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Technical Overview
Every element of these monaural amplifiers? design is new, from special high current AC connectors and line cords, through power supply elements capable of handling such power levels, to newly designed voltage gain, driver and output topologies capable of delivering practically unlimited power along with the precision and delicacy of the most subtle musical nuances.
The accuracy with which high frequencies such as instrumental overtones are reproduced by the Nº33 and Nº33H is unparalleled. This is due in part to their three stages of voltage gain and one final (current) gain stage which together achieve greater open loop gain, reducing phase and amplitude errors at high frequencies. The use of precision matched bipolar transistors throughout the amplifiers assures optimum differential amplifier performance through precise matching of operating parameters and intimate thermal tracking.
Similarly, exceptional low frequency phase coherence results in improved definition and extremely ?tuneful? bass. Low bass performance is enhanced by DC coupling throughout the amplifiers. (This is another way of saying that no capacitors are used anywhere in the signal path.) Protection against DC faults elsewhere in the system is assured by the use of servos, yielding stable operation and elimination of DC errors both within the amplifiers and at the speaker terminals. The amplifiers will correct for up to 1 volt of DC offset at their input terminals (although properly designed preamplifiers rarely have more than a few millivolts of DC offset).
While it would take an entire book to fully describe the many design choices made during the years that these amplifiers have been under development, the following highlights provide a glimpse into the development of such stunning products.
Balanced Design
The benefits of properly implemented balanced signal transmission are well known, including better signal-to noise performance, improved dynamic contrasts (both large and small), and higher resolution. Mark Levinson products have been pivotal in proving the superiority of balanced signal transmission over the years, having featured balanced input capability since the first amplifier to bear this trademark (the legendary ML-2).
The Nº33 and Nº33H take a radically different approach to balanced amplification, however, including fully balanced operation from input to output. Even single-ended signals (if used) are converted to balanced at the input and handled in a balanced fashion thereafter.
The uniquely balanced design of these amplifiers offers an unprecedented range of dynamic contrasts and utter freedom from noise, allowing the music to emerge effortlessly into your listening room, free from the usual patina of electronic reproduction. Significantly, the music seems to exist independent of the equipment used to reproduce it, rather than merely being part of a complex electronic tapestry. While this characteristic must be heard to be appreciated, some of the underlying technologies can be explained.
Each of these amplifiers is actually two separate amplifiers in one chassis. Beginning at the input, the voltage gain stages in each monaural amplifier include two symmetrical, balanced amplifiers. This unusual ?double-balanced? design offers superior rejection of common-mode noise and distortion, and provides a truly balanced load impedance to the preamplifier (thereby optimizing preamplifier performance).
The output of the third and final voltage gain stage of each of the two balanced amplifiers is converted to a single-ended signal, achieving significant common mode noise reduction in each. This conversion results in a pair of opposite polarity, precisely matched musical signals which then move on to the two output stages of each monaural amplifier. (In effect, the voltage gain stages of the Nº33 and Nº33H are ?double? balanced, while the output stage is ?single? balanced.)
Magnetic interference within sensitive voltage gain stages, which otherwise might be caused by the tremendous currents circulating within the amplifiers, is addressed in several ways: by the orthogonal arrangement of output sections and input stages for minimal coupled interference; by steel and aluminum shield plates to further attenuate electromagnetic fields, and by careful, symmetrical circuit layout.
Output Stage
The unprecedented degree of symmetry in the handling of the music signal outlined above explains (in part) why these amplifiers seem to ?disappear,? leaving only the music. This symmetry continues through to the output stage for several reasons: to provide a symmetrical current path for the alternating current that drives the loudspeaker; to eliminate corruption of system ground by output currents; and to enhance the amplifiers? common mode rejection ratio by balancing the impedances of the amplifier?s output terminals. All these technical considerations further improve the amplifiers? sonic neutrality.
Performance, reliability and longevity are all enhanced in the Nº33 and Nº33H, since no single output device is ever called upon to deliver more than its ?fair share? of the work. Sixty bipolar output transistors are distributed throughout each Nº33, consisting of fifteen complementary pairs in each of the two output stages (non-inverting and inverting) within each monaural amplifier. Having lower output requirements, the Nº33H uses forty bipolar output devices per monaural amplifier. All output transistors have been sorted and carefully matched, ensuring that the task of providing output current is shared equally by each device.
The Nº33 and Nº33H exhibit drastically lower distortion than other amplifiers, without the terrible inefficiency and resultant thermal and power management problems found in traditional Class A biasing systems. We developed a sophisticated, continuously variable adaptive biasing scheme for the output stage of these amplifiers that responds to the instantaneous demands of the signal being amplified. This innovative circuitry delivers three times the Class A power its idle current consumption would allow in traditional biasing schemes. Moreover, unlike alternative biasing schemes, the output devices of the Nº33 and Nº33H are never reverse-biased under any signal conditions.
Tight control over all output devices is reflected in the authoritative control these amplifiers exert on the loudspeaker?something which must be heard to be appreciated. For example, experimentation and computer simulation resulted in a specific vertical spacing of output devices on each heatsink to ensure matched operating temperatures for all output transistors. The attention to detail in every aspect of the design, typified by this vertical spacing of output devices, results in tightly controlled output stage operating parameters and superior musical realism.
Extremely High Power
In addition to improving upon the subjective sound quality of the legendary Nº20.6, yet another design goal of the Nº33 was to provide enough power to be completely unchallenged by even the most demanding loudspeaker loads. The Nº33 is the most powerful high end amplifier in the world and will not be the limiting factor in the performance of any system.
The enormous power supply behind the Nº33 gives it unmatched authority and control over the loudspeaker, enabling any loudspeaker (regardless of its electrical characteristics) to achieve its finest performance. The Nº33 is rated at 300 watts continuously into 8 ohm loads. While other audio amplifiers have claimed this amount of power (or even more), none has achieved the ability to deliver as much real power to loudspeakers as the Nº33. The Nº33 is capable of continuous power output of 2400 watts into a 1ohm load if provided with an adequate high voltage, high current AC power source. (Under ordinary musical conditions, which do not require such enormous power on a continuous basis, the Nº33 will function properly with ordinary household AC lines.)
By contrast, the Nº33H is rated to deliver 150 watts coninuously into an 8 ohm load, ?doubling down? to a continuous power output of 1200 watts into a 1ohm load (again, assuming that the AC outlet can adequately replenish the power being used). In the real world of complex speaker impedances, the Nº33H delivers tremendous power effortlessly, mimicking the performance of the Nº33 up to the point at which the greater voltage swing of the Reference amplifier allows it to leave the smaller amplifier behind. Comparisons made within the rated power of the smaller amplifier show the two to be uncannily alike?as you might expect, given the similarity of their designs.
High power, high end amplifiers present a special design challenge. After all, one must both create a huge reservoir of power that is of the highest quality and control the use of that power with the precision and finesse required to accurately reflect the subtleties of music signals. Historically, it has been difficult to achieve both of these goals simultaneously.
Both of these issues have been addressed comprehensively in the Nº33 and Nº33H. The fully balanced implementation already discussed outlines the extraordinary efforts made in controlling a presumed ?perfect? power supply. The remaining challenge is to create a massive power supply that genuinely exhibits those ?perfect? characteristics.
Power Supply
As stated above, even the larger Nº33 will perform properly with ordinary household wiring. Still, we expect that some music lovers who invest in these Reference amplifiers may want to provide special high powered AC lines (at least 200 volts @ 30 amps) so the amplifiers can develop their full continuous output power capability regardless of how demanding the loudspeaker load may be.
A glance at the connectors, line cable and fuses of the Nº33 indicates that the real power capabilities of the amplifier are unlike any other. Every potential ?weak link in the chain? was addressed to ensure maximum performance.
Each monophonic Nº33 contains two large 2.45 kVA toroidal transformers, laying the foundation for two independent bipolar power supplies to support the fully balanced configuration of the amplifier. These transformers are self-shielding (meaning that their magnetic fields are self-contained), greatly reducing the potential for noise and interference from the power supply. They also offer greater efficiency than other transformer designs, reducing unnecessary power consumption and heat.
The Nº33H uses a single 3.417 kVA transformer with separate windings for the two bipolar supplies. This approach allows the amplifier to be smaller than it would have to be with two transformers, without incurring any performance penalty at all. (Unlike stereo amplifiers, there is no danger of introducing crosstalk through the common power supply of a mono amplifier.) Like the Nº33, this transformer is self-shielding, oriented to minimize the influence of any stray magnetic flux, shielded within a heavy gauge steel column, and extremely efficient. As a result, the foundation of the Nº33H power supply is both extraordinarily powerful and stunningly quiet.
Each amplifier rectifies the secondaries of the transformers using a total of eight diodes rated for seventy amperes each. In both cases, the rectification diodes are bolted directly to the power supply bus bars to maximize their current capability.
The Nº33 employs twelve 39,000 µF computer grade electrolytic capacitors with extremely low ESR for filtering and storage in each amplifier. Each of the four power supply rails has 117,000 µF of capacitance for a unit total of nearly one-half a Farad of capacitance and 1200 Joules of energy storage. At half the power, the Nº33H uses four 60,000 µF capacitors (one for each rail) with similar characteristics, providing power supply capacitance and energy storage proportional to its lower output requirements.
The efficiency of the power distribution systems within both amplifiers is enhanced by the use of tinned, oxygen-free copper bus bars that are far superior to PC board traces or point-to-point wiring.
AC Mains Regeneration
In addition to the power supply elements described above, power supply refinements never before available in any power amplifier are employed for the sensitive voltage gain and driver stages of these two amplifiers, resulting in unmatched reproduction of subtle details and independence from poor quality AC power.
Typically, AC power mains carry a significant amount of high and low frequency noise that can (and frequently does) degrade the sonic performance of audio equipment. Rather than using even the smallest reflection of a ?dirty? AC mains supply, the Nº33 and Nº33H use an AC regeneration system to provide the cleanest possible power. In effect, these amplifiers derive their own AC power, creating an ideal source for the critical voltage gain and driver stages.
For the first time in an audio amplifier, portions of the positive and negative DC voltages in the main power supply are siphoned off and used to drive a special oscillator that generates a pure 60 Hz sine wave. This distortion-free sine wave is then amplified and transformer coupled (for further isolation) to a traditional full-wave bridge bipolar power supply. Amplitude fluctuations of the AC mains are eliminated, and the oscillator output is adjusted for precisely chosen operating parameters of the downstream DC regulation circuitry.
The result is an absolutely pure and stable DC power supply for all voltage gain stages that is derived not from the ?dirty? AC mains supplied from the wall, but from our own pristine 60 Hz sine wave. This approach isolates the sensitive voltage gain stages from external AC interference and enables them to realize their full potential.
This design feature renders any external power conditioning device useless. This is just as well, since there are no such devices in existence that have the capacity to supply adequate power to these amplifiers.
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