The KR Audio Kronzilla DM (Dual Monoblock) Power Amplifier
by Dick Olsher
 |
Does Size Matter?
Towering over 13 inches, the T-1610 aka Kronzilla, represents the most substantial audio power triode put into production since the days of the Western Electric 212E. And while, in some quarters, it has earned the appellation of the Beast from the East, its effect on an unrepentant tube-aholic such as yours truly, has mainly been instant adoration. From its brass base and meticulous internal construction, to its hard glass envelope, this hand-crafted colossus is a thing of beauty. Oh yes, during handling of the tubes, I have to admit to a slight fear of letting this $2750 tube statuette slip through my fingers and go crashing to the floor. But I got over it once they settled securely into their sockets. And yes, size does matter: the Kronzilla is capable of producing 22 watts of pure, unadulterated, single-ended triode power. A small tube simply does not possess the anode heat capacity to safely sink and dissipate the thermal load. In addition, The T-1610 features a new ribbon filament construction and KR Audio's patented distributed cathode with 64 cathode elements.
During the early 60s, with tube technology under assault from semiconductor devices, engineers at GE unveiled the Compactron tube, a concoction of several tube types housed in a common envelope and aimed initially at compacting radio design. At about the same time, RCA was promoting its most advanced research product - the nuvistor - a miniaturized, metal encapsulated, tube with ruggedized performance for military applications. By the early 70s, tube technology R&D dollars dried up as it became crystal clear that the transistor tide could not be stopped. Tubes had lost the war - at least for mass market applications. It was left to a visionary, such as the late Dr. Riccardo Kron, to rekindle tube technology after about 30 years of nothing more than copy cat tube production. KR Audio was built up in the early 90s from the remains of the Tesla tube works, enlisting former employees and critical tooling. From its humble beginnings, and despite of initial skepticism and distrust, production of both old and new tube types became firmly established. Today, under the leadership of the lady boss, Eunice Kron, and with Marek Gencev as chief engineer, KR Audio is firmly in control of its destiny. The company has retained its core expertise in tube manufacturing and has continued to expand its own line of high-end audio amplifiers.
Tube production at KR Audio remains a unique and fascinating mixture of artisanal hand labor and special tooling. 
Take, for example glass shaping, which is not only labor intensive but requires considerable skill. The whole manufacturing process takes days to finish. Of course, these manufacturing methods would not do for large-scale mass market needs, but are ideally suited to meet high-end audio demands at production rates up to about 300 tubes per month.
Much effort is spent on ensuring a perfect vacuum, as a high vacuum is considered to be the most critical aspect of a thermionic tube. Each tube is evacuated to a residual pressure reading of one billionth of a Torr – a hard vacuum that is several orders of magnitude superior to standard industrial production techniques. Each tube undergoes multiple QC inspections and tests. Batch lots are tested for static measurements, and then dynamic functions to assure years of trouble-free operation. Tube life has been extensively tested and is said to typically exceeds 10,000 hours. KR Audio confidently guarantees its tubes for one full year.
The Amplifier
So far I have focused my attention on the most visible asset of the Kronzilla DM, namely its twin towers of power, which on my pair of review samples were protected by the optional but extremely stylish "Butterfly Wing" tube guards. It's now time to turn our attention to the bigger picture and take stock of an amplifier that was designed to unleash the Kronzilla's full power. It is be a bit surprising to realize that the signal path is entirely solid-state (JFET and MOSFET) up until the output stage. First of all, KR Audio's core business is tube production. And since the renaissance of SET in the US during the early 90s, I can't recall anything but all-tube SET designs. Single-ended triode (SET) power amplifier designs are a throwback to the 1920s, having virtually disappeared from the marketplace by the late 40s, displaced by the engineering "common sense" of push-pull designs. To be sure, a hybrid topology represents a departure from the classic, vintage tube approach, but is in keeping with KR Audio's design philosophy of minimizing distortion and maximizing musical resolution. KR Audio considers the use of both tubes and transistors to give audiophiles the best of both worlds.
The power supply uses solid-state rectifier bridges and a beefy filter network. The output stage consists of a pair of the T-1610 connected in parallel and operating in class A1. Cathode resistors are used to auto-bias the power tubes. Even though cathode or auto bias is not as efficient a scheme as fixed bias, as some of the B+ high voltage (500 V) is "wasted" across the cathode resistors, it ensures stable operation over time. The output transformers were custom-wound for this application, and feature both 2 and 4-ohm output connections. However, only one set of output taps is provided and the change from the factory setting of 4-ohm to 2 ohms requires an internal adjustment.
Kronzilla meets BassZilla
As the virtues of SET amplification gained a wider audience, more and more audiophiles have naturally been tempted to invest in the technology. Some manufacturers, eager for a wider market reach, have understated the requirements for matching speakers. I'm always shocked to discover the insanity of a manufacturer at CES demonstrating a 2A3 based SET with an 89 dB sensitive loudspeaker, and with a straight face hyping the resultant sound. No thank you, I'm not into overload distortion. Such an amplifier can barely eke out two clean watts and is really meant for super efficient speakers. Neither is efficiency alone a guarantee of compatibility. The impedance magnitude should also be sufficiently benign to minimize current demands from the output stage. I have been able to easily drive a 2-watt amplifier into overload - even with a 97 dB efficient speaker as my own Lowther BassZilla. Another myth being circulated is that tube watts are more powerful than solid state watts, and therefore go further in producing adequate SPLs. Well, if you believe that, may I interest you in the purchase of a certain bridge in Brooklyn. It's true that SET amps clip more gracefully and may give the impression of having more horsepower than similarly rated solid-state designs. But in reality, low-power amplification runs out of steam, and often, when coupled with typical modern speakers. The average audiophile speaker sensitivity is probably in the range of 86 to 89 dB, and as a minimum requires a 25 wpc amplifier in a typical domestic listening environment. Coupling such speakers to lower power amplifiers runs the risk of severe dynamic compression and distortion. More extreme loads demand even more power. A case in point is a pretty quiet electrostatic loudspeaker (ESL) such as the Sound Lab A-1. It needs to be fed in the range of 100 to 200 wpc for proper operation. I've seen the statement that by their capacitive nature ESLs are "wattless" transducers and therefore do not require powerful amplifiers. Again, utter nonsense. A capacitive load is very current hungry and although much of the power is not dissipated resistively in the load, it is kicked back and must be dissipated in the amp's output stage.
The above discussion highlights the fact that an amplifier is a power transducer and that the amp-speaker interface needs to be evaluated as a system to ensure compatibility. Enter the Kronzilla DM. It is able to sink over 40 watts per channel of SET power into a real-world audiophile speaker load. That effectively cuts through the Gordian knot that has frustrated so many audiophiles looking for SET nirvana. You no longer have to chuck your current speaker system to enjoy SET amplification. The Kronzilla fears no speaker! While I auditioned this amplifier with the Lowther BassZilla, the power reserve was such, that I'm confident it will do as well in the context of average sensitivity speakers.
The Kronzilla's most endearing sonic attribute proved to be its utter coherency. When a reviewer lapses into an analytical mode, attention is usually focused on the bass, midrange, and treble ranges of a particular product. We may rave about the midrange but find fault with a treble that lacks air or a bass range that lacks definition. The point is that with most amplifiers we are able to decompose the presentation into distinct frequency bands. The situation is analogous to that of a multi-way loudspeaker that appears to speak with multiple voices. In a case like this, the auditory system is able to make out the voicing differences between the tweeter, midrange, and woofer. With the Kronzilla in the system, it became impossible for me to isolate performance differences, top to bottom. It consistently spoke with single voice, and in this respect emulated the coherency possessed by a full-range driver. The bass range was distinct and powerful, and that same delineation of the harmonic envelope was preserved, starting with the fundamentals and extending through the upper partials of each transient. Neither were there any speed differences over the entire frequency bandwidth, and the distortion spectrum did not change during the attack and decay of musical transients. The dynamic range was superb, especially the linearity and realism with which transients unfolded from soft to loud.
What did the Kronzilla sound like? That's a difficult question to answer. It would be much easier to first state for the record what it did not sound like. It never sounded like a vintage tube amplifier. It wasn't soft sounding at the top. Neither was it warm or lush sounding in the midrange. It did not emphasize the mids to the exclusion of the treble. It totally lacked the tube brightness ubiquitous to so many midfi push-pull tube amplifiers. It lacked the overly liquid harmonic textures that are common to 300B based designs. The picture I've painted so far is of an amp that is tonally neutral and free from tubey colorations and distortions. After experimenting with changes in the front end, it became clear to me that the Kronzilla was reflecting the sonic character of front-end components though the speakers. If I wanted to enjoy midrange warmth and palpable 3D imaging, all I had to do is insert my favorite tube preamp in the chain. The Kronzilla faithfully reproduced that sort of character through the speakers. Switching to a solid-state line stage gave me maximum detail resolution at the expense of losses in soundstage depth and focus. The overall character of the sound could be tilted in a particular direction by making deliberate component choices upstream. Neutrality is not a bad thing per se, but it does require active user intervention to spice up harmonic textures to one's level of expectation. My tastes being what they are, I could never imagine myself enjoying the Kronzilla though a solid-state preamplifier. And if your taste also gravitates toward vintage tube sound, then the use of a tube preamplifier would be mandatory with the Kronzilla DM.
Conclusion
The Kronzilla DM's sonic performance makes for a fascinating amalgam of tube and solid-state virtues. An SET on steroids, it is capable of exceptional coherency, speed, purity, detail resolution, and dynamic linearity. It's power reserve makes it a suitable match for the average audiophile speaker. The Kronzilla's neutrality and lack of coloration hold up a mirror to the sonic character of front end components, and mandates careful selection of the matching preamplifier.
There may be a few SET designs out there that match the power reserve of the Kronzilla, but at much more extravagant price points. We've all seen 10-watt 300B based amps priced at $10,000 and beyond. It seems that in the ultra high end arena, the current cost per SET watt is on the order $1,000. In that case, the Kronzilla should have been priced at something like $40,000. At its asking price, the Kronzilla DM represents a stunning piece of industrial design and a bargain to boot.
Manufacturer
KR Audio Electronics sro
Nademlejnska 1/600
198 00 Prague 9
Hloubetin
Czech Republic
Website: www.kraudio.com
Email: kr.audio@tiscali.cz
Features and Specifications
US Retail Price: $16,200/pr.
Tube complement 2 x T1610 in parallel
Maximum Output Power 42 WRMS (THD=5%)
Output Impedance 2, 4 Ohms
Frequency Response 20 Hz-20 kHz (-3dB)
Feedback Zero
Damping Factor 2.8
Input Sensitivity 1VRMS / 90kW ( at 41W )
Input Impedance 100 kOhm
A/C Power 115/230V 50/60Hz
Power Consumption 260VA (at maximum output)
Operating Temperature 5C - 40C
Dimensions W 385 x D 415 x H 550 mm (w/ projecting parts &
controls)
Weight Approx 36Kg