sub-bass extension matches the THX789, while slam isn’t as thick and assertive, it is deep-reaching and tight. The TA-26 provides a slightly more mid-bass biased presentation, assuming a more organic tonality over the dynamic and clean THX789. The A8000 benefits from this, sounding more natural and even-handed. It sounds controlled and very impressively textured from the TA-26, to the extent that I often preferred this pairing over my solid state amps. The midrange sounds more natural on the TA-26 as well. Where the A8000 sounds a bit shouty and over-articulated on SS sources, the smoother, warmer TA-26 really helps to tame sibilance and aid structure. It sounds smoother and more forgiving. You sacrifice the gorgeous resolution and separation of the SS pairing but gain a more pleasing tonality and improved layering and frequency balance in turn.

Treble sounds a bit more linear on the TA-26, notes are smoother and have slightly more body. Attack is almost as defined as a SS source but not quite. The biggest advantage of the THX here was with regards to extension and headroom. It had noticeably more background detail and nuance going on. Surprisingly, the TA-26 pairing with the A8000 provides a much larger soundstage than the THX789. Possibly, this is due to the high-end roll-off providing the impression of a more distant background. Either way, this pairing greatly impressed me, while the technical performance isn’t flattered, the A8000 would no doubt sound magical from a hybrid amp setup and is driven admirably by the TA-26.

Verdict –

I don’t want to make it sound like I’m making a case for tube over solid state amps as this is simply not the case. If you are on a strict budget and want one amp that does it all, solid state is the way to go. Their low noise, output impedance and high current output means they will sound better with the vast majority of gear than a tube amp. Even if you were to enjoy the colouration of a tube, hybrid designs that mostly solve these fallibilities are expensive and the other quirks such as the high OI and low current will not provide a good experience with a lot of gear. However, if you do have high impedance dynamic driver headphones or a specific stack in mind like the HD6XX headphones then, under these specific circumstances, even a moderately priced tube amp like the TA-26 can provide a better experience than even the best measuring solid state amplifiers on the market.

As far as the TA-26 itself goes, this is a superbly constructed piece of technology with a look and feel that matches its old-school sonic character. Though I wish it picked up less AC noise, it is altogether a well-realised and cost-effective introduction to tubes, especially if you do not require the complete modularity of something like the Bottlehead Crack and are looking for something a bit more modern and elegant than the Darkvoice. As previously stated, the unique properties of OTL tube amps means users with the budget to add one to their collection will find some uniquely strong pairings – and the TA-26 is a compelling example of this. Combined with a set of high-impedance cans, it makes for a reasonably priced, truly unique and romantic listening experience that you’d be hard pressed matching with other amp types.

The TA-26 is available from Apos Audio (International) for $319 USD at the time of writing. Please see our affiliate link for the most updated pricing, availability and configurations.