Power Amplifiers

Mark Levinson No 53 Monoblock Power Amplifier

ARTICLE INDEX

The Design

The No 53 has four amplifiers (called amplifier blocks) that have a very unusual configuration. Each amplifier block pair has two "switches" in the output stage that consist of two MOSFET transistors per switch, connected in parallel. So, there are a total of eight switches (sixteen transistors) to produce the output signal.

As to the No 53's overall design, there are four sub-sections: the power supply, the analog input stage (balanced and unbalanced), the modulation section, and the amplifier output stage. The power supply was described in the preceding section (Page 2), and has a 2.8 kVA toroidal transformer and 188,000 µF of capacitance. The input stage is fully balanced, and if you use the RCA unbalanced input jack, the signal is converted to balanced and remains so throughout the amplifier circuit. The modulation section is where the interleaving circuit is located. It has four isolated modulator sections, one each to modulate the four amplifier blocks.

Below is a photo of the inside of the No 53, with the amplifier boards removed, to show huge inductors, the local power supply capacitors on either side of the inductors, and the main power supply capacitors at the bottom. The second photo shows the modulator PCB, which has more than 1,500 parts.

mark-levinson-no-53-power-amplifier-photo-inside-chassis-side-view

mark-levinson-no-53-power-amplifier-photo-inside-chassis-modulator-board

In the Levinson No 53, each amplifier block operates with each switch (two output MOSFET transistors) delivering the entire waveform, both positive and negative portions. Each switch is turned on and off 500,000 times per second, but they are "interleaved" so that they are not switched on at exactly the same instant.

Summarizing the activities of each of the two switchers in each of the four amplifier blocks, here is what happens: For amplifier block 1, switch 1 comes on at 00 going positive, and switch 2 comes on at 1800, going negative. Then in amplifier block 2, switch 1 comes on at 450 going positive and switch 2 comes on at 2250 going negative. For amplifier block 3 we still get the phase shift but the direction of the voltages is reversed, so switch 1 comes on at 900 going negative, and switch 2 comes on at 2700 going positive. For amplifier block 4, switch 1 comes on at 1350 going negative and switch 2 comes on at 3150 going positive. The interleaving process results in an effective switching frequency of 4 MHz.

Those large inductors in the photo shown above, allow the switches to be on at the same time. This is very important, because in conventional switching amplifiers, the output devices (there are usually a total of two output devices in the amplifier) are never allowed to be on at the same time because this would produce "shoot-through" current that would cause the amplifier to fail. Making sure that the two output devices are never on at the same time results in "deadtime" which causes distortion. Manufacturers try to keep the deadtime as short as possible, but it is never 0, so there is always some deadtime, with resulting distortion.

At the crossover point (where the waveform goes from positive to negative, or negative to positive), the switches in all four amplifier blocks are on at 50%, which results in a net current flow of 0. However, the fact that they are on, results in no crossover distortion.

Two amplifier blocks are arranged in parallel, and deliver the output in phase with the input. The other two amplifier blocks are also in parallel, but operate out of phase with the input through the use of a phase inverter at the input stage, and their output is, therefore, out of phase. The in phase output is connected to the + speaker binding post (red), while the out of phase output is connected to the - speaker binding post (black). This results in a fully balanced output.

Shown below is a block diagram of a conventional Class D switching amplifier output stage vs. the Levinson with its IPT circuitry. The presence of the inductors separating the two diodes is what allows the switchers to be on at the same time. You can see how large those inductors are in the first photo above, of the inside of the chassis.

mark-levinson-no-53-power-amplifier-standard-class-d-and-ipt-block-diagram

Here is a spectrum from a switching amplifier that uses two output devices, switched at 500 kHz. There is no interleaving involved. This is the spectrum before filtering.

mark-levinson-no-53-power-amplifier-standard-class-d-output-before-filtering

Compare that spectrum with the spectrum from a No 53, shown below (spectra © Harman International), also before filtering.

mark-levinson-no-53-power-amplifier-before-filtering