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At $549 the 50wpc direct digital amplifier is the most aggressively priced 100er. Conceptually it would also seem the most ambitious. With 1 x USB, 1 x coax and 2 x Toslink inputs and one surprise Toslink output—there are no analog inputs to keep with the direct concept—it doesn't convert from digital to analog in any of the usual sense. Here pulse code modulation elegantly turns pulse width modulation all still in the digital domain which now switches the output transistors directly. Hence no headphone output either.
Built-in DSP operates at 3Gbps to oversample PCM data before applying a digital-domain 24-bit volume control. "Efficiency for the DDA-100 is 90% and its built-in FIR filter creates 100% phase-linear speaker output because unlike an IRR filter, a FIR filter suffers no phase distortion."
The DDA-100's elegantly minimalist styling seems self-conscious of the fact that it won't be joined by other NuForce product (though you could run their CDP-8 as transport only to waste its presumably superior D/A converter in the process). It thus must represent the company by its lonesome. Is that why particularly in the silver livery it looks so extra dashing? Upon spotting the digital attenuation spec—de rigueur if one no longer performs traditional D/A conversion—the tech-savvy will already have begun to chant the bad karma mantra of resolution decimation. But quoting from Jon Iverson's Stereophile review of NAD's $2.000 M51 DAC, we learn that "... the extreme headroom afforded by the 35-bit architecture allows for a DSP-based volume control that does not reduce resolution. Even with 24-bit high definition signals, the output can be attenuated by 66dB—very very quiet—before bit truncation begins." Clearly NuForce's 24-bit control exploits similar thinking. It's precisely why all incoming signal is upsampled before the attenuator ever gets to it.
"The DDA-100 upsamples to 3Gbps to do PWM conversion—a good class-D amp switches at 300-400kHz—and digital attenuation is handled by 24-bit DSP with 100mbps speed and a 512-tap FIR filter."
DDA-100 is our attempt at bringing high-end audiophile sound quality down to a mass-market retail level. It is meant to be a state-of-the-art digital integrated amp at a reasonable cost which targets consumer's digital home audio lifestyle (computer, Apple TV and other digital sources). There is no DA conversion inside. The S/PDIF signal converts directly to PWM which becomes the power amplifier output. Adding a headphone output would have meant gluing a completely independant DAC circuit into a very elegant DSP-based DDA architecture. That would have departed from the original concept and moved up cost. Plus any headphone output would have been quite inferior to at best equal our existing uDAC2. The DDA concept is a single-chip solution of which we here use 2 chips to deliver a solid 2 x 75 watts into 4Ω. That's RMS power by the way. It means dynamic music power really becomes 2.8 x 75W = >200 watts."
I did say ambitious. Let's take a closer look at both contenders.