La poursuite de la meilleure amplification est un thème récurant dans le monde du high-end audio. Les nouvelles technologies présentent de nouvelles approches, tout en s'inspirant du passé. C'est dans cet esprit que l'amplificateur Mark Levinson® № 534 a été développé : une conception basée sur la tradition classique de Mark Levinson avec en point de mire un son profond, une scène sonore élargie , un placement pointu de l'image sonore, des basses profondes et tendues et des aigus délivrés de toutes agressivité.
The pursuit of perfect amplification is a well-known theme in high-end audio. New technologies present new approaches, while looking to the past provides inspiration for the future. It was in that spirit that the Mark Levinson® № 534 amplifier was developed: a design advised by the traditions and art of classic Mark Levinson amplifier designs, and with a specific target sound: a deep, expansive soundstage, accurate image placement, extended low bass response with good pitch definition, and detailed high frequencies free from “etching.” To that end, the Mark Levinson engineering team started with the following requirements:
Harmonic distortion <0.02% at 1kHz and <0.2% at 20kHz
Bandwidth extending from 0.2Hz to 120kHz, and beyond if possible
Critically damped square wave response, with leading and trailing edges free of any slewing artifacts, and a slew rate of at least 100V/µs
Maximized open-loop linearity and bandwidth, with global negative feedback in the range of 12-20dB and without nested feedback loops
This fully differential, fully discrete, dual-monaural Pure Path amplifier drives virtually any loudspeaker effortlessly for impeccable imaging, musicality, and openness. A direct-coupled signal path; a highly linear, low-feedback design; and voltage gain and drive stages operating in class A, are joined by the modern system integration capabilities provided by Ethernet, RS-232, and USB for monitoring and network control. Housed in a 6000-series aluminum chassis, like all Mark Levinson equipment, the № 534 is designed and handcrafted in the USA.
Mark Levinson Pure Path Circuit Design
Since 1972, Mark Levinson has been dedicated to the uncompromising art of sound, with the guiding principle of musical purity above all else. To achieve that goal like never before, Mark Levinson engineers scoured company archives, ultimately developing a circuit-design philosophy called Pure Path. On a conceptual level, its hallmark principles include a discrete, direct-coupled, fully balanced, dual-monaural signal path that delivers unrestricted, uncompromised sonic purity.
Far from merely arranging high quality components in an intelligent manner, Pure Path is the meeting of science and art: Mark Levinson engineers are tasked to create the best possible measured AND subjective performance. To quote Albert Einstein: “Everything should be made as simple as possible, but not simpler.”
Pure Path highlights in the № 534 include Folded Cascodes, an extremely high bias current, and usage of very little negative feedback.
The Folded Cascode
Mark Levinson components are designed around a Pure Path amplifier circuit concept that makes heavy use of a circuit design element called a cascode. Originally created for improving the bandwidth of vacuum tube circuits in radios, Mark Levinson engineers deploy cascodes in Pure Path circuit designs to both improve bandwidth and enhance linearity. A cascode combines two transistors so that they operate as a single composite device that functions like a single transistor, using the wanted characteristics of each component transistor, and rejecting the unwanted characteristics.
Mark Levinson engineers began their design of the gain stage with pairs of JFETs (junction field-effect transistors), chosen for their low noise and high gain. JFETs achieve these characteristics in part by being large, and with their large size comes nonlinear gain – or distortion. To mitigate this, BJTs (bipolar junction transistors) were added to the circuit. The BJTs exhibit very low input impedance and very high output impedance, which translates to excellent bandwidth and linearity. By creating a cascode of the JFETs and BJTs, they created a design with the best characteristics of both sets of devices.
Engineers than connected the output of the cascode to another transistor of the opposite “gender.” Current flows through it in the opposite direction to “turn around” the direction of the signal current. This special connection is called a Folded Cascode and it is a hallmark of the Pure Path circuit: high gain with low noise, wide bandwidth, and excellent linearity.
Gain Structure
The audio circuitry in the № 534 has been optimized for the best possible open-loop performance. As a result, very little negative feedback is necessary, and the amount employed could be adjusted to achieve certain subjective or objective results. The open-loop gain and bandwidth of the circuit were painstakingly set using precision resistors and compensation capacitors, and additional precision resistors were employed to compose the feedback network.
