DNM 3D Six preamplifier

It isn't enough to say that engineer Denis N. Morecroft is one of contemporary audio's few visionaries: He's one of a very few mature designers whose passion for doing things a certain way hasn't abandoned him in the least, and whose well-argued convictions seem stronger than ever. Thus, as others cave in to commerce—the tube-amp designer who offers a solid-state product just to help his dealers fill a price niche, the source-component manufacturer who rails against digital audio one day and starts cranking out CD players the next—DNM Design remains the likeliest of all modern companies to stay its course.

The DNM 3D Six preamplifier ($13,495), the latest realization of Morecroft's ideas on domestic audio, embodies literally all of the design distinctions that have originated at DNM throughout the company's 30 years of existence: low-mass conductors, nonmetallic casework and parts, true star grounding, "spaced pair" signal paths from input to output, three-dimensional circuit layouts, slit-foil capacitors, and the use of subminiature resistive-capacitive networks to tame impedance reflections.

The 3D Six's model designation refers to the number of individual 25V sources in its outboard power supply: three dual-mono boards, for a total of six discrete sources. Prior to this review sample, my experience of the DNM 3D had been limited to the Primus version ($7995), an entry-level product (for DNM), whose scaled-down outboard power supply incorporates a single 25V source, and which I wrote about in the March 2008 edition of my "Listening" column. DNM also offers the 3D Twin ($10,595), the outboard supply of which contains—you guessed it—two discrete voltage sources.

Like most earlier DNM preamps and amps, the 3D Six is considerably smaller than the high-end audio average, with casework and fasteners made entirely of acrylic and nylon: Given Denis Morecroft's conviction that recorded sound suffers distortion from eddy currents created within conductive materials by the audio signal itself, the 3D contains as little metal as possible.

In common with other solid-state preamplifiers, the 3D Six's active circuitry resides on various removable subboards—actually double boards, designed to allow complementary gain devices and supporting parts to be laid out in three dimensions, for consistently optimized spacing and minimal electromagnetic interference. The active boards all plug into a passive motherboard that occupies most of the floor space of the chassis. The motherboard also plays host to various passive subboards dedicated to source selection and attenuation, as well as to a sprinkling of miscellaneous switches and sockets. Under the motherboard and parallel to it, a separate power-supply distribution board determines which outboard power supply can be used: one distribution board suits either the Primus or the Twin; a different one is required for the Six (footnote 1).

In standard trim, and regardless of power-supply options, the DNM 3D has inputs for three line-level sources, labeled Radio, Tape, and Direct; two more, labeled Aux 1 and Aux 2, can be installed by a DNM dealer at extra cost. In each case, signal input is by means of a 5-pin DIN socket in which pin 1 is unused, 3 and 5 are hot, and 2 and 4 are individual, channel-specific grounds rather than the usual common-ground pair of phono jacks. (Note that the Direct socket is next to the preamp's Output socket—yet whereas the labels for the other two inputs are directly beneath their respective sockets, the label for the Direct socket is above it, as is the label for the Output. Perhaps it's just me, but seeing the words direct and output next to one another is something I find almost boundlessly confusing.)

Input sockets (also 5-pin DINs) are provided for up to two phono sources, requiring that one or two pairs of plug-in phono boards be installed on the motherboard. The user can install either moving-coil or moving-magnet boards, switching between them by means of a front-panel toggle. Rather than a rear-panel ground lug of the usual sort, the DNM 3D has a pair of 2mm pin sockets for tonearm or turntable chassis ground leads.

In addition to the aforementioned phono toggle, the 3D's front panel has a similar toggle for tape-monitor switching, plus a 3.5mm headphone jack (directly driven by the line-amp boards), a pushbutton Mute switch that illuminates when activated, a skirted knob for source selection, and two more skirted knobs for channel-specific attenuation: the volume-control scheme I prefer above all others, for easy channel-balance adjustments without the need for additional parts. A final touch: Snugged between the latter two knobs is a two-position toggle for selecting between stereo playback and a mono blend. Amen!

As with the slightly downmarket 3D Twin, the 3D Six's outboard power supply is built into a full-size DNM acrylic case. Voltages are knocked down by a hefty Noratel frame-style transformer, fastened to the supply's own motherboard with a pair of nylon plates that seem intended to absorb mechanical strain. The trannie's two 27V output leads are directed to a single bridge rectifier, with the 0V center tap referenced to a central point on the star-ground board. DC from the rectifier is smoothed by a pair of DNM's own chunky T-Network slit-foil capacitors before traveling to a trio of dual-mono plug-in boards, for regulation and impedance control.

Three 45" ribbon cables exit the back of the power supply, each initiating its journey at one of the three subboards, and each ending with a 10-conductor plug pledged to a corresponding socket on the 3D preamp. There's also a pair of sockets on the back of the power supply for routing the audio signal to the power amplifier, but these don't function in a way that owners of older British separates might expect. Whereas the shielded cables that carry DC power to Naim preamps in particular are also used to carry the AC audio signals from them, thus making the outboard power supply the grounding center of the overall amplification system, the DNM ribbons are for DC only: Signal-transfer sockets on the back of the 3D Six power supply are provided only for the sake of convenience, for installations where cable- and gear-placement constraints make it difficult to do things any other way, and their use depends on two things: bringing the signal from the preamp to the power supply with an extra cable dedicated to that purpose alone, and installing a DNM Power Supply Transfer Board on the underside of the power supply's motherboard. Although my review sample was equipped with said board, I didn't try the alternate connection scheme, as it held no hope for a performance advantage.

Footnote 1: Power-supply distribution boards can be swapped out by a dealer or a very adventurous user, but it can't be done terribly quickly—which makes it extremely difficult to use a single 3D to compare, say, the Primus and Six power-supply options. Consequently, for this review, I borrowed two 3Ds.
DNM Design
US distributor: Austin HiFi
603 West 13th Street, Suite 1A-348
Austin, TX 78701
(512) 236-9100