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Positive Feedback ISSUE 73
may/june 2014

MBL Corona C51 Integrated Amplifier
text by Wojciech Pacuła, images by Wojciech Pacuła and mbl. translation by Andrzej Dziadowiec

MBL started in 1979 with a design Radialstrahler mbl 100 called. This name should ring a bell for many of you—tall loudspeakers shaped like a rocket missile, with unique drivers to generate sound. They look like a sheaf of metal petals (what MBL calls "lamellas") arrayed vertically in a circle to radiate sound omnidirectionally. In a well-adapted listening room, with top electronics, preferably from MBL, they sound truly spectacular and unlike any other speaker. 1982 saw the first preamplifier with the MBL logo, followed by a power amplifier in 1989.

This is a German manufacturer through and through, i.e. showing a German work ethic and care for long product and service lifetime. With offices in Berlin and a factory in the idyllic Eberswalde, 30 miles from the German capital, the company employs a total of 50 people. ‘MBL' is an acronym from the names of its "founding fathers": Meletzky, Beinecke and Lehnhardt, with Wolfgang Meletzky being Radialstrahler's designer. This long-time chief MBL designer built his first portable radio at the tender age of 11. This was followed by a series of DIY speakers during his high school years. He eventually left MBL in 2005 to found MC (Music Culture Technology), where he offers more conventional design solutions.

The Corona line, which includes the C51 integrated amplifier under review, was designed under the guidance of the new company head, Mr. Christian Hermeling. It currently represents MBL's entry-level line, followed by the Noble and top Reference line. Previously, MBL's product lineup started with the Classic series. The Corona is not, however, its direct successor as products from the Classic line were relatively—for high-end, that is—inexpensive, two or three times cheaper than their counterparts from the new entry-level line. Design and assembly solutions are also quite different between the two lines. MBL now use enclosures that are as solid as those used in products from the more expensive models, but with a different shape. These are no longer regular rectangular boxes, but their top is gently curved down at the sides, with a distinctive flat surface in the center. This is where all the controls are located, including a large display. While the C51 comes in two main color finishes, black and silver, more options are available for metal details on the front panel, including the volume knob. They can be ordered in gold or chrome (the company calls the latter "palinux", which may denote a special alloy) and there is a choice of black, chrome or gold finish of the top panel central section that sports an illuminated MBL logo and display dimmer button.

Some of the technical solutions employed here are also new for this manufacturer. The C51 is an integrated amplifier that combines a powerful, linear power supply and analog class D power amplifier. Christian Hermeling prefers to speak of ‘class D' in inverted commas. In his interview for the "Mono & Stereo" magazine in 2010, and hence before the launch of the Corona product line, he described class D in the following words:

Nearly 95% of all PWM class D amps on the market are sounding much too bright, too thin and artificial. This would not be our target. But similar to our different thinking about our loudspeaker, we are working on a different method with PWM amplifiers. So with this class D design concept we will have a totally neutral distortion behavior. We are still in the design phase, but maybe in the future, for some of our products we will release this new technology.

Besides the class A output stage […] we also have direct push pull and isolated gain cell technology in our amplifiers. Direct push pull means that we do not have any differential input stages where we would split the signal into positive and negative waves. We are directly driving it from the input of the transistor both phase in and off phase so we have a much faster and much detailed signal transfer but the main thing is the isolated gain cell. Every transistor has some nonlinearities that change the performance over current, over voltage, over temperature and we have simulated and measured all of the misbehaviors of the transistor and we compensate all the misbehaviors and this is the so called isolated gain cell. The transistor that makes the sound and the amplification in the amplifier is held absolutely constant so the voltage on the transistor and the current will never really change the performance of the transistor. This is a really unique point you will not find this in any other product. […]

Matej Isak, Exclusive Interview with MBL Akustikgeräte, Germany, "Mono&Stereo" 2010, see HERE.

Jürgen Reis, current head of design and technical department at MBL, claims to have designed a PWM amplifier that has the advantages of class D without exhibiting its problems. The latter include low damping factor, vulnerability to variable speaker impedance and its impact on amplifier's frequency response, the impact of cable inductance and—perhaps most importantly—problems associated with THD distortion. In a typical class A or AB amplifier THD increases proportionally with power output increase. In class D, it is the other way round—THD decreases with increased power output to reach minimum at maximum rated power output. THD level quoted in the C51 specifications is measured at the maximum rated power. Jürgen Reis refers to his design as the Linear Analog Switching Amplifier (LASA) and refrains from calling it "class D".

And what about the name ‘Corona'? It has to do with the amplifier's external design and is highlighted by two elements—a standby switch and large 40-mm display dimmer button on the top panel. They are encircled by a white illuminated ring that forms the shape of a "crown".

Sound

• In The Mood For Love, OST, Wong Kar-wai (director), Virgin France 8505422 8, CD (2000).

• Art Farmer and Jim Hall, Big Blues, CTI/King Records KICJ-2186, "CTI Timeless Collection 40", CD (1978/2007).

• Bajm, Ballady, Pomaton EMI 8 55988 2, CD (1997).

• Black Sabbath, 13, Vertigo/Universal Music LLC (Japan) UICN-1034/5, 2 x SHM-CD (2013).

• Czesław Niemen, Dziwny jest ten świat…, Polskie Nagrania Muza/Polskie Nagrania PNCD 1570, CD (1967/2014).

• Deep Purple, Now What?!/Now What?! Live Tapes, EAR Music 0209064ERE, 2 x CD (2013).

• Depeche Mode, Soothe My Soul, Columbia/ Sony Music/RiTonis, ProXLCDr/P.0006, SP CD-R (2013).

• Diary of Dreams, Elegies in Darkness, Accession Records A 137, "Limited Edition", CD (2014).

• Frank Sinatra, Songs For Swingin' Lovers!, Capitol/Mobile Fidelity, UDCD 538, Gold-CD (1956/1990).

• Heart & Soul, Heart & Soul presents songs of Joy Division, 2.47 Production CD EDITION 033, CD (2013).

• John Coltrane, Coltrane's Sound, Atlantic/Rhino R2 75588, CD (1964/1999).

• The Cure, Disintegration, Fiction Records 8393532, CD (1989).

• The Doors, The Doors, Electra/Warner Music Japan WPCR-12716, CD (1967/2007).

• The Pat Moran Quartet, While at Birdland, Bethlehem Records/Victor Entertainment VICJ-61470, "Bethlehem K2HD Mastering Series, No. 20", K2HD, CD (1957/2007).

• Vangelis, Spiral, RCA/BMG Japan 176 63561, K2, SHM-CD (1977/2008).

An amplifier of this class, so nicely built, coming from a company with a long tradition and—last but not least—with this kind of price tag, is bound to be met with high expectations. In C51's case a few things—actually a whole lot of things—are given right from the start, even before playing the first album. First of all, its looks. The machine has all the features of German audio products, including large, shiny metallic surfaces combined with a deep black finish. It looks very serious and is really well thought out. Secondly, build and assembly quality. It is truly exceptional. What we get here is not merely good craftsmanship but also own technical solutions, which are an important ingredient of what we might call a "product". Last but certainly not least, the sound—a key, if not the sole, factor for many audiophiles (let me repeat that I think of a "product" as the sum of various components, of which the sound is one of the most important, but not the only one). The sound is very interesting and largely defines what is currently possible in the field of class D amplifiers.

The MBL's sound is open. Earlier on, I quoted Mr. Christian Hermeling who believes that class D amplifiers sound cool, cold and analytical. He must be talking about other design solutions than those I have come across, as my experience is exactly the opposite. In my opinion, no longer than two years ago most amplifiers of this type, with a few exceptions, could be characterized as sounding fairly warm, with a rounded treble. This was the sound offered by amplifier modules from B&O, Tripath and other manufacturers (except for a fully digital TacT amplifier). Even if there were differences between particular designs and their applications, they would all follow an easy-to-identify "pattern".

While the C51 exhibits some of the sonic characteristics that are distinctive of this technology, its sound is much more refined. What is most evident is its openness and expressiveness. It is no longer a definitely "tubey" sound, with all the major elements of the vacuum tube technology stereotype. The presentation is not darkened, with eased attack and homogeneous tonality. Or at least not as much as its predecessors'. I think that in a blind listening test, without knowing what's "under the hood", a large number of listeners would be convinced that it is a class A amplifier, from the Accuphase and Luxman camp. In this comparison, the class D amplifiers I referred to earlier would be close to class A, but rather that represented by Sugden.

Imaging is no less important. Products from this manufacturer are famous for their incredible soundstage, most striking with MBL omnidirectional speakers. It is the electronics, however, that has the task of shaping the signal in such a way that the speakers, very transparent on their own, have SOMETHING to show. You can clearly hear that the C51 designers' attention was focused precisely on this aspect. No matter what kind of speakers the amplifier will be paired with, we get a big, expansive soundstage. Events take place in the whole space between the speakers and beyond. Far up, the soundstage does not shrink to a narrow line, but rather spreads out in a wide semicircle. One can also be pleased with soundstage saturation. It is not even so much the amount information as the accompanying "air", which is very suggestive.

While it is stereophony that benefits most from that, mono recordings also come out quite interesting, like the new remaster of Niemen's Dziwny jest ten świat…. The amplifier portrays substantial large-volume phantom images. That is, they are "large in width", and is not but a faded afterimage of what originally had a certain weight, depth and height. Speaking of height, it is this aspect of imaging that is usually completely ignored by the sound engineers. It's because they don't "believe" that it exists at all and that a stereo recording, based on differences between the left and right channel, is also capable of capturing any differences in the vertical direction. Anyone who has ever had a chance to listen to a high quality audio system knows that this is simply not true, and that it is perfectly audible that some instruments are located higher and some lower, no matter which frequency range they occupy. The MBL amplifier clearly shows these imaging details.

It is much to the credit of its strong, saturated midrange and upper bass energy. The whole 200-600Hz range is "boosted" and this is where most of the action happens. It is manifested by a large volume, strong images and dense midrange. On certain recordings, this causes a thickening of vocals and their "strengthening" from the bottom up. Beata Kozidrak's vocals on Bajm's Ballady, slightly on the light side, here gained something extra "behind" them that "equipped" them with density and meatiness. The latter also made the bass drum extremely strong and suggestive. It was cool to hear so great-sounding tracks from Black Sabbath's 13 and Deep Purple's Now What? Live Tapes. Followed immediately by electronica on Vangelis' album Spiral. That is a real value as most audio systems compress the sound and make the bass drum seem to be far in the mix, irrelevant. Part of the problem, of course, has to do with the recording itself. Most sound engineers compress the bass drum sound on purpose, so as not to "disturb" later mastering work. But it is the compression introduced, most notably, by speakers and then amplifiers that has even more impact on this kind of sound. The C51 seemed to sound more dynamic and, hence, more impressive.

The thickening referred to earlier did not muddle the sound as much as it "energized" it. Since there was no evident bass hardening and contouring the attack, the sound was meaty and full. And while I earlier distanced myself from the tube stereotype, I will now say that the MBL sounded like a good tube amplifier. I am not saying "tubey" but tube. Distinctive and exhibiting a specific combination of sonic characteristics. Good characteristics, I might add.

There are, however, two sonic aspects that make it sound not like a tube amp. As I see it, they both have to do with its high power output. These are the—already mentioned—high dynamics and selective treble. Both of them seemingly expected from and normal for a powerful solid state amplifier, they are not often seen combined with the set of characteristics mentioned above. Most rarely in tube amplifiers.

The dynamics results directly from mid-bass presentation, especially its density and agile weight. This builds up a large image in front of the listener, without blurring the low registers. Treble, on the other hand, is slightly sweet, i.e. smoothed out, and selective, which echoes what is audible on the bottom end. It is the treble that most clearly distinguishes the C51 from other class D amplifiers. The top is neither sharp nor emphasized, yet it shows significant energy. This opens up the soundstage and helps to improve the reverb and instruments' acoustics.

The way the amplifier handles both the dynamics and treble is very refined. I can find no other word to describe what I heard on Niemen's album. In the track titled Wspomnienie, the rhythm provided by a maracas-sounding percussion instrument was placed by Mr. Jacek Gawłowski, who was responsible for the remastering, slightly up front, before vocals. This results from a specific tone control setting. The German amplifier showed a dense percussion texture in front of Niemen, without obscuring the vocals that were most important in this recording. Soundstage depth was exemplary here and each of these elements had its own plane, including other instruments in the background. The timbre of "maracas" was excellent—neither sharp, light nor dull. Not so long ago, class D amplifiers operating were not capable of such a feat. The C51, together with the recently reviewed Jeff Rowland Continuum SII, are now catching up with the best amplifiers operating in class A and AB.

Conclusion

Technology dictates results, there's no escaping that. Only in mass-produced, inexpensive, not to say cheap, audio components the differences are blurred to such an extent that it is difficult to tell one from another. Products that are featured in "High Fidelity", those that bring a deeper dimension to music that is no longer limited to just melody or rhythm (depending on listener's preference), show significant differences. The real mastery is demonstrated by those manufacturers that are capable of turning the sonic characteristics of a given audio technology to make them work to their advantage. Although the reference point—the sound of unamplified musical instrument or human voice as heard live—is the same for all, it can be achieved in a number of ways.

The MBL designers made use of Class D advantages, adding to it something new that expands the boundaries of what used to possible with that technology. The result is a meaty and dense sound with high dynamics and open treble. Its greatest asset is the combination of these characteristics. The sound is imaged in a huge space, has a large volume and is well differentiated. However, instruments' textures are not particularly distinct nor is the depth of individual images. Tit for tat.

Design

The machine is a real beauty. It has its own characteristic "chic", manifested by gold-plated details and true piano black finish. This is an integrated amplifier with a PWM class D output stage. The front panel is adorned with an aesthetically pleasing green fluorescent display screen. It is essential for communicating with the C51. The five buttons above and volume knob below are only identified by on-screen labels. The buttons are used to activate the mute mode, turn off the power amplifier and enter the menu in order to select an active input. Besides button labels, the screen is used to display, in large font, the currently selected input and volume level on bar graph below. The machine is turned on and off with a small illuminated button on the left side of front panel, featuring MBL logo. A large button with MBL logo on the top panel is used for display dimming or switching it off.

‘Top panel' is not quite precise description in the case of this unit. It suggests a uniform flat surface, which is only true here about a metal part in its center. The sides are curved down to give all products in the Corona line their distinctive look.

Although this is an amplifier, no cooling vents are in sight. They are only to be found at the rear, but they are not too big. The RCA connectors visible below are of high quality, a real deal. Gold-plated speaker binding posts are quite solid, although not of the same quality. There are six line inputs that are divided into three groups. The first group includes a single pair of balanced XLRs. A pair of plugs above them may be pulled to install an extra pair of RCA input connectors (analog) or digital inputs. The latter need an optional DAC board. The second group includes CD1, CD2, and Processor inputs (e.g. for home theater). Adjacent to them are two more pairs labelled Tuner/AUX1 and AUX2. While they look the same, their input impedance is different, 5k Ohm vs. 50k Ohm, respectively. It looks like the lower impedance inputs offer a shorter signal path, without an input buffer. I think it is hence preferable to use the CD1/2 inputs, even if our player has a tube output stage, as in my case. Please note that I quote impedance parameters from a leaflet supplied with the amplifier. Materials downloadable from the manufacturer's website claim the RCA inputs impedance of 9k Ohm.

Above the usual IEC socket with a rocker switch there are two Ethernet ports that can be used to connect several MBL components into one functional whole. Adjacent to them is an SD card slot for software upgrades.

The C51 is a very solid machine. The enclosure is made of different materials: acrylic (front panel), thick aluminum profiles (sides and top) and steel (bottom and rear). The two curved sides of the top panel open down on hinges. The whole amplifier is very heavy. It combines a class D output stage with linear power supply. The output stage has its own dedicated power supply, with separate secondary windings for each channel, while the preamplifier and microprocessor circuits are also powered separately. Hence, we see two toroidal power transformers. Both are solidly shielded—the company literature mentions MU-metal shielding. Additional shields separate the input voltage filtering and logic circuits as well as audio signal circuits.

Selection of the input signal is by input relays. The XLR input has a dedicated PCB with input buffers on the Analog Devices AD797 opamps. The preamplifier circuit on a larger PCB features the NE5534 and more AD797 opamps. It seems to employ a balanced topology and the signal from RCA inputs is symmetrized. Quality capacitors and resistors abound throughout, the latter being of the oversized SMD variety. Volume control circuit is an interesting affair. One might expect a resistor network but I couldn't find it anywhere. Instead, we see a classic rotary potentiometer with a long shaft to allow control via the front panel, soldered directly to the preamplifier board at the back of the unit.

The largest main board houses a filtering capacitor bank—the manufacturer evidently treats the issue of power supply seriously. Low-current circuits have their own dedicated power supply. Bridge rectifier diodes are mounted to a solid heat sink. This is one more surprise as class D amplifiers usually do not require such a large cooling surface. The output stage is tiny in comparison and consists of two small PCBs with small T-shaped heat sinks, mounted to the large heat sink mentioned above.

The remote control comes in aluminum enclosure. It is used for volume and input control, and for display dimming. It can also control an MBL CD player. As the buttons are close to one another and uniform in shape, it is not particularly comfortable to operate.

Technical Specifications (according to the manufacturer)

Rated Power

• 2 x 300 W (Stereo 4 Ohm)

• 2 x 180 W (Stereo 8 Ohm)

• 2 x 400 W (Stereo 2 Ohm)

Input Impedance CD1/CD2/Symetric (RCA / XLR): 5 kOhm /18 kOhm

Input Impedance Tuner/AUX2 (RCA): 50 kOhm

Input Sensitivity (RCA/XLR): 100 mV/200 mV/1 W/4 Ohm

Signal / Noise Ratio (Rated Output): 124 dB/300 W/4 Ohm