Enjoy this masterpiece of audio technology: Mark Levinson 32 reference, the best preamp ever (solid state technology):
Nº 32 Reference Pre-Amplifier
Nº 32 Reference Pre-Amplifier
Historical Perspective
When phono was the primary source and most circuitry was single-ended, an outboard power supply was the most obvious characteristic of high-end preamplifier designs. Even today, many low-gain line stage preamplifiers call for an external power supply despite the fact that balanced circuitry is generally far more effective in dealing with electrical noise.
In most cases where balanced line stage preamplifiers employ an external power supply, the cost of the additional chassis far outweighs the benefit. However, many companies ignore this in order to give high-end customers a product that fits their perception of how a high-end preamplifier should be configured.
The Nº 32 Reference Controller's machined chassis houses five important "housekeeping" subsystems which support the preamplifier circuitry:
* AC power supply
* AC power regeneration
* DC power regulation
* Control circuitry (for input selection, volume and other microprocessor-controlled functions)
* Communications (control links to other components).
AC Power Supply
Two master supplies are located in the center section of the Nº 32 Reference Controller. One supply is dedicated to control and communications while the other is used for the audio circuits. A combination of inductors, capacitors, and resistors pre-filter the AC line and isolate the two master supplies from one another.
The power supplies use custom-designed transformers featuring multiple secondary windings which substantially improve isolation between separate circuit blocks. Separate rectification, filtering, and regulation circuits supply DC to the control/communications circuits, a Nº 25 phono DC supply output† and the 400hz oscillator circuits used for power supply regeneration.
† The Nº 32 features optional phono modules. For customers who wish to use an existing Mark Levinson Nº 25 or Nº 25S phono preamp with the Nº 32, the Reference Controller chassis offers the same DC output as would be available from a separate PLS-226 power supply.
An external ground terminal is provided for systems where the AC mains receptacle lacks a separate ground pin.
AC Power Regeneration
The Nº 32 Reference Preamplifier employs a technique called AC power regeneration in which the DC supply is derived from an AC source generated within the device itself. This technique is expensive but very effective. In the Nº 32 Reference Controller, a 400Hz oscillator generates the AC source for the preamplifier's audio power supplies (replacing the 50/60Hz reference provided by the local utility).
One advantage that 400Hz offers over 60Hz is that transformers are more efficient (they generate less heat) at the higher frequency. Another advantage is that filter capacitors are refilled more quickly, leading to a smoother supply of DC from the filter capacitors. Equivalent filtering from a 60Hz supply would require much larger capacitors (with higher Effective Series Resistance at high frequencies). And, since 400Hz oscillators and power supply components are commonly used in aircraft electronics, these high-quality parts are readily available.
The 400Hz oscillator supplies an input signal to separate left and right channel power supply amplifiers. The oscillator amplifier outputs, in turn, supply clean, stable (400Hz) power to discrete DC regulators located in the adjacent left and right towers. Each of these towers contains a completely independent power supply; each powers a single preamplifier channel independently.
DC Power Regulation
Each tower is a machined block of aluminum, containing a power supply for its respective audio channel. An isolation transformer, mounted inside the solid block, further de-couples the audio regulation stages from the 400Hz oscillator amplifier. Soft-recovery diodes rectify the output of the transformer, which is then filtered and regulated using a low-noise, high-speed voltage regulator amplifier designed specifically for this task. Separate cables supply the DC output to each channel. Although two meter-long DC cables will be supplied with the Nº 32, lengths of up to 8 meters can be fabricated to suit special installations.
Power Supply Regulation
The DC power coming into the Reference Preamplifier from the Reference Controller is even further regulated with yet another low-noise, high-speed voltage regulator amplifier. Critical blocks such as the volume control and the optional phono modules receive additional local supply regulation. Liberal use of local bypass capacitors assures an optimal electrical environment for critical audio circuit components.
Control Circuitry
Microprocessors grants modern preamplifiers an unprecedented level of control flexibility, but microprocessors are noisy devices which can pollute low-level audio signals if care is not taken to keep processing noise out of the signal path. In addition to being located in the controller box (physically separated from the preamplifier chassis), the controller module resides in its own shielded box, and is removable should a hardware update becomes appropriate. In addition to communications (discussed below), the Nº 32’s microprocessor controls volume changes, signal routing and other switching functions.
It is easy to name inputs, select mono modes, set mute-level, or program offset- or gain-levels for individual inputs from the front panel. It's even possible to adjust phono cartridge loading with the remote control (or from the front panel). Making the right loading choice has never been simpler or more convenient.
Power and Control Connections
Each channel is connected to the Reference Controller by a ten-conductor, heavily shielded DC cable. Four of the conductors deliver regulated DC power for the preamplifier's audio circuits, while the other six carry DC control signals.
Each channel's low-voltage microprocessor is active only while receiving control signals - its sleep mode keeps it dormant and totally silent at all other times. A low voltage, differential-drive control signal, buffered by opto-isolators in the preamplifier chassis, issues commands. Except for the fractions of a second when the microprocessor is transmitting a command, there is nothing in the DC cables connecting the controller and preamplifier chassis other than pure DC. As a result, the Nº 32 offers full microprocessor control while introducing absolutely no noise inside the preamp chassis when music is playing.
Communications
Like other Mark Levinson products, the Nº 32 uses a software-based operating system that can be upgraded without removing a single screw. New software can be downloaded into the preamp using the RS-232 port. The RS-232 port also allows AMX, Crestron or other similar control systems to communicate with the Nº 32.
Mini-jacks on the Nº 32 Reference Controller's back panel offer DC trigger output (5-12VDC, pulse or level), DC trigger input, and remote IR input control.
User Interface
The Reference Preamplifier chassis has no buttons or knobs. The Reference Controller execute all commands through simple front panel controls that belie the sophistication of its advanced user interface and functionality. The Nº 32 exceeds the efforts of other designs by effectively acting as a dedicated preamp for each source. In other words, it performs as if it were a single-input preamp..
Following in the tradition of classic analog preamp design, the essential controls for volume and input selection are prominently positioned on the control panel. Secondary functions such as balance, mute, or recording source selection are accessed using small buttons on the central panel. You can adjust parameters for each source to insure optimum performance. The input selector toggles among only those inputs that are active, presenting only those sources actually connected in the system. Each source can be named in the display and full control of all functions, including mono/stereo and polarity selection, is available from the remote.
Reference Preamplifier Chassis
The preamplifier casework is extraordinary even by Mark Levinson standards and, like the other aspects of the Nº 32’s design, reflects a totally new approach. The chassis is a custom aluminum casting, machined and finished with a conductive Irridite (clear chromate) coating. This design approach assures a rigid, stable environment where vibration and microphonic effects are controlled.
The left and right channels are separated by a solid aluminum divider wall, formed as part of the casting, sealing each channel into its own environment to prevent the electromagnetic noise of one channel from degrading the other. Since each channel is effectively isolated from the other electrically and physically, the Nº 32's stereo separation is equivalent to that of two separate mono preamplifiers.
Inputs
The Nº 32 features eight inputs, three utilizing balanced XLR connectors and five utilizing single-ended RCAs. The optional phono modules boast two additional inputs and may be purchased for use with either balanced or single-ended connectors.
Unused inputs have both their signal and ground connections opened to prevent unwanted noise from seeking a path through the well-grounded preamp chassis. An R/C network hinged at 1MHz provides RF filtering for each input.
An instrumentation-grade "T" switch topology keeps inter-input crosstalk exceptionally low in the Nº 32. Even in an unrealistic worst-case test scenario, with adjacent inputs not assigned or terminated, isolation is greater than 120dB. With other source components connected (as they would be in actual use), isolation improves to a startling 140dB. This is approximately 100 times quieter than our excellent No380S. This outstanding performance is due to a special relay switching scheme and circuit layout, as well as brass shield bars tied to the system ground.
Circuitry
The Nº 32 is the first Mark Levinson product to use Arlon 25N® for the circuit boards for the active preamplifier and phono module circuits. This superior material offers superb dielectric properties and helps the Nº 32 achieve remarkable performance with complex audio signals. Arlon 25N was chosen only after extensive listening tests revealed that it was audibly superior to all other materials in this particular application.
Physical layout is an important performance factor in audio circuits. A quick visual inspection of the left and right channel circuit board layout will reveal that they are symmetrical only in a general way. Individual parts are not located in an exact mirror-image to the opposite channel. From this, one may correctly conclude that the layout was developed by hand for electrical rather than visual symmetry. We have learned that this is the only way to approach circuit layout, using exhaustive listening tests as the measurement of success -- it doesn't matter how pretty a circuit is if it isn't audibly superior.
Both balanced and single-ended inputs are provided on the rear panel. Single-ended signals have common mode noise (i.e. noise that is present on both signal and ground) rejected at the input stage. Fully balanced instrumentation amplifiers are employed as the gain stages and each input can be separately adjusted using the software to offer 0, 6, 12 or 18dB of gain. Using this feature, the Nº 32 allows the volume control to work in its optimum range even when its sources have substantially different output levels.
Optional Phono Modules
For customers using analog disc sources, optional phono modules may be installed in the Nº 32 Reference Preamplifier chassis. Left and right phono modules are enclosed in mu-metal shield boxes that slide into the preamplifier on either side of the divider wall separating the channels. This location is ideal for power supply and signal paths while being close enough together to allow easy connection of tonearm cables.
Each phono module comes fitted with two, independent input connectors to accommodate systems where two tonearms or separate turntables are in use. Phono modules are available with both inputs configured for either single-ended (RCA) or balanced (XLR) connectors. Each phono input is independent of the other, allowing custom setup and optimization for two different analog sources.
Setup and customization of the Nº 32 phono is an analog lover’s dream. A defeatable infrasonic filter, resistive and capacitive loading and gain can all be selected independently for each input. A fine trim adjustment for balance lets you compensate for the small channel imbalance found in virtually every cartridge. What’s more, all of these variables may be adjusted from the remote control. Now, the process of setting up the preamplifier for best phono performance, regardless of the choice of cartridges, can be done more easily and with greater confidence than ever before.
The Volume Control
Volume control is one of the primary functions of a preamplifier; unfortunately, it is also a source of compromise in many designs, especially since less-than-optimal implementation can degrade audio signal quality. The Nº 32 introduces a new active attenuator; a discrete design that offers an unprecedented combination of precision and neutral sonic performance. The attenuator modules are constructed on their own four layer Arlon 25N circuit boards where local power supply regulation and bypass capacitors assure the sensitive switches of clean power and optimum isolation.
An array of precision resistors provides attenuation in 0.1dB steps down to -57.0dB, where step size is increased to 1.0dB. In all, our technology provides over 65,000 steps, allowing us to make the Nº 32's "stepped attenuator" act and sound like a continuously variable control. One can always select the proper playback volume, regardless of system configuration or program selection. And, because of the accuracy of the Nº 32's display, playback levels are reliably repeatable.
Outputs
High-performance buffer amplifiers form the output of the Nº 32. These circuits feature low noise and distortion, low output impedance, high current capability and broad bandwidth. Two pairs of balanced (XLR), as well as two pairs of single-ended (RCA) connectors are available for the main outputs. Single-ended and balanced outputs are independently buffered to allow simultaneous use of both connector types without sonic compromise.
Buffered record outputs are provided via one pair of balanced (XLR) and two pairs of single-ended (RCA) connectors.
The Sound
Exhaustive listening tests were conducted over several years to select topologies, component parts and operating parameters for the Nº 32 Reference Preamplifier. Venturing a description of the sound, we find ourselves reaching for superlatives. Our usual words and phrases seem inadequate to the task.
The Nº 32 is spectacular in an unexpected way; it is a masterpiece of complexity and subtlety. It does not call attention to any single attribute, as some products do, only to disappoint over time. Just as a fine wine reveals more of itself throughout the night, or from one year to the next, so too does the Nº 32.
Music-loving, careful listeners will find themselves discovering new and subtle meaning in familiar recordings. With time, recordings of great performances reveal more of themselves through the Nº 32. The Reference Preamplifier sorts out complexity the way the proper light can reveal the artistry of a great painting. New and interesting aspects of the music reveal themselves with each playing of the recording, promising new secrets waiting to be discovered.
The Nº 32 is for the serious music devotee, whose audio system is the means to an end, not the end itself. While it can sound breathtaking, it is designed for a sophisticated, refined and patient listener. Please take an evening to enjoy the new Reference. We are confident you will hear expressiveness in your favorite recordings that you have never heard before.
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Edited by oliverstoned - April 23 2006 at 15:18
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