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Technical Overview
Every element of this amplifier?s 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 Nº 33H is unparalleled. This is due in part to its 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 amplifier 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 amplifier and at the speaker terminals. The amplifier will correct for up to 1 volt of DC offset at its 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 Nº 33H was under development, the following highlights provide a glimpse into the development of such stunning products.
Vertical Orientation
Several factors contributed to the decision to employ a radical vertical design in these amplifiers. First, an amplifier capable of sustaining such high power levels will necessarily be physically large. Second, the need for optimal distribution of power disqualified a two-chassis-per-channel approach. Third, achieving optimal operating temperature required controlled heat dissipation. Lastly, we wanted to minimize both the floor space and the visual intrusion otherwise required by such a large chassis.
The solution to all of these concerns was to expand vertically, in a chassis design that consumes the same (or less) floor space as the Nº 20.6. A massive power supply forms the core of each amplifier, with the audio circuitry forming the outer shell. This arrangement minimizes the distances from supplies to circuits, optimizing power distribution. Another advantage of this approach is that the largest surfaces can be used as heatsinks. In conventional designs, the top and bottom plates are often larger than the heatsink areas, yet contribute little to this important function.
The striking appearance of the Nº 33H reflects the balance of authority and refinement exhibited by its sonic performance. It adapts well to home environments, with a sculpted appearance more evocative of art than of high tech electronics. In fact, this design has already come to be regarded as a new ?classic? in design, thanks to the architectural elegance of its implementation.
Power Supply
The Nº 33H 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.) 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 Nº 33H power supply is both extraordinarily powerful and stunningly quiet.
Nº 33H rectifies the secondaries of the transformers using a total of eight diodes rated for seventy amperes each. The rectification diodes are bolted directly to the power supply bus bars to maximize their current capability.
The Nº 33H 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 system 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 this reference amplifier, 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 Nº 33H uses an AC regeneration system to provide the cleanest possible power. In effect, it derives its 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 at this level of purity.
The Art of 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 Nº 33H takes 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 this amplifier 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.
This amplifier 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 Nº 33H 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 amplifier, 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 Nº 33H, since no single output device is ever called upon to deliver more than its ?fair share? of the work. The Nº 33H 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 Nº 33H exhibits 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 Nº 33H 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 is 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.
Effortless Power
The Nº 33H is rated to deliver 150 watts continuously into an 8 ohm load (300 watts into 4ohms). In the real world of complex speaker impedances, the Nº 33H delivers its power effortlessly.
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 Nº 33H. 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.
Conclusion
The Nº 33H Monaural Power Amplifier is the absolute reference for audiophile listening. It delivers effortless power in an elegant design that conveys the entire musical experience. From the subtlest nuance to the boldest crescendo, the Nº 33H will remind you why you fell in love with music in the first place.
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