June 3, 2009 We’ve been living in a digital age for a while now and many of us access all our music from digital formats like CD and MP3. But has anyone managed to perfect a purely digital amplifier? NAD believes it has with the Master Series M2 Direct Digital Amplifier.

The NAD Master Series

The Master Series concept was introduced in 2005 as a natural evolution of NAD’s core values of performance, simplicity and value. Each component in the series offers performance, both measured and subjective, that showcases their latest technology and NAD’s exhaustive design/testing processes. A key element to the Master Series is that any successful new technology employed in the models, eventually trickles down to less expensive NAD products, much like motor racing technology filters down to production model cars.

The digital amplifier

Generally digital amplifiers sound a bit harsh and sterile. Combined with bright sounding speakers, the result is often a sound that’s difficult to listen to for long periods of time. One of the biggest advantages of a true digital amplifier is their ability to handle a digital PCM (Pulse Code Modulation) signal directly, avoiding the unnecessary signal conversion and processing present in analogue designs that can introduce noise and distortion. The block diagram (see gallery) for NAD’s direct digital audio system best illustrates this pure signal handling.

Master Series M2 Direct Digital Amplifier

The M2 boasts a SNR (signal-to-noise) ratio of around 105dB (referenced to full power) in a practical application (not just on paper!). NAD has achieved this impressive figure with circuit collaboration partner, British semiconductor company Zetex. Even with 35 bit architecture and Zetex’s DDFA (Direct Digital Feedback Amplifier) circuit, this figure was only achieved after 10 years of exhaustive research and development.

In fact the only source of noise or distortion (as minimal as it is) in the M2 architecture is the final modulation and gain stage, where a PCM signal is converted to a PWM (Pulse Width Modulation) signal and amplified to create the output stage (to run to your speakers). The M2 varies the pulse width being applied during the modulation process ensuring the amplitude at the output is always precisely reproduced. This step is where the 35 bit architecture is so important. Accurate amplitude reproduction is critical to sound quality at this point in the process.

The output stage is where the DDFA circuit shines. The approach, best described as noise shaping error correction, uses a reference PWM signal and any deviation of the modulated PWM signal is considered an error. This error information is then processed to compensate any subsequent modulation cycles. This constant monitoring and correction of error information gave rise to the name Direct Digital Feedback Amplifier during the M2’s development process.

As complex as the M2’s internals may sound, the outside look of this amplifier is typically NAD – clean, simple, functional and elegant. A built in ultra high resolution (48, 96, 192 kHz) A/D converter accommodates any analogue source you may have like a turntable or AM/FM tuner, and both balanced and un-balanced signal sources are catered for. The M2 also features three separate, low noise switch mode power supplies. One for each of the 250 watt amplifier channels, and one for control.

NAD makes numerous references to subjective testing when referring to the M2. Hopefully it won’t be plagued with the harsh, clinical sound that digital amps normally have. We’ll be able to find out when it ships in the next few months, at USD$5,999.

Tim LeFevre

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