A Class D Exploration and Comparative Review of PS Audio, Wyred 4 Sound, and XTZ

Living on the Edge - Class D & Three

Just imagine sitting in a minibus with a bunch of audiophiles, overtaken by two 911s and one 2019 Subaru, while you are playing the alphabet game for Class-D amplification :

Apollon, BelCanto, Classé, Devialet, ...WROOM..., Jeff Rowlands, Kiii , Lyngdorf,.. WROOM .., Mola Mola, Nord Acoustics, Onix, ... wrooooooom ... PS Audio,  Wyred 4 Sound, XTZ. 

Like Subaru, the three manufacturers noted last, have recently designed amps, which neither target clients interested in German engineering, French flair, British exclusivity nor Italian grandezza. They aim for brand-loyal owners, who are interested in reliability, experience and performance. Who are looking for power amplifiers, which are top of class when it comes to performance vs. price. Maybe a convincing trade-in-program is appreciated as well? 
Hence, there is an important question left: How do they sound? 

The Stellar S300 from PS Audio, the W4S ST-750 LE and the XTZ A2-300 EDGE all use Class D amplification from ICEpower A/S, which manufactures on an industrial scale in Asian countries while conducting administration and engineering in Denmark. Denmark is a small - although the Kingdom of Denmark contains Greenland, the biggest Island on earth - European country between Sweden and Germany, most famous for its cuisine (Noma), their Danish Dynamite (national football selections m/f), a bunch of exciting modern crime auteurs and one less than stellar quote from Shakespeare (Hamlet, Marcellus, 1.4). 

In our audiophile context we must not forget the Technical University of Denmark (DTU), which has given rise to cohorts of engineers, that have built the foundation of Danish excellence in Class D technology, driven by companies such as Lyngdorf, Purifi or ICEpower A/S.  

ICEpower A/S proposes its Off-The-Shelf solutions for all types of amplification, some aimed at industrial customers or audio manufacturers and other on DIYers. Similar to its competitors Pascal (also from Denmark), Hypex (from the Netherlands), Infineon (Germany) and TI (USA), it has a special focus on high-end audio. A more detailed view on the state of Class D is at the end of the review. After a short sidestep on Class D definitions and perceived limitations in sound quality, I'll then present some details on the amplifiers, even more facts on the amplifier boards used plus some background on the manufacturers. Subsequently you'll get a thorough listening contest and an evaluative verdict how these amps sound to my ears.

Class D - Definition, Quotes, Advantages and Difficulties

Albeit Jun Honda & Jonathan Adams describe Class D as

"basically a switching amplifier or PWM (Pulse Width Modulation) amplifier ... In this type of amplifier, the switches are either fully on or fully off (thus the D equals digital moniker) , significantly reducing the power losses in the output devices. Efficiencies of 90-95% are possible. The audio signal is used to modulate a PWM carrier signal which drives the output devices, with the last stage being a low pass filter to remove the high frequency PWM carrier frequency." (1)

From Wikipedia:

"A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices (transistors, usually MOSFETs) operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly switching back and forth between the supply rails, being fed by a modulator using pulse width, pulse density, or related techniques to encode the audio input into a pulse train. The audio escapes through a simple low-pass filter into the loudspeaker. The high-frequency pulses, which can be as high as 6 MHz, are blocked. Since the pairs of output transistors are never conducting at the same time, there is no other path for current flow apart from the low-pass filter/loudspeaker. For this reason, efficiency can exceed 90%." (2)

I decided to give you both the EE expression and the full wiki on Class-D definition, as this move keeps me safe & sound  and the readers informed on the subject. While none of the definitions directly visualize in my mind's eye, I have found a helpful diagram.  

Block diagram of a Class D amplifier. "The audio signal in the comparator is converted into a pulse-width signal by the triangle signal. This contains all the useful information and then controls the amplifier part only as much as is necessary. At the output, the high-frequency components are filtered out by an LC low-pass filter, leaving the amplified audio signal behind." (translated by deepl.com) (3)

Simplified for the non-EE audio enthusiast, Class-D amplifiers have three important components, the input stage (analogue or digital), the PWM module and the output stage that uses Low-Pass filters to eliminate any form of undesired distortion or artifacts. The advantages of Class D are small footprint, low power consumption, high efficiency and cost advantages at industrial scale. This is due to ongoing advancements in Class-D-design, nurtured by growing demand in telecommunications and automotive industry. Back into high end audio, Class D quality depends (as most of us may agree) on high quality components and design/implementation.

Bruno Putzey (HYPEX, Mola Mola,  Kiii & Purifi) has put it very smart this way:

"Modern Class D amps, in particular mine—ahem—sound good not because they’re Class D, but in spite of it. I can’t repeat that often enough. Left to its own devices, a switching power stage tries to do just about anything except amplify audio. You choose Class D to save energy but it’s all elbow grease after that. People don’t realize how much more challenging Class D is compared to Class AB. It’s truly an order of magnitude." (4)

That might explain why B&W in Switzerland still points out the obstacles it has overcome with its Classé Class-D power amps:

"A known problem with Class D sound is related to the fact that the long dead band period not only produces unpleasant distortions, but also provokes engineers to make increased use of feedback circuits to compensate for the distortions. Increased use of inverse phase feedback reduces harmonic distortion (THD), which is a standard measure, but increases intermodulation distortion (TIM), which is not a standard measure. These TIM are largely responsible for the flat soundstage, the low resolution and the lack of round sound in the high mids and high frequencies and thus for the typical harsh sound of Class D amplifiers." (5)

Modern Class D designs consequently concentrate on advancements (new chipsets like ICEedge or eGan-FETs) restricting the effect of dead time on harmonic distortion (THD) and the minimization of TIM, which appears to be an important reason why Class D has often been characterized as subpar to Class A or AB amplification. 

For the modules used in this review, multiple measurements are available in the documentation on the ICEpower website*. I have selected frequency response and TIM charts for visualizing the 300AS1 and 700S2 performance (6)(7)(8).

Hardware Under Review

The Class D power amps  provided as review models from the manufacturers were all equipped with ICEpower boards from the AS series. Each model sports them in a different configuration.

XTZ's amplifier makes use of the convenient solution 300AS1, combined with the "hanger" module 300A1, which is powered from and attached onto the DC-bus output of the main amplifier board. Consequently, the device generates up to 2x150 Watt RMS/8 Ohm while having a minimal ecological footprint.

In contrast, the Stellar S300 from PS Audio employs two 300AS1 boards in a true dual mono design. Its twin open-loop Class-A MOSFET circuit input stage is called "the analog cell" and allows the otherwise discreet build modules to produce a special sound signature.

Finally, the W4S ST-750 LE exhibits a single ICEpower700AS2 integrated stereo amplifier with balanced output stage & fully balanced 3rd generation input stage, that “significantly reduces the detrimental effects of a noisy source through balanced circuitry, audio grade capacitors, and a quad-paralleled FET buffer". (9)(10) 

This ICEpower board uses the ICCX chipset, a design from 2004, which is ICEpower's latest technology aside from the 2016 presented ICEedge chipset. It might be of interest, that the crosstalk of the 700AS2 is quite good (Figure 15 of 700AS2-datasheet) (11), but not nearly infinite as we would expect from a real dual mono design.

PS Audio Stellar S300: 

• Dimensions: 17” x 3” x 13" (cm: 43,2 x 7,62 x 33) including connectors, 
• Weight: 13 lbs / 5,9 kg
• Price: Starts at 1499 USD direct from PS Audio (home trial and trade ups available in the continental US), from 1599 € in Europe (ebay - seen 04/02/2018)        
• Design: Classic true dual mono design w/ 2 ICEpower 300AS1 modules / 2 analog cell input stages. 
• Input:  XLR balanced and RCA unbalanced inputs
• Output: Per channel 2 pairs of Copper based nickel plated binding posts
• Hum: No Hum @ 12 O’clock on the NAD C165BEE

Wyred 4 Sound ST-750 LE 

• Dimensions: 15.75"  x  4.625" x 13.75" (cm: 40 x 11,75 x 34,9), 
• Weight: 13 lbs / 5,9 kg
• Price: Starts at 1599 USD direct from Wyred 4 Sound webpage. From 1695 GBP in UK (eliteaudiouk.com)        
• Design: Single board integrated stereo ICEpower 700AS2 Class D amplifier with balanced output stage & fully balanced input stage.
• Input:  Neutrik-XLR balanced and RCA unbalanced inputs
• Output: 1 pair of fully insulated binding posts per channel, optional WBT binding posts and Kimber TCX wire are available for +250 USD
• Hum: Very light hum @ 12 O’clock on the NAD C165BEE

XTZ A2-300 EDGE

• Dimensions: 7.9 x 2.3 (2.8) x 11.2"  (cm: 20 x 5,8 (7) x 28,5), 
• Weight: 5 lbs / 2,3 kg
• Price: 495 € + shipping direct from XTZ webpages. According to XTZ available worldwide on request, incl. US & Canada       
• Design: ICEpower discret build 300AS1 Class D amplifier with integrated power supply, ICEpower 300A1 Class D board attached, Input stage with adjustable impedance
• Input:  RCA unbalanced inputs
• Output: 1 pair of Charming Music Conductor copper binding posts per channel 
• Hum: Light hum @ 12 O’clock on the NAD C165BEE

NAD C275BEE (old guard)

• Dimensions: 17 1/8” x 5 1/4” x 13 3/4” (cm: 43,5 x 13,3 x 35,2), 
• Weight: 32.2 lbs / 14,6 kg
• Price: bought 2014 for 879 €
• Design: all discrete build Class AB amplifier, Class A Input stage with adjustable impedance
• Input:  RCA unbalanced inputs
• Output: 2 pair of fully insulated binding posts per channel
• Hum: No hum @ 12 O’clock on the NAD C165BEE

A detailed specs list with links to the manufacturer you will find HERE.

Datasheets for the ICEpower boards are here (300A1, 300AS1, 700AS2, 1200AS1 / 1200AS2).

While a Wyred 4 Sound device is typically viewed as having value due to its modest chassis design, that does not hide high end audio's aura as a particularly masculine domain, PS Audio conversely emphasizes the challenge "how to build a beautiful, full-sized chassis, and still have budget"(12). Paul McGowan has noted that "The chassis design took nearly a year. It was the key to making this line. Our other chassis are waaaaay more expensive. This one’s cool!"(13)

It's pleasant in the way, that despite its size it does not take up too much visual space with its timeless elegance. You can keep your breath and you won't be affected, negatively or positively, looking at this neutral style. Plus, it enables much more appreciation in one's living room than we may imagine. It's just that, with power amps, built as a stereo amp or a pair of monoblocks, the WOW! factor of the cabinet is - for me - usually quite limited. Especially, if one is aiming for gear that doesn't kill your budget, but primarily lives up to your audiophile sonic demands.

To the contrary, the A2-300 EDGE is a handsome, small unit, featuring an elegant brushed dark aluminum case. With this neat and tiny outfit, the XTZ, compared to its full sized competitors, gives you the notion of a lightweight hobbit in a smart tuxedo. Small, powerful, smart, elegant. This style is extremely convincing in my view.

Its petite size enables one to opt for a 19''rack mount, single or double, both using one rack space. Here's the point: one could cover 4 of them with the chassis of the NAD amp, and still have only 1/4 of its power consumption at idle. Sounds convincingly economic.

Flashback: Paris - A short story of (not) buying audio equipment

About 5 years ago I was contemplating a new power amp for my very transparent and detailed sounding B&W 805S stand mount speakers. I selected 3 shops in Paris, which were supposed to offer the 800 product range on display, where I’d look to find a matching amp. 

A Saturday free of family obligations enabled me to pursue my quest in the Parisian November rain. Unfortunately, I experienced some uncomfortable encounters with several professionals of the audio trade, which left me completely estranged to my goal of spending up to 2000 Euro on an integrated amp. Separates were an option, but equipment fit to drive my B&W 805s speakers with verve & vigor was the main goal.  

I was told how foolish I was to think my small amount of money could buy anything adequate for these high end transducers. If I doubled the investment, I heard, I might start to get a matching amp from NAIM, but that would be the bottom of the line line, and a McIntosh at 6k would be much more adequate. 3 shops visited, 3 times more or less the same outcome. High end components are supposedly in need of even more expensive high end components to match, they said. Class D was not even mentioned as a possibility and I had not a single recommendation suiting my price point. Having high quality speakers suddenly had a prison flavor. Trapped in high end audio, I felt.

I called it a day, checked reliable resources on the web, and made my decision for  insolent cheap NAD separates. I was lucky to get them  for around 1500 Euros and spent the remaining budget on room treatment. By no avail, has this combo ever failed to deliver musical pleasure. The NAD components C165/275 BEE, a design from 2012, were nowhere near the most recent developments, even 5 years ago. The Pre-amp 165Bee offered 2 tape inputs, a well reviewed phono stage, LFE out, one straight plus one adjustable output to the power amps, and a remote, which I barely use. 

It Just Lacked Everything Digital!!!

The power amp was a sour sherry to swallow. Design wise, it may have been very competitive if one aimed for shoeboxes in size 47 or black desktop cases for hypersized E-ATX mainboards from Supermicro. Its front plate takes up three times the space of my CAPS Zuma, which, at first glance, seems a wee bit odd. I found my peace with that, not only because it’s a tad bit more generously built than most of the newer models, but the NAD combo delivers a very natural sound to everything I have thrown at it. Until now, I have filed every flaw I've heard under room problems (1 Room, 15 sqm., 2 doors, 6 windows) not solved yet, bad sourcing, bad mastering or limitations in my DAC, rather than crediting the flaws to the very solid NAD combination. 

My system includes an iFi iDSD Micro BL DAC (Non MQA-Firmware), a modified CAPS V3 Zuma (I7-4790T/16GB RAM) using JRMC 24.0.75 on WS 2016 w/AO. The files for critical listening are served from an SSD inside the CAPS, both powered by HDPLEX 100W LPS. B&W 805s speakers are connected with Dynavox Perfect Sound twisted speaker cables. The cost of all cables is less than 150 Euro. My XTZ 10.17 EDGE subs, Gieseler Klein DAC and Uptone Audio Regen were disconnected.

Here's a little meme about my system. 

The Manufacturers & Their Truths

XTZ in Sweden is a manufacturer that is known for its direct sales of passive and active speakers, particularly for the affordable range of excellent subwoofers. The A2-300 Edge is the second amplifier model the company has produced, and its excellent performance should be put in perspective with its bargain price.

Wyred 4 Sound, a company from California/US, already in its second decade of building HiFi components.  Founder EJ Sarmento also exhibited his speaker creation @ RMAF 2018. because of this you can purchase a complete high end audio chain seamlessly from W4S. Wyred 4 Sound has used ICEpower amplification, as in the ST-750 LE, since 2007.

PS Audio from Boulder/Colorado/US is an established company for Audio and Power components, and digital products not only with the Direct Stream series, but since 2017 it offers the Stellar series of DAC/Pre and power amps, from which the S300 is the "cheapest" device.

Let Them speak truth! Thank you to Bill Leebens, EJ Sarmento and Hampus Morberg taking their time to respond to my questions.

Q1: Could you please describe your company in three lines we can't read on your website?

PS AUDIO (Bill Leebens):  "PS Audio is a family-owned company dedicated to the hi-fi family. We focus on high-value products that will last for decades, and that owners will enjoy using and be proud to have in their homes. Our insistence upon exemplary customer service is part of our focus upon treating PS customers like family."

Wyred 4 Sound (EJ Sarmento): "Wyred 4 Sound is a company built on value and performance. We control manufacturing processes and customer support to deliver the best possible experience."

XTZ (Hampus Morberg): "In three words, it would be "eXtreme Transparent Zound" This is not written on the website atm." 

Q2: Would you mind to talk about your "special sauce" for perfecting the pre-manufactured Class-D amplification?
 

Wyred 4 Sound: "Our design focus is on presenting a natural sound. One that is fast, dynamic, quiet and smooth. Discrete circuits and balanced designs help us achieve such goals. Our desired signature for all of our amps is neutral. We believe that the amp should not alter the presentation of the signal.  Its job is to amplify what it sees on the inputs. It is up to the artist to determine how the music should sound so an amplifier that sounds as if it is not there is ideal to us."

XTZ: "Technically solutions like stable cabinet to reduce resonances, internal twisted cables to lower cross talk and an input stage board with e.g. impedance adaptation. To get optimum performance when combining our amplifier to the vast amount of exciting audio equipment on the market. The rest is XTZ black magic ;D"

PS AUDIO: "Our Analog Cell is an open-loop Class-A MOSFET circuit, and is used to present the ICE Power module in its best light."

XTZ on its worldwide distribution model: "We mainly have dealers in Asia and Europe, however we do ship our products to all other continents also. Prices are the direct conversion from Euro to USD basically. And yes, we are able to send each and every product range to NA. For us, it is not effective to have a large warehouse in NA at the moment, Thus we are managing each NA order differently, shipping price is depending on the logistic complexity."

PS AUDIO on its Use of Class D - "Modules from ICE, Hypex, and others allow a wide variety of manufacturers to produce products that otherwise may have been beyond their design and manufacturing capabilities. However, just as with DAC chips, the implementation of the module determines ultimate performance. PS’ decades of experience enable us to take the same “ingredients” as other companies, and yield better results, just as a skilled chef can produce a banquet from humble ingredients."

Wyred 4 Sound's outlook on Class D - "I (...) believe Class D amplification -is- the wave of the future. It allows manufacturers to ‘go to market’ quickly with a cost effective and reliable solution. Because the size, weight and cost are all important matters moving forward."

Methodology and Choice of Review Tracks

Listening to audio equipment isn't really scientific unless one embraces academic approaches and methods. ABX and double blind tests were not part of my listening approach, as I believe, the majority of readers relate to a coherent systematic approach to critical listening of equipment, rather than a scientific test scenario that doesn’t match their personal realms. As this might be a controversial point of view, I took my time and space to enucleate my approach in detail.

During this review I did not test USB cables, DACs, Streamers, Software or Ethernet connections. I had 4 amplifiers do their best to amplify a once-digital signal into my modestly treated listening room. I had no possibility of conducting blind listening tests. 

The devices performed, after a long phase of break-in (up to 300 hours for each), under exactly the same conditions, neither in upstream components nor the room settings. I did the critical listening for each item under review on two occasions, with more than a week between listening sessions. Each session/device was calibrated with REW 5.1/UMIK-1 in the listening position with Pink Noise (-12db/2-channel) that maxed out at 71db. Noise floor was always below 43db, usually around 41 db. Max. loudness with the "low DR group" was around 99 db.

My aim was to set a performance baseline with my NAD Class AB power amplifier. The NAD matches almost all the technical specifications of the ICEpower driven amps, apart from their weight and the output power of the 700AS2.

With that baseline, I had to listen critically, to hear how different the CLASS D amps performed. I used this systematic approach, being aware of the flaws of human hearing and my different moods and biases. I have to live with them, and they are more often than not, part of my personal decisions. I sometimes envy people who can be happy based on measurements alone ...

Research on the usual high fidelity hot spots (14) all over the web had convinced me, that a weakness of Class D can be found in the representation of acoustic music, especially with strings. 

This is an audiophile commandment:  Class D can't sound warm, immersive or exciting because it’s "over-detailed"(15). Too much harshness, especially on female vocals, and often lacking definition in bass were on that list as well.

My music selection took this into account and therefore encompasses two important criteria:

1. The Class-D weaknesses:  Acoustic music, Strings, female voices, bass definition
2. The better part of the test music may be accounted as matching different audiophile styles 😉

I selected 4 groups of music to be used with the amplifiers:

1. The Chamber Pop Group - Acoustic music with female voices & strings
2. The Live Section -  Mostly acoustic with female voices, stand-up bass, Saxophone, clarinet, Violin/Cello, Piano and lots of Accordion, some grains of electric guitar, percussion and drums.
3. The Full panoply group: one tune that work with a philharmonic orchestra and one, that maxed out the 24 track recording equipment. I noticed during the process that one choice, the Quatuor Debussy was better assigned to the Chamber Pop group. My first choice, Jello Biafra & D.O.A.s "Full Metal Jackoff", was abandoned, as I finally felt it could have possibly -even after 30 years- been too offensive to some.
4. finally, the Low DR group: strings (again!), electronic and rock up to DR7

The Critical Listening - Group by Group 

All files processed were bought in 16-bit flac (24-bit when available) from Qobuz.fr, except Apocalyptica, Bob Mould, Penelope Houston and  Radical Dance Faction, which were ripped using EAC from the original CD. All files had been played at their original bit depth and sample rate.

The full information on tracks, covers, streaming links, reviews and links to the artist or their labels website can be found HERE. A short subtext on song choice & artists is in the subtext of each cover photo - you'll find in this "diaporama"

.

1. The Chamber Pop Group

• This Mortal Coil - You And Your Sister
• Radical Dance Faction (RDF) - Hope 
• Quatuor Debussy (QD) - Flamenco 
• Penelope Houston - Innocent Kiss

A midi-fied guitar sample and female voices were nothing short of evidence how exciting a Class AB Amplifier can perform, if it is fed with quality signals. The NAD provided delicious voices, a spacious soundstage, good pace and no incoherence in the highs. The XTZ was less exciting at times, when it should have provided a proper outlet for RDF's almost airy production. Dynamic and direct, it misses the bass trap punches and does not separate the instruments in the way the W4S did. Particularly, the S300  embraces the listener with the whirling saxophone. In comparison to the PS Audio, the ST-750 does not offer as much delicate pronounced tones, yet performs vividly and punchy. The cello and the guitars from Flamenco were slightly less intense as with the S300, but dynamically better than with the XTZ and the NAD. Sweetness, as it can be found in the QD's violins, were present with all amplifiers, although never as touching as with the S300.

Penelope's voice was clear and seducing with XTZ and NAD, again the staging of the XTZ is subpar with W4S and its detailed placement. Alone the musicality of the S300 is unrivaled from its competitors. That S300 gives these small “chamber” settings the air that they need to celebrate the artists and producers virtuosity. The W4S sounded seriously neutral, and less involved. It's a different kind of pleasure to listen with it. Certainly a step up from the NAD and the XTZ with more limited separation and spaciousness. 

2. The Live Section 

• Melody Gardot - March for Mingus (live)
• Nina Simone - Sinnerman (Live In New York/1965)
• Peirani, Paier, Carstensen, Gizavo, Parisien, Valcic, Lê & Baldych - Libertango (Live)

The tracks for the live section were selected for their intense and extremely precise impression of the captured performances, for the voice in despair of Nina, the insane saxophones of Irving Hall and the relaxed bass solo of Sam Minaie. 

The NAD & the XTZ made me feel upbeat, particularly the XTZ monoblocks which oversized the stage by a factor of 2. I felt the instruments settled between the walls and speakers, just the sax solo and the finale in March for Mingus then scratched a bit on the borderline of overly perceived sharpness, while the W4S gave an excellent presentation of Melody Gardot's stage without going overboard. But, her captivating voice sounded a hush to dark, in my view. Thus, the cadence of the Libertango was less joyful than with the S300, which in turn did not show a single moment of steepness in March for Mingus and enabled it to be the artist's intended ode to her music.

Nina Simone's 1965 evening in New York enables any system to shine. It features separation of instruments, a spacious setting for the handclaps and a haunting performance. I would not set the W4S behind the Stellar's performance, on the contrary, only the NAD and the XTZ lacked a bit in vocal expression, I felt. On Libertango, a set which always leaves me breathless and with the notion that it has found its end much too soon after almost 7 minutes, the excitement mostly comes from the air which whirls enthusiastically onto the listener. A sensation that is foremost provided by the S300, even the more powerful ST-750 can't move the membranes in that precise way. The NAD and the XTZ perform more neutral in this exercise. At this point, I need to confirm, that all amps have performed to my great excitement, especially with the selected music. Some perform better than others, and the better is the enemy of the good.

3. The Full panoply Group

• BBC Philharmonic Orchestra, Mena, Roscoe - Albeniz, Suite Española No. 1, Op. 47 (Arr. R.F. de Burgos for Orchestra): V. Sevilla (Sevillanas) 
• Queen - Bohemian Rhapsody

The BBC's Philharmonic Orchestra, conducted by Martin Roscoe, accomplishing Albeniz's Sevilla (Sevillanas), delivers a panoramic performance of joyful Spanish dancing tradition, mellifluous and symphonic, with grace and finesse in the gentle undertones.

The NAD is well tempered. Unfortunately the XTZ monos were unable to cope with the necessary separation of instruments. Again, the S300 outperformed the W4S. The castanets were perceived as more passionate, the woods as more voice like, altogether more pronounced and delicious than the very neutral ST-750.

The second selection, Freddie Mercury's Rock symphony "Bohemian Rhapsody" forced out, for my taste, the best sound from EJ Sarmento's W4S amplifier. It was concentrated, always dynamic, consistently controlling the overwhelming dramatics between Bismillah and Beelzebub. The more neutral attitude, in my opinion, serves the re-mastered version of this song very well. NAD and XTZ were excellent as well, but less immersive as the PS Audio, that gave me goose bumps from Brian May's guitar attacks.

4. The Low DR Group

• Apocalyptica - One 
• Massive Attack - Man next door 
• Bob Mould - Lost Fait

Apocalyptica's latest version of "One", the Metallica masterpiece, was recorded live at Tavastia, Helsinki. The group shows a clear structure (on video) and in the soundstage, the C275Bee reproduces the exact positions of the four Cellists, the up-right bass and the drums in the middle. It has no problem with their speed, but the soloist's virtuosity could be a tad more pronounced. XTZ equally showed good staging, again the vast stage left me perplexed with the monos, and the sound definition was not up to the NAD's performance. The W4S exceled through the dynamic passages. I could feel these euphonic sounds in front of me. However, with the PS Audio the music resonated with me. The strings sounded caring and energetic, then intensively reproduced the musical story from despair to the torment of this lonely recollecting mind.

In "Man next door" Massive Attack uses iconic moments of rock history to underline another desperate situation. The music this group has generated through programming and synths meanders through the soundscape, as do the samples of Bonzo Bonham's legendary kick drum sequence from "When the levee breaks". Again the XTZ monos can't offer as much excitement as the others. I feel that the sound quality is closer to its competitor when using electronic music. The NAD shows no weakness here, and the W4S expands the sound stage and has more breath in this piece than with acoustic instrumentations. The PS audio marvels again, every note has an impact. This is pure musicality on a cold and fearful tune, despite its insufficient dynamic range.

Finally, Bob Mould's Lost Faith finds its dynamic force through the voiced antagonism between acoustic misery and rejuvenated electrified energy. NAD and XTZ fare well with this piece, W4S impresses me on the power rock parts, while the Stellar sounds more touching on Mould's gloomy reflections. One last modest advantage for the ST-750.

All listening in full sight and complete absence of expectation/confirmation bias 😉

Verdict and Reflection 

NAD C275BEE (Class AB/2014/800 Euro)

The NAD has a powerful & neutral presence, with audiophile recordings the combo is sometimes, not always, capable of just beaming one into the music. It shows a well developed width to the sonic edges. The soundstage is more than acceptable in height and depth as well. I perceived the music as not overly detailed and a bit dark, which blends in well with the 805s speakers. The C275BEE tends to punch the bass a good bit, which gives the impression of power, but a lesser sense of structure. With the NAD & the 805s, which only reach down to 49hz, I made the decision to invest in two subwoofers for enjoying a deeper and tighter bass presentation at lower volumes. I tend to believe the NAD is a very solid performer for the budget, but it's far from perfect. Given the price point, I'd be lying if I said that I expect more. As usual, most often in life you get what you pay for, and the NAD C275BEE is purposefully not called master class by its engineers. That is a title reserved for the newest NAD M models, so why bother ...

XTZ A2-300 EDGE (Class D/2018/495 Euro)

The XTZ impresses with its vitality and transparency. The dynamics were better than the NAD. The separation of instruments were better in the stereo version though. While the bi-wired double mono amplification brought an enormous width in the room, the middle part of that huge soundstage just came tumbling down and the instruments and voices were starting to overlap. The precise and once energetic bass notes were mangled, and that unfortunate performance left me stunned. I had the impression that with the mono setting/bi-wiring installation the sounds should have been better. It may well be related with the mono/stereo setting module. Testing different settings just mirrored the problem. This was a sad result, as I had high hopes for this amp to challenge the W4S and PS Audio, especially with that double mono installation. It didn't play out that way. At the price point just below the 500 Euro mark, the XTZ A2-300 is a snap value as a stereo amplifier. I'd recommend the single stereo amp if you love it's smart design, it's sustainability and you are small on budget & room size. It is certainly a winner on the price/performance scale.

WYRED 4 SOUND ST-750 LE - MK II series (CLASS D / 2018 /1599 USD)

The Wyred 4 Sound performed excitingly powerful, extremely neutral and detailed. In my opinion, its forte is electronic and rock music with lower dynamic range, a scenario where it clearly outperforms the NAD. It was not as exciting in detail presentation, multi-dimensionality and air filling vibrations as the PS Audio and on par with NAD in that domain. Its bass performance was clearly better than the NAD’s, but in my perception lacked the contour and the fresh strength of the lower tonalities. even though warmth and immersion aren't its biggest strengths, it's definitely an great value for people who prefer powerful, clear and neutral performing gear. 

PS AUDIO STELLAR S300 (CLASS D/2017/1499 USD)

Finally, The PS Audio sounded excitingly well and very different than the NAD from day one, but gave its best performance only after the 300 hour mark. I accepted the manufacturer defined need for break in, even I am still skeptical on the subject (The XTZ for example weren't supposed to have any break in time, but I did it in anyway for  2 weeks in a row) as I can't find any convincing explanation from experts about the subject. Subjectively, however, it works just fine 😉

In my perception, the PS Audio gave my chain a real push forward, especially with material that marvels at a high dynamic range. I felt the notes exhaled into the soundscape with air to breathe and gently resonating into my room, creating a huge immersive soundstage. In conjunction with the B&W 805s, this amp performed probably the way that Paris Audio salesmen implied as the reason for tripling my investment. 

If you've ever enjoyed the sound and atmosphere of Kenny Burrell's Midnight Blue from Analogue Productions Remasters Series SACD, you may understand what I'd like to describe about the sound of this Class D amp. 

I did not miss my subwoofers when listening to the Led Zeppelin drum kick sampled by Massive Attack. Their Cure sample made me visualize Robert Smith sitting in the kitchen sink. The air pushed by Melody's horn section or the accordion quartet at the Berlin philharmonic simply shook me. Its soundstage, to me, occurred as immersive and three dimensional, a certain level higher and deeper than the W4S, and not as wide as the double mono XTZ configuration.

It was the first time that I noticed the vanishing of my speaker's position without a fundamental allocation of French wine. Indeed, this is a very subjective cognitive association, which was set in motion by this amplifier's peculiar performance with the 805's. I haven't known before, what my moderate audio chain was able to produce: the IFI iDSD black is an exciting performer at its price point, the NAD preamp, I have read during this research, had been  given praise by respected reviewers when It was fresh on the market, but still is a fully analog preamp for just 700 Euro. I did not use expensive cabling nor USB signal purifier/enhancers. Actually my Regen did raise some nasty noise in conjunction with all class D amps, which sidelined it for the time of the review. 

The sonic effects have been repeatedly dittoed during the writing process with my DSD playlist sans upsampling. It just does everything better than the other amps and adds enthrallment, warmth, speed, punch line and musicality into my modest chain. I’d like to recommend the Stellar S300 as a reference in the class for power amps up to 1500 USD, and I’d be happy to hear an amplifier below and above that mark, which pleases me the same way while listening to my music collection. 

I attribute this perception to the true dual mono design and the Class A analog cell input stage. With Class D, as Bill Leebens from PS Audio has put it, the careful choice of components and design matters most. I have read repeatedly that McIntosh tube amps make the 805s sing, and I’d be curious to know if that given collaboration would really make such a difference in SQ to suffice 4-5 times the price of the S300's 1499 USD? 

The Stellar S300 in combination with the highly resolving B&W 805s - in my opinion - lives up to the promises of its designer and performs sublime and extremely musical. 

I couldn't hear any shortcomings addressed to Class D amplification with that combo and my upstream chain. Being naturally curious, I still aim to find out if the ICEpower 1200AS boards excel better than their smaller siblings, not only on design but on SQ as well. Hopefully with both, stereo amps and monoblocks. By the way, If you're into monoblocks powered with Class D, you may want to listen to the M700 from the PS Audio Stellar series. The M700 features a higher power output provided by an ICEpower 700AS1 board, but the same alluring analog cell as the stereo S300. PS Audio told me it sells nearly the same quantity of M700s as S300s. Fascinating!

Follow-up Note: 

Something is rotten in the state of Class D - A personal assessment on the industry's information approach

ICEpower A/S, the designer & manufacturer of the Class-D amplifiers reviewed in this article, had been, from 2008 until 2016, a fully owned subsidiary of Danish consumer electronics manufacturer Bang & Olufsen, succeeding the previous seven years of co-ownership with founder Dr. C. Nielsen.(16) In 2016, Danish private equity fond Industri Udvikling's (IU) financed a management buyout from B&O with a 44% shareholding in the ICEpower Holding A/S (17). They (IU) report (18) "In 2017, a new strategy was added, which included focuses on developing more complete sound solutions"* ICEpower products are designed in Denmark and manufactured in Malaysia and Thailand. Operating in a market with an estimated  annual growth rates around 15% (19), ICEpower still has to regenerate from transformation, and their dwindling financial figures for 2017 despite their merger with Audio Bricks from Sweden in 2016 (21) (decrease > factor 2,2 since 2014/2015)(20) may explain, why the appeared to be extremely contained and discreet on disclosure about forthcoming (actually announced since 2016) products or future developments.  

About 2 years ago, I read a homestory (22) about ICEpower and the newest developments in its orbit. Since the end of 2016, the newly developed ICEedge chipsets were destined to make an enormous impact on the audiophile amplifier market. When the1200AS amplifier series finally hit the street in April 2018 (23), it was supposed to be "the most powerful amplifier to date, while at the same time minimizing noise and distortion" (23) having an ultra-low noise floor and superior sound quality. While some small announcements about the new products were seen towards the end of 2017 (24), the first wave of public presentations and call for testing came after the NAMM 2018 (25), and resonated in the first 1200AS2 modules sold under the "Purepower" moniker by Mivera Audio (26). Suddenly raving reviews (better than ... xxx . and equal to .. Pass Labs 250.8 ...)  could be found throughout the audiophile fora & fauna (27), stimulating a great number of Class D threads everywhere, especially 1200AS related, which all too often were repeating the hearsay, underscored by the impressive tech specs.

High-end manufacturers like Apollon Audio, Nord Acoustics or Rouge Audio Systems then announced ICEedge based models for the second half of 2018. While we can see a "true dual mono" 4-Channel (sic!) stereo amplifier build from two 1200AS2 stereo modules on the Apollon website (28), no monoblocks using the1200AS1 module can be found aside one listing I found at EBay (31/03/2019) until the end of March 2019. I read in another forum, they were due to start shipping near the end of March 2019. Therefore, I asked the ICEpower press office, with which I have been in communicating for this article since the start of March, if there had been an unintended delay in RFM-date for that module, and if there would be announcements about it this year, which they convincingly left unanswered.

The policy of communication I have experienced with them had been somehow strange: before I sent the first series of serious questions to them, they mentioned the possibility of an invitation to their headquarters close to Copenhagen. OTOH, their quick & factual responses allowed me to retrieve important clarification on the 300AS1 and 700AS2 boards, which are used in the amplifiers on review. 

To the contrary, ICEpower's press officer friendly declined to respond about informative subjects like the application of eGAn-FETs in future designs, the future use of ICEedge design in the 300AS/700AS product range, couldn't advise resources for financial information nor granting access to an ICEedge datasheet, because "The datasheet for ICEedge is only available for very large customers". Still today, 2nd of April, when I've received a response to several questions I had asked the week before, particularly about the status of the 1200AS1 boards, including one question about visiting their HQ for an article on ICEedge and the 1200AS module, they responded with a general statement and  my 1200AS1 related questions are unanswered.

During my research, I had to notice surprisingly, how I was blue-eyed & foolish, when reading these lines about the A2-300 from XTZ on the manufacturer's homepage: "Besides the performance it is also the sound quality in which the latest ICEpower® Class-D technology outweights the traditional ways of amplification" (29). I went there in November 2018, because I had to upgrade my XTZ 10.17 subs with their newest amps, that were announced with the line "by making use of the latest ICEpower® technology, the RMS power output increases to 700W" (30). Let's put it like this, just after inquisitive questioning with ICEpower in March, I was fully convinced that their latest technology is only used in the 1200AS modules, and that neither the 300AS1 nor the 700AS2 are anywhere near that ICEedge technology. 

XTZ have confirmed later, during the Q&A, that none of their products make use of the ICEedge technology. Hampus from XTZ Illuminated this curious case, as to why they surf on the marketing effect of ICEedge, to me as one obvious error in my overcomplicated perception: "The XTZ EDGE series is a family name for our products that uses the cutting "edge" technology, Simple as that. We believe that amplification-technology has come very far and that we use modern advance & technology compared to many older designed amplifiers."

The Wyred 4 Sound ST-750 LE power amplifier correspondingly, while making use of the 700AS2 fully integrated stereo design, had been “unfortunately” announced since last year as ICEedge based (31). During March and our Q&A, EJ Sarmento then confirmed my findings on the ICEpower 700AS2 module (to be non-ICEedge) as correct, and subsequently changed his information policy for this device. While ICEpower does advertise ICEedge as first new advancement since the introduction of the ICCx chipset 15 years ago, and confirmed to me: "700AS1 and 2 uses the ICCx Class D chipset. 300AS1 build discreet, so it is not using the ICCx chipset" and that for the 300 AS1 module was not technically updated since it's invention 2014, but it had a name change in 2016, as does the 700AS1, which notes also minor design changes in 2017. "Only minor changes and smaller safety EMC improvement. But it is smaller changes that isn’t relevant for end customers."

These quotes sound refreshingly "un-edgy", though Wyred 4 Sound have now used a different, more elastic approach on the facts: "For the ST-750, we decided to use the new 700AS2 module from ICEpower.  Originally thinking this followed the same design platform as the 1200AS module, we thought it would be good to inject some newer circuitry to our lineup. Although it is not made with the new ICEedge topology, this module is the newest non-ICEedge circuit and was the logical stepping stone for ICEpower to use before completing the ICEedge standard found in the 1200AS1/2 module." (32)

I wonder, if we audio enthusiasts as consumers really deserve this kind of factual yoga??? Obviously, there was a vacancy in the actual ST product range, and the 700AS2 was the logical choice ...! If you read carefully about module pricing when looking for case & amp manufactureres, you may notice a difference of 250 USD between the 1200AS2 and the 700AS2.  … We all have to make choices, at some point ...

Important: I had my critical listening mostly near the end of February & beginning of March, and finished it before I have received the first replies to my Q&A with the manufacturers. 

The findings about sound quality weren't influenced by my feelings that came forth during the last 2 weeks. I was contemplating much too long how to process all this information I had found out of the ICEpower universe, while I was actually hoping to present you some interesting listening impressions on the (newest) Class D technology. I have now followed my inner self, that puts personal integrity before my usual kindness, sometimes to my very regret. 

This review consequently had to be shaped differently, showing more disclosure on the state of the industry players involved than on the revelation of sound aspects. 

Nonetheless, I have also been “excited” in terms of audiophile principles, while listening to that particular equipment! 

Plus: My dogs have confirmed to me, with their permanent presence in my listening room while listening to my playlist up and down relentlessly on inevitable intense intonation, that my perception regarding quality of sound must be trusted, undisputed.  

… They have so much better & advanced hearing than my wife in the kitchen 😉

Tom V.

Footnotes:

1. https://www.eetimes.com/document.asp?doc_id=1274757#  
2. https://en.wikipedia.org/wiki/Class-D_amplifier 
3. http://www.bwgroup.ch/2017/03/17/2791/ 
4. https://www.soundandvision.com/content/bruno-putzeys-head-class-d#CXp7PKRtFihBG7E8.99  
5. http://www.bwgroup.ch/2017/03/17/2791/  
6. The AS-Series brochure - https://icepower.dk/download/1627/  
7. 300AS1 Datassheet - https://icepower.dk/download/2420/  
8. 700AS2 Datassheet - https://icepower.dk/download/1624/  
9. https://wyred4sound.com/products/power-amps/mono/mamp  
10. https://wyred4sound.com/products/power-amps/stereo/st-mkii-series  
11. Figure 15 of 700AS2-datasheet  
12. https://www.psaudio.com/stellar-gain-cell/  
13. https://www.psaudio.com/pauls-posts/stellar/  
14. https://audiophilestyle.com/forums/; https://www.audiosciencereview.com/forum/; https://forum.audiogon.com/discussions; https://www.audiocircle.com/; https://www.stereophile.com/; https://www.miveraaudio.com/forum/; https://forums.audioholics.com/forums/; https://www.diyaudio.com/forums/; https://www.avnirvana.com/; https://www.gearslutz.com/board/; https://www.whatsbestforum.com/, https://www.audioshark.org/amplifiers-8 ; https://pinkfishmedia.net/forum/  
15. https://primalunablog.com/2015/01/06/7222/  
16. https://en.wikipedia.org/wiki/ICEpower  
17. http://www.industriudvikling.dk/portefoeljevirksomheder/1860/icepower-as/  
18. http://www.industriudvikling.dk/media/394919/IndustriUdvikling_presentation_2018-19.pdf  
19. https://honestbusinessman24.com/tag/class-d-audio-amplifier/  
20. http://www.industriudvikling.dk/media/394919/IndustriUdvikling_presentation_2018-19.pdf, P. 30  
21. https://en.wikipedia.org/wiki/ICEpower  
22. https://www.audioxpress.com/article/amplifier-series-a-new-generation-icepower     download - https://www.audioxpress.com/files/attachment/2643 
23. https://www.audioxpress.com/news/icepower-announces-its-most-powerful-amplifier-module-to-date  
24. https://www.nordacoustics.co.uk/single-post/2017/11/23/New-BO-ICE-Power-1200AS2-Module-arrives-for-Evaluation-First-module-to-use-the-new-ICE-Edge-chipset-600W-into-8Ohms-1200W-into-4Ohms  
25. https://icepower.dk/news/story/icepower-is-back-at-namm-2018/  
26. https://www.miveraaudio.com/forum/purepower-amplifier/purepower-icepower-1200as2-amp-listening-impressions  
27. https://www.whatsbestforum.com/threads/modwright-vs-audiaflight-power-amps.25579/  
28. https://www.apollonaudio.com/icepower-1200as2-based-class-d-multichannel-hi-fi-amplifier/  
29. http://www.xtzsound.com/product/edge-a2-300
30. https://www.xtzsound.eu/product/sub-12-17-edge  
31. http://www.10audio.com/wyred-4-sound_dac-2v2se-10th-anniversary.htm and https://www.eliteaudiouk.com/product-page/wyred-4-sound-st-750-le-stereo-amp  
32. https://wyred4sound.com/products/power-amps/stereo/st-mkii-series