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This review first appeared in the June 2013 issue of hi-end hifi magazine of Germany. You can also read this review of NAD's C390DD in its original German version. We publish its English translation in a mutual syndication arrangement with the publishers. As is customary for our own reviews, the writer's signature at review's end shows an e-mail address should you have questions or wish to send feedback. All images contained in this review are the property of fairaudio or NAD - Ed.

Reviewer: Jochen Reinecke
Sources: 160GB iPod Classic 5 with Pure i-20 dock; Pro-Ject Xpression III with Ortofon OM 30 Super, Audiolab 8200CDQ
Amplification: Audiolab 8200CDQ preamp, Dynavox TPR-2 preamp, Trends Audio TA-10.2 SE, Yarland FV-34C III, Abacus Ampollo power amp
Loudspeakers: Neat Momentum 4i monitor, Nubert nuBox 101 with AW 441 subwoofer, DIY transmission line with F120A widebander
Cables: Goldkabel Profi interconnects, Ortofon SPK 500 and Real Cable OFC 400 speaker cables
Review component retail: €2.500

Surprise! Over the past 20 years the racks of hifi lovers have undergone numerous generational shifts. In the beginning classical amps with phono stage and a few high-level inputs dominated. Slowly USB inputs showed up here and there. Today some form of digital interface is nearly de rigueur with any latest-gen integrated amplifier. And now NAD comes along to offer an amp which in stock trim only offers digital inputs. Rigor mortis analog? This eliminacious friend goes by the name of C390DD, puts out 160wpc and lightens the wallets of the willing to the tune of €2.500.

Conceptually the DD is a trickle-down of the M2 colleague Jörg took through its paces three years ago. The core of the C390's circuit is a British Zetex chip which incorporates D/A conversion, gain and drive and is augmented by a second chip which oversees the sensing and correction of the digital feedback loop. The obvious attraction here is the processing of purely digital data. The Zetex chip converts all incoming PCM signal into 844kHz resampled PWM at 7-bit resolution for 2 to the 7th power = 128 different possible widths. This pulse current is amplified by switched transistors, then passed to the speakers through a low-pass filter governed by Zetex's noise-shaping error correction aka digital feedback. Deviations of the original signal are compared to a reference pulse and corrected after which the reference pulse is filtered out.

This deck thus comes with impressive computational brain power. And wherever number crunching encounters multi-function chips with DSP features, end-user adjustments tend to be legion. Ditto the NAD. Via menu one can switch in a 6-band filter to counter room modes; swap channels without physical cable changes; invert signal polarity; or sum to mono. Further down the menu layers there's even a commendable impedance matching function since class D is known to not be entirely load invariant. Here seven different values (2Ω,4, 5, 6, 7, 8 and >8Ω) are possible.