s many regular readers of "High Fidelity" magazine already know I am a big fan of tube amplifiers, with special emphasis on the SET (single-ended triode) flavor, therefore also on high-efficiency loudspeakers. These are not, of course, designs without flaws, but such products simply don't exist in audio world. Our hobby, passion, or whatever you'd like to call it, is in fact about finding appropriate combination of (mostly) sonic features that suits one's preferences best. My own just push me toward above mentioned solutions - if I could afford it I would be absolutely happy for the rest of my life with remarkable Kondo Souga paired with the best high-efficiency loudspeaker I could find. Unfortunately I can't afford it. Maybe someday...

Despite my clear personal preferences, somehow thus far my contacts with products of one of the Polish brands specialized in high-efficiency speakers were strangely limited. In fact the only time I spend some time with one of their speakers was maybe a dozen or so years ago in Warsaw, in Audio Forte's showroom. I'm not even sure exactly which model was it that I auditioned back then, but certainly one featuring a single wideband driver. I remember, however, that I took with me the amplifier I owned at the time, the Amplifon WT 30II, and both me and my friend who was accompanying me, were hugely impressed after listening to some acoustic jazz. These were not versatile loudspeakers though, as they didn't play equally well rock or symphonic music. And yet, with acoustic music, classic string quartets and so on they delivered absolutely beautiful, natural, smooth, palpable and immersive performance. As you might have already figured it out the brand in my story is Acuhorn. You can find several reviews of their products on “High Fidelity” pages but all of them were prepared by Wojtek.

After the last of them, the one of Acuhorn 15 high-efficiency loudspeakers, a 2-way design with a big woofer and ribbon tweeter, that Wojtek rated very positively, I decided it was time to contact manufacturer to ask for a review sample for myself. Soon after that the loaned pair arrived, and the test will soon appear in HiFiKnights.com (not to duplicate texts with Wojtek here). In fact the speakers came with a bonus – the stereo tube amplifier, S2. Even though it is a classic “niche” product it is also a very interesting one.

Why do I call it a “niche” product and why is it interesting? Well, because this is an OTL (Output Transformer Less) design. The output transformers are key components of almost every tube amplifier there is and they largely determine the quality of its performance. Why are they so important? Well, because the transformer itself is a source of distortion. Depending on its class/quality a distortion level differs (the better transformer the lower distortion level). The best ones introduce only minimum level of distortion thus reproducing high quality sound. The problem is that making them is difficult and expensive, hence their price is very high, not to mention how much these best one weight (yes, much of the weight of tube amps comes from output transformers).

Probably that's why decades ago following the principle that states that an audio device should be as simple as possible, someone came up with an idea of getting rid from tube amplifiers the very elements that always introduced some distortion. In theory sound without output transformers should be clearer, more transparent, more precise, and it also meant reducing production cost – lots of advantages, wouldn't you say? The first OTL amplifier was introduced to the market in the 1950s.

Despite clear sonic advantages this type of design did not conquer the market. Why? Because such amplifiers are not too versatile - they can not work with loudspeakers with impedance dropping very low. Today most speakers behave this way – drops to 2 and even 1 Ω are nothing unusual, which is something an OTL can not deal with. Secondly, such devices produced enormous amounts of heat – they worked in Class A, and to achieve “decent” power output they featured several, sometimes even dozens of tubes per channel. The gigantic amount of heat shortened lifespan of tubes significantly and sometimes even cause spectacular failures.

Of course, over the years some problems were eliminated and some excellent amplifiers were made. You might know that the Canadian Tenor Audio used to specialize in this type of amps. Today one can still find some OTL amps on the market and one of the brands that still make them is German Einstein Audio (see Einstein The Silver Bullet OTL), but they are “niche” products - usually deliver a limited output, and there are fewer OTL-friendly loudspeakers on the market, than 50-60 years ago (when some with impedance of several hundred ohm were even made).

My closer contacts with such designs (by “closer” I mean a chance to listen to them in my own system) was limited so far (if my memory serves) probably to just one headphone amplifier, the Eternal Arts HLP. This amplifier, also having some limitations (it doesn't work too well with so popular today very low impedance headphones), made it to a short list of my personal favorites, along with such fantastic devices as Ayon HA-3, Bakoon HPA-21, Questyle CMA800R or Trilogy 933. It should not surprise you though, that I gladly accepted a chance listen to the Acuhorn S2, and that my expectations were high.

On one hand Acuhorn S2 Stereo looks quite inconspicuous but on the other, at least for tube aficionado, curiously. Inside surprisingly small and light carton box I found a neat, black, flat (just 2.5 cm high) device without any transformer housings protruding from its cover. The cover, along its left edge, features four tube sockets. There is nothing, no manipulators, no LEDs on the front nor sidewalls. On the rear panel I found (from the left) a small knob (volume control), one RCA input, two pairs of speaker outputs, IEC socket and a backlit red on/off switch.

The casing rests on four tiny, rubber feet – these are so small that for a moment I wondered if there actually were any. After taking a look underside of the device I found these feet plus a fan that forces better air circulation, to better cool the whole circuit including tubes. To complete the picture of this device, I shall add that in these four sockets one installs four highly acclaimed 6C33C triodes, which, as declared by the manufacturer, deliver output of... 3 watts per channel. This clearly shows that this amplifier is intended primarily for pairing with Acuhorn's own high-efficiency loudspeakers, or others but with similar characteristics. This forced me to use Acuhor 15 to test this amplifier although to better realize its sonic features I switched it a few times with two other, fantastic 300B SET amplifiers - my own modified Art Audio Symphony II and Air Tight ATM-300 Anniversary.

After listening to several tracks one thing became clear to me so maybe I should just start with it so that you immediately know what to expect. Having compared S2 with two other bit more powerful amps (both delivering circa 8W per channel), all paired with the same high-efficiency speakers it became clear to me that the amplifier under review is intended to play primarily acoustic music. I believe that the combination with, for example, Nero 125, which is an Acuhorn's single-driver loudspeaker could be an optimal solution for fans of string quartets, vocals and acoustic jazz, especially the older recordings, such as Miles Davis, Ben Webster and so on would most surely sound amazingly well. Later I found out that a lot of contemporary jazz, for example, ECM releases, sounded equally great played with Acuhorn 15, so with Nero these would work too, I guess.

With this type of music S2 was doing a great job with Acuhorn 15, though to achieve a satisfactory volume level I had to push it up almost to the end of the 21-step scale (how far you have to go depends also on the signal level from your source). Fans of rock, metal, Mahler, or other monumental music should probably look for another amplifier (most likely a powerful solid-state considering music preference). Rock music lacked some dynamics and power, and large classical music didn't have proper momentum despite setting the volume to maximum.

Several elements of this presentation were still quite impressive - with classical music I enjoyed exceptional clarity and transparency of a detailed sound, also timbre of instruments seemed very natural. But such a huge, complex “instrument” as philharmonic orchestra must exercise proper dynamics in both macro and micro scale and without it, without momentum and high-energy it simply doesn't sound quite right, even during quiet listening. When it came to rock the already mentioned advantage of S2, that is, high clarity and transparency of the sound, not necessarily worked to the advantage of this type of recordings. These are rarely well recorded and Acuhorn let me hear that very well. So when I realized that I finally decided to focus on music that this device, in my opinion, was made for - acoustic recordings, vocals and so on. Although, as it turned out bit later, S2 still had a few surprises waiting for me.

Once I started listening to Miles Davis, Coltrane, Ben Webster, and some (slightly) more recent productions such as, Keith Jarrett's albums, I simply couldn't stop. S2 delivered a performance somewhat different from probably all other amplifiers familiar to me (tube and solid-state). It showed performers from a certain perspective. Most tube amplifiers bring the presentation at least a bit closer to the listener, sometimes increasing phantom images too, thus creating an impression of a close, even intimate contact with musicians/vocalists. Acuhorn presented all the events on the stage either on or behind the line between speakers but never closer to me.

And yet, I had no problem with a complete, unconditional immersion in the music, as the presentation was not any less realistic. I quickly noticed that other, than usual suspects in case of SET amplifiers, elements of the presentation were actually responsible for that.

Firstly, the soundstage - S2 renders quite large one, but not huge. It is wider, if the recording allows, than the distance between speakers, and also front to back distance is rather impressive – that's a common feature with good SETs, and even some push-pull designs (I mean those really good ones, such as, for example, Audio Tekne's).

Imaging is also very convincing, although achieved by rendering full, weighty bodies of individual instruments rather than particularly precise contouring of each of them. The size of individual instruments is accurate – they are not larger than they should be, which happens when other amplifiers push the sound closer to the listener's position. The amplifier conveys ambiance of recordings in a very realistic way. It takes proper care of all phases of the sound – there is a fast attack followed by correct sustain and finally a long decay. The said initial phase of the sound is not just fast, but rather immediate, and yet attack is not hard on the edges nor dry.

Many SETs are not capable of reproducing such immediacy of the attack, although exceptions to this rule occur, but only among far more expensive amplifiers than S2. Here the presentation was quick, precise, vigorous, but never aggressive (the latter features are another common element with SET amplifiers). That is why the drums, even if not always accurately powerful, sounded really good, convincing. It was in part a result of a good differentiation of the bass primarily in terms of tone, though, and the dynamic range was not bad either. S2 will not provide you with grand dynamic performance, but at the micro level it is doing a good job. When it came to the album with the traditional Chinese (I think) music played with bamboo instruments it sounded very natural. And although I have no idea how these particular instruments actually sound like this presentation made it easy for me to distinguish them (despite the fact they were all made of bamboo!) and inspired me to imagine how they could look like :).

Just as good SET amplifiers, the S2 rendered a delicate, but at the same open and tuneful treble. For me, it is very important that I can “hear” a lot of air around instruments and the Polish amplifier delivered plenty. The metal cymbals were presented in a slightly different way than I'm used to. There was namely less accent on the sticks hitting cymbals and more on the decay phase which made them sound a bit softer. It was different to the sound delivered by my reference system but I didn't perceive it as a “worse” version. It felt more like a slightly different style of drumming. After all, among drummers there are those who hit very hard, but also those who operate sticks in a more gentle manner – the S2 in most cases interpreted music in the same way as the latter type of drummers. And the longer that I listened to it, the more I liked this interpretation – one should remember that whatever audio system one uses it always offers some version of the recorded material, which one might like or not. This was not the best option for rock music, but it worked very well for most jazz recordings.

Yet another, beautiful experience S2 provided me with was the way it rendered female and male vocals, both jazz and classical ones. And as before I spent many hours listening to good old jazz, now with an equally great pleasure I listened to numerous vocal recordings. These were rendered in a particularly natural way, with amazing accuracy of timbre and texture, with abundance of smallest details giving away secrets of singing techniques, and even small imperfections of some recordings that made them sound even more natural. The already mentioned good differentiation provided me with enough information so that I was able to tell different periods when individual recordings on “The 50 greatest tracks” album performed by Luciano Pavarotti were made.

S2 wasn't particularly peaky in terms of quality of the recordings, because it played even the oldest, a little noisy ones as beautifully as the best contemporary ones. Every time it was a voice that focused virtually all my attention, and each time it sounded particularly rich, clean, expressive and well textured allowing me to enjoy maestro's artistry and rare talent. Like all high-end tube amplifiers, Acuhorn S2 beautifully conveys the emotional side of music. Addictive, engaging, it turns listening into something more, into a wonderful musical experience. If you are looking for a device that will allow you experience music, Acuhorn S2 is what you're looking for.

I've mentioned before that S2 managed to surprise me in the course of this test. Well, one evening I sat with a laptop on my lap working while Roon chose randomly track after track and played them via S2. For some reason this functionality is called a “radio” but Roon actually chooses tracks from user's library, different artists, genres, formats, qualities and so on. In theory the sequence is random (it seems to me that the tracks are selected based on latest playback record, not really randomly). For some reason this time Roon focused mostly on the Al di Meola albums, with both acoustic and (mostly) electric music.

And it was an electric track that distracted me from my work. Possibly some credit be given to a late hour when the not so efficient Acuhorn amplifier could finally deliver proper volume of the sound. Other thing is that the late night listening sessions fall into a different category. The environment around gets really quiet, some claim that even the power grid provides higher quality current at this time of a day (night) and surely man's perception changes, at least mine does. And it was such a late hour when Al's electric guitar and the accompanying band including an electric bass, drums and keyboard surprised me with their tasty, immersive, electrifying performance.

Because even though this was an electric music, but rather of a lyrical type it suited the S2 perfectly. It was easy to appreciate how resolving this amp can be, its very good differentiation, micro-dynamics and detail retrieval. The guitar was maybe lacking a touch of aggressiveness that it should have had, but its sound was properly "fleshy", rich and saturated. The bass extension was slightly limited but its mid and upper range, where in fact both the electric bass and drums mostly operated, delivered a lot of energy, and bass was lively, fast and tight – highly enjoyable. It didn't take long before my feet started tapping the rhythm.

Soon I didn't fell like letting Roon to select music for me anymore so I just added more Al di Meola, John McLaughlin, Path Metheny and Lee Ritenour albums to the playlist. Yes, that's not hard rock, nor even rock music so my statement claiming that S2 is not really made for fans of hard music stands. But, as it turned out, under appropriate conditions, Acuhorn is able to plays some electric music in a convincing, even immersive way.

Summary

The Acuhorn S2 amplifier is a product seemingly intended for a specific group of music lovers. It requires pairing with loudspeakers easy enough to drive and a source that provides at least true 2V signal. Even when these conditions are met you should be aware that most of the time you will operate in the upper range of volume scale but it won't have any negative effects on sound quality – I noticed no distortion or compression even at maximum volume level. S2 beautifully plays acoustic music and vocals, that sound very natural, but electric music sounds also quite good on it as long as you don't play it too loud. Well, you probably won't be fully satisfied playing hard rock, but electric jazz and blues should offer you a lot of fun.

This is an extremely musical device, delivering rather warm, but at the same time clear, transparent and resolving performance. It differentiates recordings well in terms of both, tone and dynamics, although with the latter it deals better on a micro scale. Treble is delicate, airy which reminded me of SET amplifier performance. The bass might not be so well extended but apart from its lowest part the most of this range is fast, tight, vigorous – which allows listener to enjoy, for example, drums. The biggest strength of this amplifier is obviously the midrange - hence the acoustic music is what sound best, most natural on S2.

The presentation is also very coherent and smooth. Acuhorn S2 is, in my opinion, an amplifier for people who love to spend their evenings sitting in a comfortable chair, sipping their favorite poison, enjoying jazz and vocal music played at reasonable volume levels. If you are a “party type” you should look for something else. But as long as music is your way of relaxing S2 might be what you're looking for.

From the practical side of things - not only this amplifier will cost you (relatively) little (compared with good SETs), but also replacing the 6C33C triodes will be much, much cheaper than doing the same with a quartet (or even just pair) of 300B, 2A3, or 45s. So without spending a fortune on an amplifier you might still enjoy a beautiful, lush tube sound, and when it comes to that, just replace tubes without stretching your home budget and keep playing music for many more years.

Wait! I almost forgot. Once you buy this amplifier you'll become a proud owner of a device Made in Poland!

Acuhorn S2 Stereo is a stereo (available also as monaural version), Class A tube power amplifier without output transformers (OTL) with a volume control. The OTL principle made it possible to enclose the whole circuit in a small (320 x 460 x 26 mm) and lightweight (3.5 kg) black metal housing.

The amplifier's appearance is quite original – it is a very low profile, flat aluminum box devoid of any elements on the front. On the upper surface, along the left side there are four tube sockets hosting well-known Russian triodes - 6C33C. The device rests on four tiny, rubber feet.

They are just big enough for the fan mounted in the bottom plate of the device to be able to suck the air from underneath the amplifier to cool the whole circuit that gets really hot under load. The fan's operation is not completely silent, but the level of noise produced by it is negligible while listening to the music. One can hear it only in the moments of silence between tracks. On the narrow rear panel manufacturer managed to place a small, 21-step volume knob, a pair of rhodium-plated RCA jacks, two pairs of robust speaker connectors, IEC socket and a backlit red on/off switch.

Specification (according to manufacturer):

input voltage mains autoranging 90-264Vac
maximum input power 460W
OTL tube amplifier type
single ended SE 6C33C triode in class A
stepped attenuator volume control
grid transformer drive
input sensitivity 2V
output power tube 2x3W (mono mode 6W)
internal cable occ copper
terminal rhodium
weight 3.5kg
dimensions W 320 x D 460 x H 26mm