Mark Levinson No.53 Reference monoblock power amplifier
Although it has a switching output stage, the No.53 is not a class-D amplifier. While the original rationale for the class-D technology was its efficiencylightweight, cool-running amps that could produce a great deal of powerthe No.53 is big and heavy, resembling the No.33and it runs warm even at idle. It weighs 135 lbs, with claimed outputs of 500W RMS into 8 ohms or 1000W into 4 ohms, 20Hz20kHz, at no more than 0.1% total harmonic distortion.
What's wrong with class-D
Without going into too much tech detail, class-D, aka "switch mode," amplifiers are more efficient than class-A or -B designs because their output transistors are either full on or full off. Rather than produce a higher-voltage version of the input signal, a class-D amp produces a high-frequency pulsed rectangular waveform: its transistors alternately connect the output to the positive and negative supply rails, with no in-between state. If the width of each pulse can be made proportional to the input signal's instantaneous level, the power delivered to the speaker over time from this "pulse width modulated" (PWM) signal will be equal to that of a conventional amplifier. When this PWM signal is low-pass filtered, the theoretical result is the original signal at a much higher voltage, just as you get from a class-A/B amplifier.
Of course, producing those proportional pulses in the first place and then perfectly filtering the squarewave are no mean feats. What's more, the high-frequency pulse signal will "broadcast" RF energy at upward of 300kHz.
Mark Levinson's innovations
The No.53 uses what Mark Levinson calls a patented, multistage "very high speed switching amplifier technology," Interleaved Power Technology (IPT), which is claimed to offer significant advantages over prior switching-amp topologies.
In a conventional class-D amplifier, one output transistor connects the output to the positive voltage rail, another connects it to the negative voltage rail. It is very important that when one transistor is turned on, the other must be turned off and vice versa, otherwise the positive and negative voltage rails will be short-circuited and the amplifier will self-destruct. However, because one transistor will be turned off before the other is turned on, there will be a short period of "dead time" when neither is turned on. This is analogous to crossover distortion in a conventional class-B amplifier.
ML's engineers set out to develop a new PWM output stage designed to overcome these limitations. The transistors switching the positive and negative voltage rails to the load in Levinson's "class-I" topology are each connected to the load via a large air-cored inductor and to the opposite voltage rail via a diode. The result is a stable amplifier with an effectively doubled switching frequency. Modulating the duty cycle of the 500kHz switching frequency of each transistor results in a 1MHz PWM output signal but without producing anything like the usual amount of ultrasonic noise. So minimal are the ultrasonic artifacts in the No.53's output, it is claimed, that, rather than requiring a sonically degrading brick-wall filter, all that's needed to remove them is a simple notch filter.
The No.53 uses four interleaved class-I/IPT stages in a balanced bridge configuration, to produce an effective PWM switching frequency of 4MHz. This allows the No.53's signal bandwidth to be extended to 100kHz.
The No.53's interior is divided into four sections: the single-ended and balanced inputs (SE signals are converted to balanced and remain so throughout the No.53), a modulation section, the amplifier itself, and the power supply. A six-layer printed-circuit board containing about 1500 parts incorporates the four isolated IPT modulator circuits, as well as the proprietary Link2 and MLNet system-control functions and the No.53's protection circuits. The output stage includes multiple high-voltage, high-current, high-frequency vertical MOSFETs and eight air-core (nonferrite) inductors said to be virtually immune from saturation at high current levels.
The power supply is at the base of the tower, and includes a large toroidal transformer, 188,000µF of capacitance in the main supply, and an additional 105,600µF of "local" capacitance. While one of the supposed disadvantages of amplifiers with switching output stages is a low damping factor, due to the output low-pass filter, ML states that the No.53 has a very high damping factor, which allows the speaker to see a "virtual short circuit path back to the amplifier" that limits the effects of back EMF and results in tight, deep bass.
The result is a powerful amplifier capable of enormous peak current delivery, "tremendous" headroom, operational stability, improved electrical and thermal efficiency, and substantially reduced mass.
Setup and Use
The No.53 is about as tall (20.9") as it is deep (20.4"), but only 8.4" wide, with carrying handles cleverly integrated into the curved front and rear faáades. Nonetheless, carrying one was not easy. Once they were in place, it was easy to make connections on their open, uncluttered rear panels. In addition to the RCA and XLR inputs, each panel has two sets of sturdy speaker terminals fitted with wide, flanged, easy-to-grip plastic nuts. There are also low-voltage mini-plug trigger inputs and outputs for signal-sensing turn-on, Link2 inputs and outputs, and an Ethernet port to allow you to connect the No.53s to your computer network. Doing so allows you to connect to the amplifier's internal Web page with Internet Explorer, where you can modify the network setup, pull down a window to select one of four levels of intensity for the front-panel display, get basic status information, and track system-related error messages. In the event of a problem, this page will also be used by Mark Levinson Customer Service.
The ML Net protocol allows you to control two or more network-capable Mark Levinson products simultaneously from their Ethernet ports by setting up master and slave units. If the system doesn't work right, make sure the master and slaves are "properly daisy-chained." A front-panel status LED blinks differently, depending on what it's trying to tell you.