Review: (updated) darTZeel CTH-8550 integrated amplifier (with Siltech)

The $25,000 USD gold, and Ferrari-red, solid-state integrated amplifier from Switzerland exuded raw sex appeal as it sat in my listening room oozing quality. With it’s staggeringly addictive color scheme (a point of opinion, and contention I know), muscular casework and that rubbery, nubby-feeling volume Pleasure Control for tactile feedback, the darTZeel CTH-8550 could be a bit much for some people at times, but oh baby was it posh. Edward Ku at Element Acoustics had made the amp available to me for review, and I was looking forward to having a few drinks, and letting it take advantage of me.

Blurring the lines between valves, and solid state?

The state of solid state

I’ve developed a preference for tubes in my sonic wanderings over the last several years, and my reference system is comprised of said valves: those glowing glass, bulbous knobs that inhabit amplifiers, and pre-amps of a certain bent. But regardless of that, I’m becoming unabashedly brazen at inviting amplifiers like the darTZeel home.

I don’t think anyone would disagree that high-end audio manufacturers are making huge strides in solid state Class-A, and Class-D amplification, and Hervé Delétraz, the wunderkind amplification engineer behind the design of the CTH-8550, has shoehorned not only some massive, overbuilt components into this gold, and red behemoth’s chassis, but a delectable potpourri of audiophile-grade caps, resistors, and transistors too.

All guts, all glory.

Tonally, I found the 8550 to be beautifully voiced, with a real effortlessness, power, and breath/space imbued into playback. Trying to find fault with any one area of amplification of the signal proved pretty much pointless regardless of what type of music I threw at the 8550. If I had to nitpick, maybe not as much bass extension as I’m used to with tubes? But, the reality is that it drove through everything. Like a Cadillac on a backwoods dirt road, it just soaked up every signal imperfection or flaw, and made it smooth, glorious, and stately. I caught up with darTZeel head honcho Delétraz in late April to talk about the design process he embarked on when tackling the 8550, and here are his responses:

RA: What were the main circuit design/performance goals when you sat down to build the 8550?

HD: “The CTH-8550 was a little more powerful than the NHB-108 stereo power amp.
The goal was to retain the sonic virtues of the NHB-108 yet with providing more output power. This new design eventually led to the big NHB-458 flagship mono blocks.”

RA: Did you approach this project in a different way from your previous work at all?

HD: “The answer is almost entirely in the code name of the CTH-8550: CTH stands for Close To Heaven, 8550 stand for 85% of the quality brought by separates (NHB-108 + NHB-18NS) at 50% of the price.
So yes, the approach was a little bit different than for previous designs, which were more ‘no compromise’ designs.”

RA: How much influence does the critical listening tests you, and your engineers engage in while designing, and building your amps impact the overall circuit design?

HD: “As for any piece of equipment, what you can see at the end, is if the customer is satisfied by the experience; the bigger the smile on a customer’s face, the better we know we succeeded in the design process. Listening sessions are of paramount importance for us, and if we can’t get this smile, we return to our design, and look for what is wrong.
However, do not believe we only design by ear. Performance is a key parameter in order to obtain that kind of smile; the way we differ is we do not necessarily measure the same things that our competitors do. After 40 years of experience, I have some ideas about which kind of measurements actually affects the perceived sound, or not.”

RA: I’ve heard some tremendously musical solid-state amplifiers of late, in both Class-A, Class-A/B, and Class-D. What is your take on the current state of solid-state amplification now, and moving forward?

HD: “As I like to say, I think we may be able to design an amplifier where the sound would be independent from being Class-A, A/B, D, or even transistor or tube. The main reason these designs have particular sonic signatures depends more on the designers themselves than the operating class or technology used. I  said “the main reason”, since there are some basic differences you can’t escape. If I will have some time one day, I will try to build different classes of amps that sound the same… just half kidding!”

RA: Do you have any thoughts on what high-end audiophiles could expect to see from solid-state amplifier designers in the years to come?

HD: “Well, solid-state electronics proved to be more compact, more efficient (read “less power hungry in idle mode”), more reliable, more convenient to use and less expensive than their tube counterparts.
Do they sound better than tubed gear? I think audiophiles, or, if you allow me to use this term, rather music lovers, will always look for a sound reproduction which will bring them closer to the emotional event; closer to the music itself. Tubes are known to offer those attributes. We at darTZeel aim to bring such emotional experience through solid-state audio equipment, without the troubles that tubes are also known for.

“If I could answer in a very few words, I would say that audiophiles would love to buy an amp that they will still love ten years later.”

What’s in the box?

The guts of this fully, dual-mono integrated amplifier are a neatly tidied mass of carefully matched, resistors, transistors, caps, circuit boards, and cabling dominated by two separate toroidal power transformers, the huge one handling the 8550’s output stages, the other, mini-one pulling duty for the pre-amp stage. There’s technical, and specification information available for what darTZeel did to wrest the most from their circuit designs, and they have made it available, here.

Attention to detail.

This amp has a rated output of 220 watts into 8 Ohms, and 350 watts into 4 Ohms, and a frequency response of 10 Hz to 100 KHz of +/- 0.5 dB.

A look that instantly appealed to some, and was off-putting to a few.

Listening sessions

For vinyl listening sessions, I used a modded Rega RP6 with a Denon 103 running into an Auditorium 23 Standard SUT (Step-up Transformer) plugged directly into the darTZeel’s MM phono section. Transducers for this review were silver-wired, and silver-voice coil equipped Audio Note UK AN-E/SPe HE two-ways.

Basie-Williams-LPI started with a Japanese pressing of The Greatest! Count Basie Plays… Joe Williams sings Standards, a 1981 reissue of the 1957 big-band session which I had rescued from the used jazz bin of a local record store. It still had its obi, and I snagged it on the cheap. This is a gutsy LP with big dynamics, and played dead quiet after being run through my RCM.

Thou Swell, off of Standards, has an incredible 3D sound stage, with startling clarity, depth, and separation to each instrument in Basie’s orchestra. The musicians were placed well behind the speakers’ focal plane, with Williams voice well out in front, and into the room.

On Another You, the sax, and trumpet possessed an almost poignant timbral accuracy to my ears, with fantastic, organic tone, and real grit to the sax’s reedy parts. Very stable sound stage, I never got the impression of any drift in placement of the players, and the drum kit… the attack speed on the snare’s leading transients was beautifully pronounced, and clearly separated in 3D-space from the rest of the kit.

On Our Love is Here to Stay, the amp was giving up mid-range to die for; the liquidity of Williams voice, and the notes of the flute just flows through the mixture of instruments without ever getting smeared or bunched. Bass is taught, and the body of the instrument is beautifully fleshed out, if a tad rounded off, and while it seemed to lack the last bottom octave I’m used to on this cut, it still sounded sublime. Basie’s piano has the organic rendition of soft bone on wire I need to hear for true piano reproduction.

S’wonderful features these sweet sections where the interplay between saxophones, and trumpets could easily get mashed together, and lose the wonderful brassy color, and blat that separates the two instruments, and while the darTZeel nailed the tone of every instrument, and kept them clearly delineated, there wasn’t the big, bloomy decay that I’m used to from valves following a percussive smash on cymbals or high hat; that spectral space around the notes as they dissipate into the ether wasn’t lasting quite as long as I like.

tosca-coverOn Tosca’s lushly-produced 2xLP Suzuki, the instruments again were exquisitely spatially separated on the sound stage, with some of the best 3D-placement I’ve heard from this album (no small feat considering the complexity of the interplay among the myriad instruments, and vocals used in the production of this album). Hallmarks of playback on this LP were incredible pace, and timing  with a real drive to the track’s beat that added a propulsive stride to the sound.

Flipping sides, Orozoco is a cut that possess a positively Stygian bassline, and despite a lack of bottom-end ooomph on certain tracks, there was tons of colorful texture to vocals, keyboards, and percussion. The amp was able to render a level of resolution, and micro-detail that a number of other amplifiers around this price point that I’ve heard (for power, and pre separates), are unable to match.

Great-ReunionDespite my nitpicking, the 8550 has the ability to reveal sonic secrets writ large, it doesn’t want to just translate the signal with technical perfection, it wants to make music out of the sine wave, beautiful music. Think of a cellist whose technique is perfect, but who is unable to translate that technical prowess to emotional connection; the 8550 can play back music flawlessly, transparently, but it can also leave you a bit spent with its ability to provoke emotional responses in that playback. Quite a trick for solid state, in my opinion, and to my ears.

The Great Reunion with Louis Armstrong and Duke Ellington is an abashed jazz masterpiece to me. This Classic Records remaster, and pressing is flawless, dead quiet in the groove, and as close to perfection in a jazz album as one could hope for on many levels.

Here the darTZeel shows its ability to wrest greater image density from the music; Armstrong’s horn is all brassy-sheened blat conveyed like art with complex brush strokes on the aural canvas he paints with Ellington. The percussion, and drumming in particular, is rich with timbral shadings, and intoxicating syncopation, and polyrythms. You can hear every subtle inflection of Mort Herbert’s fingers on the strings of his bass from the moment of contact on Just Squeeze Me.

Hawaii high-hat

There’s a level of detail available from this LP that the 8550 seems made for. Armstrong’s lips moving against the trumpet embouchure as he belts out his solos in I Got it Bad, could almost be labeled distracting if you were not gripped by the exquisite tone of the horn as I was; the note’s extension, the portrayal of air pressure variance, and pitch changes as his fingers fly over the valves is mesmerizing, and here the darTZeel presents such a lifelike size to Armstrong’s sonic image that your brain can’t help but clearly see him well out in front of Ellington’s piano, with the other band members spread out behind him.

It’s little Danny Barcelona’s drumming on Azalea that really shines a light on how well the amp can translate complex textures. Barcelona has such a delicate brush stroke on this cut, and every subtle whisk of the brush’s filaments on the snare is fleshed out with a conviction that it defies your imagination to not picture them drifting over the drum’s skin.

Cable Change

Siltech Royal Signature Prince brings noticeable refinement to an already regally-voiced amplifier.
Darker black.

Edward decided to drop off some Siltech Royal Signature Prince speaker cables, and a Royal Signature Ruby Hill II mains cable to use with the 8550. These cables are massive, and I have to say, as with pretty much every set of well-researched, science-based designed, and built cables, they made a noticeable difference to what I was hearing from the amp. The cables didn’t add any sonic flavoring, rather they peeled back another layer of haze from the recordings, a haze you wouldn’t know was there until suddenly it’s gone.

There’s a TV show on Netflix called Archer. I love it. Watch it. You’ll laugh. There’s an episode where he buys 10 black turtlenecks, except there’s five dark black turtlenecks, and five darker black turtlenecks. That’s this amp with the Siltech cables. It’s already the dark black turtleneck, so when I say the already black background gained considerable void, allowing further explorations into low-level detail, that’s really something special.

Yes, these cables are expensive, but they’re completely in-line, price-wise, with the 8550, and for the extra performance they wring out of the darTZeel, I’d say pairing them together produces very synergistic results. A big bump to the bass weight didn’t hurt either, neither did increased instrument separation, dynamics, tonal saturation, and added speed to transients, which helped further refine an already great sound from the amp.

Digital listening

I utilized the somewhat lush-sounding Audio Note UK CD 2.1x/II that I had in for long-term review courtesy of Soundhounds for listening to CD.


Interstellar is one of my favorite films, and I still remember – very clearly – how much impact the soundtrack had that was accompanying the film. In several instances it was literally drowning out all else, including dialog, and I recall at the time thinking “Holy shit, Christopher Nolan really wants this music to be considered a living part of this film.” There’s some excellent short films, interviews, and writing that discusses the making of the Interstellar soundtrack, and exactly what Nolan, and composer Hans Zimmer had in mind as they developed the music for it.

The disc, to me, is fantastic. Loaded with thundering, emotional organ playing layered with complex harmonic structures that captivates, and pulls one into its orbit. I’ve played it for several people, and invariably they all start to become more quiet as the music progresses because they are listening so intently to the pieces that Zimmer has wrought, and the notes of the massive massive church organ employed is capable of are so unique, it’s difficult not to be focused on the music by the sheer gravity of the sound.

Much like the plot of Interstellar revolves around the interminable pull of a black hole, and its displacement of the storyline, so too does the soundtrack act like a black hole, inexorably drawing you into it. This is a disc that will show you very quickly what type of bass your system is capable of reproducing, and sustaining. The depth, and sheer scale of the organ notes seems to plummet ever further, and if your getting clean bottom-end reproduction out of this CD, then you have chosen your system wisely.

I first played this disc through the darTZeel without the Siltech cables in place, and then swapped them in, and immediately noticed enhanced tightness, and muscle – real grunt, if you will – to the bottom end. Sound stage punched out much further side-to-side, front-to-back, as well as in height. My level of engagement also increased as I was drawn further into the music. Listening to Ann Marie Simpson’s violin shimmer holographically in space between the speakers on S.T.A.Y. portrayed a convincing sense of the orchestra’s recorded space, and a true 3D experience to playback from the mix. Roger Sayer’s organ playing took on an unabashed passion, bristling with power, it seemed to be trying to muscle past my listening room walls, all the while Zimmer’s tentative piano explorations (well forward in the sound stage) was laid bare in stark tonal contrast.

American-BeautyThis is a disc that crept up on me courtesy of a friend who brought it over for me to hear. Previously I’d never heard it, and it had been many years since I’d seen American Beauty, so I had completely forgotten what an amazing job Thomas Newman had done to capture the tension, uncertainty, loss, and heartbreak that permeates much of this film. After hearing it, I promptly ordered it from my local record store.

A week later I had it, and listened to it in heavy rotation for the next few days through the darTZeel. I then swapped in the Siltech cables. Right away, with the Signature cables in place feeding the 8550, I noticed a vastly improved image weight, and depth to the bottom end throughout the CD, and the top end had more extended highs; banishing a hint of a glare that I had previously associated on Newman’s upper piano registers with stock cables. Sound stage, and 3D depth to instrument placement also greatly benefited from running the darTZeel with the Siltech cabling.

A slight smearing/sluggishness in the lower-mids primarily affecting the tablas, and kim-kim drums also disappeared, they sounded far more crisp, and delineated from one another now. Instrument texture was more resolved, and finger movement on strings was easily palpable, with Rick Cox’s fret work on the banjo, and bass in particular, benefiting from the lower noise floor the 8550 was enjoying. Attack, and decay on the pedal, and lap steel guitar had far more transient speed, and blossom to them. Cohesion of the music improved greatly too, allowing more individual nuances to break through between the performers, particularly the subtle flute work of Michael Fisher.


I understand that the darTZeel CTH-8550 integrated amplifier could be considered a divisive product, perhaps because of its flashy casework, or the not insignificant price.

I hate to touch on price when it comes to the gear I review, because everyone’s economic circumstances are different, as is our disposable income spending habits, and preferences. I know people I spoke with regarding the CTH-8550 always mentioned cost, but whenever I got someone to dismiss sticker-shock from the equation, and talk to me about the amplifier’s merits based solely on performance after hearing it in my system, they spoke of musicality, and how right it sounded regardless of the genre it was tasked with playing.

While it didn’t necessarily present the deep, blossoming bass that I typically gravitate towards (as from EL84, 300B in particular, and some 6L6 tube-based amplifiers), this racy-red Swiss powerhouse sounded far closer to the type of sound I associate with valves than just about any other transistor-based integrated amp I’ve heard has. 

With the darTZeel, I find myself presented with yet another expectation-breaking anomaly; I’m finding more and more amplifiers with Class-A, and Class-A/B, and Class-D topologies that completely defy my preconceptions of what different operating classes are supposed to sound like. Delétraz is right. I think the day is coming where Class-D and Class-A will no longer be distinguishable, and I’m really looking forward to it.

Personally, I’d prefer to think of the 8550 as something that brings people together – as it did in my home – to enjoy the experience of fantastic music playback, rather than argue over it.

But musically, the 8550 is a sonic knockout, full stop.

Manufacturer’s Response:

 Some remarks about your comparison versus tubes I think readers should know:

The delay propagation time in the CTH-8550 is uniform across the entire audio frequency spectrum and way beyond.
It means that harmonics and time decay duration are the closest possible to the real event.
As tube equipment exhibit unavoidable phase shift at extremes (due to transformers and internal tube-to-tube cap coupling), it lead to some modified harmonic response, giving this most wanted warmth all music lovers like so much.

For the same reason, bass seems a little bit leaner on the CTH-8550 because simply faster. If you would reproduce a square wave form at 40 or even 20 Hz, you would see that the plateaus on the CTH-8550 are quite flat still, while on tube equipment they will look closer to a triangle wave form, indicating massive phase shift, translating into much slower and boomier bass because energy spread over a longer time. After some time, the bass precision and detail exhibited by the CTH-8550 become obvious, and returning back to tube is not always easy.

The CTH-8550 has extended response in low frequency down to about 1 Hz at -3dB, I doubt any tubed gear can do the same. So if they seem to have “more bass” it is just because they are slower.

Thank you for reading it.

Hervé Delétraz

About Rafe Arnott 389 Articles
Editor of InnerFidelity and AudioStream


  1. Part-time,
    Most of what you say is contradicted by the fact that an amplifier with low distortion, linear response, high input impedance and low output impedance, driven within its limits, will have no sound of its own. Perfection in amplification isn’t fantasy, it’s routine, unless you have to draw a ton of current. Speak to any electrical or electronic engineer. In that context, I find the analogy with a cellist who needs to interpret, inject personality into a musical piece, a bit out of left field.

    With all due respect, maybe you wouldn’t feel so much exasperated if you had some references other than entertainment magazines like Stereophile or TAS.

    • Amps, unconnected, have no sound <– this is very true. Boring, but true.

      But connected, we have a system, and one that can have quite a bit of sound. It's the system — the interaction between speaker and amp — that is the issue. The problem is in taking anything in isolation.

      Gabriel, don't know if you're a designer or not, but you're welcome to survey some other designers — speaker designers, specifically. Have yet to meet one that says the amp doesn't matter, or won't have the opportunity to "color" the speaker's sound.

      It's not a matter of "wanting this" or "choosing it" — it's a matter of compromises. There is a reason why every "cost no object" speaker sounds different — and this is quite odd, when you think about it.

  2. “Think of a cellist whose technique is perfect, but who is unable to translate that technical prowess to emotional connection” That is called interpretation, and that’s the reason why we go to a concert and listen to an artist instead of a computer program reading the score. What an amplifier does is called reproduction. There is no reason why a playback system should interpret the music, it’s already interpreted. Why this desire to modify the sound if it’s ok to start with? If music already has a “soul” before it reach the amplifier, why want something other than “technical perfection”? My take: this fuzz about electronics “personality” distract the reader (and the reviewer) from the fact that technical perfection in electronics, at least in relation with the human threshold of audibility, is available at Best-Buy at low cost. And the high end aristocracy don’t like that.

    • You’re wrestling with an analogy as if it were factual — that’s a Category Mistake.

      And while I have no idea who the “aristocracy”are, if someone does, I want a knighthood!

      • What is factual is this belief that technical perfection is not enough, that a designer should optimize some unknown, artistic, parameter. Am I wrong?

      • Then I think we’re both using words at cross purposes.

        The issue isn’t perfect reproduction. That’s a fantasy. There is, factually, no such thing. You have approximation at every single stage. Every single one. At the very end, what you hope for is verisimilitude. But unless you’ve miked a stereo PA system, that’s going to be very difficult to reproduce without significant and dramatic interpretation — both at the input stage, and the output stage. Lets assume all of this as axiomatic.

        What you have on the output stage is … whatever you have. Single drivers. Planar drivers. Complex horn arrays. Multi-driver/multi-way dynamic drivers. Doesn’t really matter — all of them are going to massively compromise the output from the original. And they’re going to do it, in large part, because they’re not broadcasting into a vacuum, but into real space. Not just any space. Or rather, any space at all. It’s complicated.

        Each driver in each speaker, regardless of type, can and probably will respond sub-optimally. It’s just a fact. Interestingly, each sound-producing mechanism also responds differently to different kinds of amplification — some need so much voltage and current, some need not so much current and more voltage (and vice versa), some demand more than others across the audible band, some demand more at specific points in the band. Again, whatever. The point — each driver, fed with a specific kind of amplifier, can and probably will respond differently.

        Now, and here’s the kicker, it turns out that that voltage and current aren’t the only things that effect the performance of the driver. As far as I know, there’s no such thing as a perfect amplifier — one that does infinite voltage on demand, with infinite current also on demand, with perfect damping. That amp + transducer pairing has yet to hit Earth, as far as I know.

        So, why wouldn’t a designer, with knowledge that perfection is impossible — and that “close to it” is probably prohibitively expensive and physically inconvenient in terms of size, heat, and power draw — make choices that are well within the possible? And why is it a stretch to assume that those compromises have sonic impact? And that designing is, at root, simply being able to engineer toward sonically pleasing compromises?

        Anyway, I think we’re done here.

  3. Would really really love to know how it compare to the LHC-208 musically/sonically…

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