NAD M2 Direct Digital Amplifier (TAS 198)

Inventing the Future of Audio

The term “digital” is often erroneously applied to amplifiers with Class D (switching) output stages, but in the case of NAD’s new M2 Direct Digital Amplifier that word is appropriate. In fact, the M2 represents a major rethinking of audio-system architecture, directly converting standard-resolution or high-res digital bitstreams into signals that can drive loudspeakers.

Functionally, the M2 is an “integrated amplifier” that replaces a DAC, preamplifier, and power amplifier. The M2 eliminates from a traditional signal path all the electronics of a DAC as well as the active analog gain stages of a preamplifier and power amplifier. It does this by converting the PCM signal from a digital source directly into a pulse-width modulation (PWM) signal that turns the M2’s output transistors on and off. That’s it—no digital filter, no DACs, no multiple stages of analog amplification, no interconnects, no jacks, no analog volume control, no preamp. The conversion from the digital domain to the analog domain occurs as a by-product of the switching output stage and its analog filter. This is as direct a signal path as one could envision. (See sidebars for the technical details.)

NAD’s M2 is a significant departure for the company that made its reputation building simple and affordable electronics. For starters, the M2 costs $5999, a new price level for a NAD “integrated amplifier.” Second, the M2 is NAD’s first amplifier to use a switching output stage. The company had previously rejected the technology in favor of linear amplifiers because switching output stages just didn’t sound good. But the M2’s output stage is significantly different from any other currently offered (see sidebar). Third, NAD believes that the M2’s technology could eventually become the basis for nearly all of its amplification products. In fact, NAD suggested that the M2 was not designed to capitalize on Class D’s functional advantages, but rather to establish a new benchmark of performance in amplification, no matter what the technology.

Let’s look at the M2 Direct Digital Amplifier in operation. The unit looks and functions like one of NAD’s upscale Masters Series integrated amplifiers, with a row of front-panel input-select buttons, a volume control, and a display. The rear panel, however, reveals that the M2 is not a conventional integrated amplifier. Five digital inputs are provided (two RCA, one AES/EBU, two TosLink, plus a TosLink loop) along with one single-ended and one balanced analog input. The digital inputs can accept any sampling frequency from 32kHz to 192kHz. Analog signals fed to the M2’s analog-input jacks are converted to digital.

Once you’ve connected an analog or digital source to the M2 (such as a CD transport or music server) and loudspeakers via the output binding posts, the M2 functions just like a traditional integrated amplifier. You select the source from the front panel and control the volume with the large front-panel knob or from the remote control. The front-panel display shows the input sampling frequency and volume setting.

Purists will note that the M2 requires that analog signals, such as a phonostage output, be converted to PCM digital. Similarly, those who enjoy SACD will be loath to convert their SACD player’s analog output to PCM, and then back to analog in the M2.

The M2 offers a number of features not found on a traditional integrated amplifier. Pushing the Menu button allows you to select the sampling frequency of the analog-to-digital converter (for analog input signals) as well as engage an upsampling feature that converts, for example, 44.1kHz to 96kHz. Analog signals are digitized at up to 192kHz/24-bit. You can also attenuate the level of the analog inputs by up to 9dB. A “Speaker Compensation” adjustment is a five-position adjustment that “allows fine tuning of the top octave to match the speaker impedance.” An absolute-polarity switch rounds out the menu-accessible features. A rear-panel switch engages NAD’s “Soft Clipping” feature, which limits the output to prevent audible distortion if the amplifier is overdriven. An RS232 port allows external control via a PC or control system such as Crestron or AMX. The full-function remote control selects between sources, adjusts the volume, dims the display, and can also control a NAD CD or DVD player.