Passive SpeakersSpeakers
Overall Score: 87/100

Buchardt Audio S400 Review – Big, Bold, Ballsy

My Video Review:

Ah, it’s so good being back testing and listening to a nice pair of loudspeakers. I quite enjoy the active version of KEF LS50W since it is so user friendly setting up their internal streamer, linking a Tidal Hi-Fi account and then just using a smartphone as a remote for them. *WAF factor is the best I encountered thus far; it was one of the biggest reasons I ended up buying them in the first place. Obviously, a lot of compromises needed to be made to have them small, cute and easy to use. I like a lot of things about them, but wish they would do better in terms of low end, in terms of slam, I wish they had a wider and deeper stage, plus a better instrument placement in the room would be great too. You really can’t have them all in a small cabinet? Or can you?

I always wondered if at the same cabinet size, I could improve my listening experience and mitigate some of those drawbacks at the level of some mid-sized stand floor speakers.

I stumbled upon the Buchardt Audio S400 loudspeakers, that could fulfill my low-end needs and maybe increase the soundstage in my room. The more I researched about them, the more questions would appear in front of me. Why putting the tweeter underneath the woofer? Why putting a passive radiator on the back and why making the cabinet leaned back are some of the questions I always wondered about. When opportunity struck to have a listen to a pair of S400 for few weeks in the comfort of my home, I didn’t hesitate for a second and here we are.

Unboxing Experience

S400 came in a large box filled with hard foam for some extra protection measures. In tried shaking the box in the air, but the way they are wrapped in that foam will basically absorb any damaged might occur to them. The centerpiece is of course the speakers themselves; we’ve got the white version to match the white speaker stands I’m using with the KEFs. In the box you’ll also find an owner’s manual and a smaller cardboard box where four magnetic grills can be spotted and that is basically it.

Build Quality & Looks

I have a soft spot for white color and I think white is the nicest color of S400 that contrasts really well with the blacked-out tweeter and woofer. When looking from afar both the tweeter and woofer assembly are looking identical and I quite enjoy the symmetrical look and the exact size of both black circles. If you put them sideways you can clearly see that the cabinet is leaning slightly backwards to create a unique look for S400. It was done on purpose and its part of the Constant Directivity Control waveguide that Buchardt Audio created, about which I’ll write deeply about in a minute.

On the back you can spot a massive passive radiator, that in fact is much bigger that the woofer itself, that is something very different compared to traditional speakers. On the other hand, when closely inspecting the newest high-performance Bluetooth speakers from the likes of Vifa, Ultimate Ears and even JBL, you can clearly see that huge passive radiators are used in all of those that will push bigger amounts of air, that will make them sound much bigger than the cabinet size and most importantly will help a lot in delivering a truer low-end, a better control and sustain of that low end. S400 looks cool and very different compared to traditional speakers, it was done on purpose by borrowing a lot of technology from traditional speakers, from wireless speakers and by adding some unique ideas and developments by the Buchardt Audio team.

Under that passive radiator you will find some high-quality speaker terminals, you can see the serial number of your particular unit and that is basically it. Measuring just 365 mm by 180 mm, S400 is not a big bookshelf speaker, more like a small to medium size speaker that could be easily integrated into small spaces, they worked well on a PC desk too, just make sure to have at least half a meter behind them for a clearer bottom end.

In terms of looks and overall design, I find them modern and futuristic looking, they remind me of beautiful modern furniture, but maybe that is just me.

Technology Inside S400

S400 is using a relatively medium sized woofer, it is a 6” (152 mm) in diagonal. What’s interesting about it? It’s an aluminum cone with break-up optimization developed by Ulrik Schmidt, if he doesn’t ring a bell – it’s the man behind the ScanSpeak “Revelator” series. It seems that Buchardt tried numerous paper drivers but in the end settled on the aluminum one that Ulrik was able to successfully dampen to the point of eliminating all its drawbacks. In the end, a great woofer was born that exhibits great control and power handling.

Under the woofer, sits a relatively small tweeter, 0.74” (19 mm) in size, again it’s a custom one, made out of soft fabric textile with CDC aluminum waveguide. CDC stands for Constant Directivity Control that is actually that big circle around the tweeter itself. All that part is made out of aluminum and if you look closely it was designed to match the contour and shape of the woofer down to 1 kHz, making the tweeter and woofer work as a single driver at the crossover point of 2 kHz!

With the large passive radiator on the back, Buchardt team was able to reduce the cabinet size of S400 by 30%, that is one of the reasons the best Bluetooth speakers are quite small in size and big when it comes to sound. Thanks to that radiator, S400 are more forgiving when it comes to placement near the wall, it also means it will not suffer by the artifacts of the ported designs and most importantly will awake some of the bass that only bigger stand floor speakers are capable of.

As for the crossover, Burchard team selected some of the best components that will stand the test of time and will deliver a great sound performance. You’ll find inside metallized caps instead to electrolytic caps, solid copper coils instead of iron and the list goes on.

Now, as a whole, its lean back design, that CDC waveguide, the shape of both woofer and tweeter are all interconnected to create a controlled directivity of the waveguides. Most of the traditional speakers, even my LS50W were created to be listened in a “sweet-spot” or as others are calling it On-Axis listening. Once you move Off-Axis a lot of its acoustic performance goes to the garbage bin. Buchardt Audio wanted to be free on this nonsensical and archaic approach to music listening. Do you want to always stay in the middle of the couch or do you want to sit at whatever angle or position? Burchard team were able to create S400 that will sound as good Off-Axis and they do On-Axis. I strongly recommend reading the detailed description of S400 and how did they do it. I can vouch for everything that is said about the CDC and about off-axis listening. With S400, sounds are coming from all over the place, not only from the speakers themselves. Stage is mindbogglingly huge, so it’s the instrument placement around the listener. With few tracks you even might get an impression that you are listening to a multi-speaker setup.

Sound Performance

I. System Matching

To fully understand the S400 and fight back some of its cons, it was mandatory testing them with few particular sources and with a bunch of amplifiers for the best possible system matching. When it comes to digital to analog converters, it seems like a fetish of mine having at least 4 of them or even more. From tube based xDuoo TA-30, to class A output stages like Flux Lab Acoustics FCN-10, Burson Conductor 3 Reference, Audio-GD D38, to musical designs like the R-2R Denafrips Venus and to absolute linearity and ultimate detail like Matrix Audio Element X, everything was tested on the S400. In the end I liked the fastest one and the one that would increase the speed and slam of the driver, it was the Matrix Audio Element X, I will explain later on about my decision.

As for power, I used four amplifiers, two integrated and two power amps. First on the test bench was the small and affordable SMSL DA-8S, providing 80 Watts in 4 Ohms and working in class-D, I thought it should work decent enough with the S400. Although it worked pretty nice with the entry level Wharfedale Diamond 11.2, with Buchardt S400, I’ve felt a serious decrease in pace, rhythm and timing and a critical hit in the slam department that was just nowhere to be found. A bad first impression for the speakers, it almost makes me re-write the review I posted for the DA-8S. To be totally sure that I am not taken back by low burn-in hours on S400, I left them powered on for few weeks, my son did the honors for me watching cartoons all day long.

Two weeks later, I came back and listened to them again, this time on the Burson Audio Bang power amp, it is slightly more expensive but also less powerful. It offers exactly half the power of SMSL DA-8S, at 40 Watts into 4 Ohms, I thought this could be a terrible idea. Bang offers about 10 Watts of power into class-A and then moves to AB for an organic, musical and transparent sound. I was controlling it using the amazing preamp inside the Element X. At half the power, Bang sounded mighty impressive. Finally, some slam came back, speed improved, S400 started showing signs of great potential, everything was improved, except for detail that was slightly better on the DA-8S. All in all, at half the power, with this particular speaker, Burson Bang sounded pretty good. If I would not be limited by the power output of the Bang, I would call this a really good pairing.

Moving on to a Hegel H190 integrated, S400 were changed to unrecognizable, speed, thump and slam in the chest improved the most. It was like the driver inside them was exchanged with a much faster and with a much bigger one. S400 were slamming harder, sounded much bigger, I’ve heard a better control over the driver and for the first time sub-bass made its appearance while listening to the newest Infected Mushroom album – More Than Just a Name (2020).

I was 100% sure that Matrix Element X will counterbalance some of its issues, it remained only to find the perfect amp that could do the same to them. The last combo I tried was Element X + Benchmark HPA4 working as a preamp and KECES S125 as the power amp. S125 is using an oversized 1000VA transformer, offering 225 Watts in 4 Ohms, it’s more than enough driving the S400 to really loud levels. The sound was very close to that of Hegel H190, but I’ve felt sharper and clearer details, transparency increased as well, the texture of the notes was clearer, it was less grainy and stage improved as well. I believe I have found a perfect system matching that counterbalances its biggest cons and from good sounding speakers, it makes them awesome sounding speakers. With traditional speakers, system matching is important, but still not as important as it was the case of S400. They sounded unrecognizable from the first system to the last one. Please do note that the same source was used in the first and the last system. S400 seems to be very sensitive when it comes to power, from my experience I would stay away from tube amplification with them, it will make the passive radiator slower and less punchy. S400 are craving for fast sounding amps that will deliver faster decays, a good slam and a good control over the drivers.

II. Sound Signature & Timbre

Having my active KEF LS50W for more than a year, I already know what to expect from them, I know where I should sit on the couch for the best sound and I know what music will work best with them. Transitioning to Buchardt S400, I was mesmerized how bigger and fuller is the overall sound signature and how deep I can look into my records. Hell, even electronica sounded a whole lot better, feeling those bass notes with my whole body. With LS50W I have two big circles in the air from where all the sounds are coming from, rarely I can hear a note coming from the walls, from weird angles or just behind the speakers. Moving on to S400, soundstage increases to unrecognizable and those big circles in the air are just increasing tremendously in size. On some particular songs, I knew a precise spot in the air where a snare drum would hit me, on S400 that spot increased from a square meter to about two square meters, everything is just much bigger sounding. I’m getting sounds from multiple angles and surprisingly some of them hit me from behind. S400 knows how to play with imagination, increasing the stage, deepening the performance, infusing some much-needed warmth, adding a lot of low-end where a speaker of this size would normally render just a tiny fraction of it. On the other hand, S400 will not impress that much when it comes to speed and slam, nor does it want to have the sharpest outline of the musical notes. Taking into consideration all the knowledge and working principle behind their Constant Directivity Control, Buchardt Audio wants you to have this huge enveloping sound that hits you from multiple angles. When you take a fast-moving air mass and start splitting it so it would hit the listener from different angles, it is normal that the energy, the slam and impact of the original air mass would be seriously decreased. So far, this is one of the biggest cons of S400 and this is the sole reason I strongly recommend a faster sounding source and amplifier. Put a slower and mellower sounding source or amplification and S400 will have a super relaxed type of sound that will not work with faster beats or aggressive music.

More than a year passed since I have listened to a good stand floor speaker, S400 on the right setup reminds me a lot about a performance like that. Finally, I can hear some sub-bass notes not only with my high-end headphones but also with some bookshelf speakers. I find its frequency response close to perfect for my tastes and I do find the enjoyment factor to be high with Buchardt S400.

III. Transient Response

Probably the only area where I felt a less than immaculate performance compared to the rest of these chapters. Since in the high-end audio pros do outweigh the cons by a really big margin, once I listen to anything I start searching for the cons, it is much easier searching for drawbacks where I could recommend an improvement for its future designs. I don’t find the transient response weak per se, but it is not the strongest point of S400. Once you spread out the stage, spread out the bass with that huge radiator, when you lean back the cabinet, you do really get a much bigger picture compared to its size at the cost of the ultimate slam and punch that I so crave about while listening to aggressive type of music. I find its speed good, but not great. Do note that system matching plays a big role here, you can have them even mellower and slower than in my experience. Can you have them even faster compared to my system? Certainly so, I’m pretty sure about that. I plan on upgrading the S125 with two Benchmark AHB2 dual mono amps, that I am pretty sure will improve the pace, rhythm and timing and who knows, after an upgrade like that S400 could hit me in the chest like Thor’s hammer.

With a gentler punch, slam and overall speed, the decay of the notes is indeed a bit slower compared to the best speakers out there. Depending on individual tastes, this could be a good or a bad thing. Listening to a lot of jazz, soprano and classical, S400 will send you into a relaxed state of mind and if you like to relax while listening to music instead or air-guitaring and headbanging like myself, S400 would definitely be to your liking.

IV. Soundstage & Depth

OK, this is where S400 is showing its muscles and its taunting its competition. The jump from Wharfedale Diamond 11.2, to KEF SL50W and then to Buchardt S400 feels like I am sitting in a much bigger room, there is just more air around me, there is definitely more music around me, every corner is filled with it and even if I change position or move to another couch, it sounds absolutely the same.

S400 has an incredible trait of pushing a high sound pressure level anywhere in room, just make sure to feed some clean watts and seconds later you will be surrounded by music. When properly driven, I felt a much better control of the driver and as a result the holographic image is much sharper and clearer to me. Thanks to a better control, the outline of every note is clearer and I can easier spot it in the air. I owned quite a lot of bookshelf speakers including high-end studio monitors, but none of them were able to throw such a big, believable and breathable stage in front of the listener. Sometimes I just take them off the stand, look at them and admire the engineering that went into them for that big sound. I’m quite sure that the passive radiator had the biggest impact for the stage size, pushing so much air in all directions. If you want a tad clearer image in front of you, I recommend leaving half a meter, or better yet a meter from the wall, the difference is not that huge compared to a ported bass-reflex, but it’s there.

I listened to some older records of B.B. King and Dave Brubeck and the size of the auditorium was so apparent on S400, I can definitely appreciate how big is the room where it was recorded, how much distance is between those artists. With my KEFs, everything sounds tinier and not that spread out, it sounds more like a cozy jazz concert in a busy club. S400 possesses such a relaxed, ethereal and soul-grabbing performance, if only I would not be quarantined with a 6-year old kid and my wife…

V. Detail Retrieval

As I mentioned in my first chapter, S400 are changing a lot depending on the system it is connected to, it changes a lot with different sources and amplification. The cleaner your audio chain will be, the cleaner the S400 would sound in return. Stereo image and outline of a note is one thing, but detail retrieval is certainly something else. In a crowded passage, thanks to a wider spread presentation, picking up tiny details is much easier to do without stressing too much. I believe S400 offers a detailed presentation, without being clinical at all, it is more like a natural and non-aggressive type of detail. S400 will not hide imperfections in a tune, neither highlight all its drawbacks, if you know where to look and focus on, every detail can be spotted.

VI. Bass

I adore when I can spot some bass layers with my ears and feel the impact with my body. I do like fast executed bass lines, I like when I can hear it decaying naturally and most importantly, I like when it’s a detailed type of bass, pulsating high amounts of energy and increasing the mood level of the listener. When I get new transducers to play with, like these wonderful S400, I tend to drive them to their limits even past my comfortable listening levels. If all I can hear is an undistorted type of bass, then the hardest test was already passed. S400 is definitely part of the group, it can show some crazy amounts of bass, with layers and layers of it hitting one after another, playing it in a clean and undistorted manner.

My other bookshelf speakers were decent in this regard, but never impressive. I even used an active closed-port subwoofer with the KEFs, so I can feel some of that bass. On the plus side, low-end was going low awaking some rumbling bass notes, on the flip-side tonal balance and timing was out of the window. Buchardt S400 are great not only in terms of mid-bass but in terms of sub-bass too. Obviously, you can’t expect notes hitting you as low as 25 Hz, but from 30 Hz and up there is plenty of information and bass rumble to be heard and feel. Living for a year with the LS50W, made be somehow forget that there can be still few additional bass notes, some additional layers of it in a track. When I switched to S400, sometimes I would just turn my head, hearing a new bass line and a microsecond later feeling it with my whole body.

Listening to Infected Mushroom – Symphonatic (More Than Just a Name, 2020) made me realize that I would hear only half of that bass with the KEFs, the performance felt incomplete and really different to the S400, that were able to awake some bass notes that weren’t there with the KEFs.

S400 are not some speakers for bass addicts, it still follows some strict rules offering linear type of bass with just a small increase in the mid-bass section. I tend to like this effect with speakers and when writing about bass, there is little to nothing to complain about with these.

VII. Midrange

Listening to the soundtrack of A Star Is Born, Lady Gaga – La Vie En Rose, first it’s mandatory mentioning that I am not a Lady Gaga listener, like at all, I just liked the film and I liked this particular song. It’s a pretty short song, but the naturalness is through the roof in this one. I like everything about it, that sweetness of the piano, that deep sounding voice, those calm brush sticks hitting the drums. The people in the room cheering up and applauding, such a life-like performance on S400, so much naturalness was preserved, such an amazing midrange. Absolutely nothing to complain about, only to cheer, relax, have a cigar, a glass of whisky and call it a day. I particularly liked the piano o n this track having such a heavy tonality, a meatier tone and a full-bodied presentation. I want to highlight that a big part of this track is made out of midrange tones and if I feel them full and natural, then this is the way S400 sounds in the mid zone.

Listening to the newest album of Myrkur, with an abundance of musical instruments and with voices playing on different tonalities, its impossible to not be carried away with such music. Myrkur is like 80% midrange, there is a bit of bass and treble. Their album called Folkesange (2020) is carrying so much midrange presence, it is the one of the nicest albums testing out midrange capabilities of any transducer. Call me nuts but that aluminum woofer is quite natural and non-metallic sounding, quite a paradox really.

Most of the manufacturers are putting the breakup region of the crossover somewhere in the upper midrange up to the lower treble region and Buchardt Audio is no different as the crossover point is at the 2 kHz area. I tend to agree with the manufacturer stating that it’s a damn fine crossover that puts the right signals in the right places as cleanly as possible. However, you must understand that the breakup region is decreasing the energy in that particular region by few decibels, depending on the room acoustics and on the position of the listener. This is probably one of the reasons, S400 is never bright, harsh or dry sounding. Try lowering the 2 kHz region by a few dB with a parametric equalizer and you will get where I’m heading.

VIII. Treble

As for treble, I was quite impressed by the performance of that custom fine weave soft fabric textile tweeter. It’s small but powerful and thanks to that CDC aluminum waveguide it sounds crisp, detailed and quite extended in the top octave. I stressed the S400 quite a lot with a healthy playlist covering most musical genres, in return it offered a non-aggressive and a life-like treble performance. As I mentioned before there is a gentle slope in the 2 kHz area, that creates a natural start for the lower trebles. They start rising to the linearity point and remain that way even past top octave. I can measure headphones and soon I hope I will start measuring speakers too. Buchardt Audio is quoting a frequency response of 33 Hz up to 40 kHz, I don’t know how much information is there past 20 kHz, but up to that point its crispy, it’s detailed and sharp without being a burden in the long run.

IX. A Comparison

Buchardt Audio S400 (1800 EUR) VS KEF LS50 Wireless (2300 EUR/2500 USD)

In terms of build quality there is nothing to complain about them, once you touch and hold them, they both scream high-build quality and awesome aesthetics.

LS50W is a more expensive speaker, but it also incorporates a lot more tech inside, it has a Hi-Res DAC, a wireless streamer, a Bluetooth input capable of AptX codec, it also has dual mono 230 Watt amps inside. You just power them on, connect to a Tidal HiFi account with your phone and that is basically it, super convenient and hassle free. Buchardt S400 is cheaper, but will need additional electronics to work.

LS50W are using a smaller woofer the size of 5.1” and a 1” aluminum tweeter that together create a coaxial driver, as for the low-end, it has a bass-reflex port on the back.

LS50W are sounding impressively good and mighty fine for their price and for their size. I like almost everything about them, except for the limited bass response and for that small soundstage that can’t fill up a bigger room. LS50W has a small but a very precise and really outlined stereo image, it is not that 3D sounding, but has a really defined location of the notes. It creates two big air circles in front of speakers from which all sounds are coming from, except for the bass that will hit you from different spots. LS50W is quite a detailed sounding speaker and has some high-end flair to it once you listen to them for a longer period of time. It lacks a bit of midrange presence which S400 plays so vividly natural and effortless, treble information is top notch and so is the detail retrieval. KEF is also offering an app where you can tune the sound signature to your liking, you can add a subwoofer and you can change the low pass filter for the best integration with a subwoofer.

Buchardt S400 on the other hand offers a much wider frequency response, a much deeper bass which is more spread out, that pulsates really nicely with few particular songs. I felt a warmer midrange too and in general a fuller tone that worked better with a wider variety of music and audio equipment. When it comes to treble information, I would say it’s a tie, both are sounding extra crispy and extended in that region. S400 looses a bit of speed and impact compared to LS50W but it is winning massively in terms of soundstage size and depth information. It sounds like a big speaker, very unusual for that size. Its more holographic too, very 3D sounding in a way and really spread out around the listener. Sounds are coming from weird angles, from behind the speakers and are filling the room much nicer.

On the flip-side, S400 will need additional electronics to work, at least a source and an integrated amplifier which are adding to the cost. In my particular system and after trying lots of sources and amplifiers, I can definitely say that S400 are performing at a higher level than the LS50W.


I’m really glad I kick-started our loudspeaker tests and it is really good being back listening to a nice speaker that has so many tricks under its sleeve. S400 was quite a shapeshifter when it comes to system matching, if you are thinking about getting them, make sure you will be feeding them a snappy and punchy amplifier together with a linear, detailed and fast source. This is my first contact with Buchardt Audio and with a passive radiator in a Hi-Fi speaker, but I’m looking forward to next crazy ideas of Mads Buchardt and his passionate team. I’ve felt the care that went into them and the crazy engineering that worked and offered a different point of view to the traditional speakers.

Buchardt S400 will cost you 1800 EUR, you can get them from here, it includes free worldwide shipping and all imports costs too! As for the price itself, they eliminated everything deemed unnecessary to the speakers, including expensive marketing, distributors, agents, and Hi-Fi dealers. By selling directly to the consumer they eliminated 60-70% of the cost in comparison to other brands. Worth it? Hell Yeah!


  • Great design choices, very bold and usual for a Hi-Fi speaker
  • Good build quality too and lacks non-sense features like bi-wiring
  • Incredibly open wide soundstage
  • Accurate pin-point imaging and see-through transparency
  • Good detail retrieval
  • Depth information is too easy to pick-up in a record
  • Deep bass that goes down to lowest octaves, life like midrange and extended treble response
  • Among the widest frequency response in a bookshelf speaker
  • Awesome timbre and tonality
  • Awesome engineering and some clever ideas made their way into S400
  • Quite easy to drive
  • A great value!


  • Would love to have a harder punch/slam in the chest
  • System matching is a big deal with S400


  • DACs: Matrix Audio Element X, Denafrips Venus, KECES S3, Audio-GD D-28, Burson Conductor 3 Reference, Flux Lab Acoustic FCN-10, xDuoo TA-30
  • Preamps: Benchmark HPA4
  • Integrated Amps: Hegel H190, KECES E40, SMSL DA-8S
  • Power Amps: KECES S125, Burson Bang
  • Headphone Amps: SparkoS Labs Aries, Benchmark HPA4
  • Loudspeakers: Buchardt Audio S400, KEF LS50W
  • Full-sized headphones: Audeze LCD-4, Erzetich Phobos, Hifiman Arya, Quad ERA-1
  • Interconnects: QED Reference XLR, Aune AL3 XLR
  • Power Cables: Isotek EVO3 Premier (x2)
  • Speaker cables: Kimber PR8, Audioquest Type4
  • Balanced Power Conditioners: PLiXiR Elite BAC 400, KECES BP-600

*WAF = Wife Acceptance Factor

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