Halcro dm10 preamplifier
After designing an amplifier that turned much of the audio world on its head, Halcro's head honcho, Bruce Candy, turned his attention to developing a preamplifier to match what he'd already wrought. Two years later, the results appeared: the dm8 line preamplifier and the subject of this review, the $15,990 dm10 preamp, which adds a full-featured phono stage to the dm8. Would lightning strike twice, or was the dm58 a once-in-a-lifetime accomplishment for any designer?
Like the dm58, the dm10 is a strikingly original piece of industrial design. It fairly bristles with cutting-edge technology, and its appearance matches its space-age design. Matching Halcro's amplifiers, the box containing much of the preamp's innards is suspended between two pillars, resulting in a component that doesn't look as hefty and bulky as it is. Bulky it is, though; the thing is built like a bank vault and weighs a chunky 50 lbs. The clean, straightforward front panel contains all of the necessary switching, and an LCD panel shows input, stereo/mono status, attenuation, polarity, and the input in use.
As is his wont, designer Bruce Candy set some formidable goals for his preamp: unmeasurably low distortion of all kinds, exceptionally low noise, immunity from electromagnetic interference, minimal electromagnetic emissions, and bulletproof reliability. "Exceptional" regulation of the power supply was another central concern; the power-supply switching frequency was chosen to be far above the audioband, at more than 200kHz. Top-quality parts are used throughout the dm10, including Vishay resistors and FKP1 capacitors. (The fullest available technical information regarding the dm10's concept and design is available at the Halcro website.)
Each input can be assigned to any source component via switches above each set of inputs, the name of the source then being visible on the screen. Seven sets of inputs and multiple outputs are provided, including two sets available for current-source connection. Most notably, the dm10 features a fully adjustable phono stage in which gain—and, for MM cartridges, capacitance and resistance—can be varied over a wide range. When used with an MC cartridge, loading is fixed at 220 ohms and 4.7nF.
The following description of the dm10's technical particulars is necessarily sketchy, as Candy is notably closemouthed about the specifics of his designs.
The phono stage has four stages: an "ultra-low-noise input stage" with a 20x gain for MC cartridges, which can be switched out when using MM or high-output MC carts. This is followed by a high-input-impedance FET amplifier with 4x or 8x gain options. The output of this portion of the circuit is then fed to a 15Hz, third-order, high-pass Bessel filter that serves as a rumble filter. Next, RIAA equalization is applied, and the output of the RIAA stage is fed to a "low-noise input selector stage" that has a gain of 2x, depending on the gain of the selected phono input. Two mini toggle switches on the dm10's rear panel control all of this: one selects MM or MC, the other 1x, 2x, or 4x overall phono gain (my reference Dynavector XV-1S's 0.32mV output needed all the available gain).
These switches are easy to bump when reaching around the back of the unit or changing cables; I did so on a few occasions, only to find myself wondering where the gain had gone. The line stage, as far as I can tell from Halcro's white paper, also features four distinct gain stages. The dm10's volume control includes two identical stages in series.
The big Halcro features extensive microprocessor control. Each audio channel has its own controller, a second serves the front-panel display and user interface, and a fourth controls the activity of the other three processors. All switching functions are performed by expensive-sounding relays. Relay switching was chosen because it offers zero "on" resistance, infinite "off" impedance, very low "off" capacitance, and zero distortion. While the dm10 doesn't allow the user to vary the individual gain of each input, it reaches the same end by remembering the last volume setting used for each source.
Topnotch mechanical design was also a priority. A switching power supply such as the dm10's necessarily contains inherently noisy digital circuits. To solve this problem, Halcro encases the audio, display-panel, and power-supply sections each in its own internal aluminum enclosure, in addition to further extensive shielding.
There are more extras, including a headphone amp (which I didn't try, as I couldn't find my elderly AKG 240 headphones) and a 5V trigger that allows the dm10 to turn on Halcro power amplifiers by remote control. A thorough remote with excellent ergonomics is included, as is a very complete owner's manual (footnote 1). As with the dm58, there appears to be no way to open up a dm10, so the exact configuration of the electronics remains a mystery shrouded in an enigma.
Installation was straightforward: I set it on my reference Grand Prix Audio Monaco Modular equipment rack on a GPA F1 carbon-fiber shelf. The dm10's jacks look to be of superb quality, and were a tight fit with every cable I used.
Break-in time was minimal; within 25 to 30 hours, the Halcro had lost its initial, somewhat pinched sound and settled into an unchanging routine. I left the unit fully powered up, but in Mute mode when not listening. Despite this extended duty, it never became more than slightly warm. During four months of listening, the dm10 never misbehaved, produced any untoward noises, or did anything other than perform its appointed tasks with perfect precision—as well it should for $15,990.
In accordance with the "change one variable at a time" requirements of proper audio reviewing, I first listened to the dm10 strictly as a line stage. To get a grasp of the Halcro's sound as a line stage, I stuck with my familiar Aesthetix Io Signature and Manley Labs Steelhead phono stages for LP listening. Unsurprisingly, the dm10 displayed much of the familiar sonic character of its sibling amplifiers. Paramount was its supernatural quiet. The noise floors of Halcro components simply must be experienced to be appreciated, as there is nothing else like them. This silence allows for an almost unbelievable level of detail retrieval.
I've said it before, but it bears repeating: retrieval of detail alone does not equal transparency. True transparency requires that a listener be able to place all of that miraculously retrieved detail into the context of a musical event. The dm10 always placed the enormous amount of information it passed along in a context, at least with any recording that allowed it to do so.
Footnote 1: My review sample of the dm10 arrived nestled in form-fitting foam and packed in a heavy-duty Anvil-style road case. The preamp was further wrapped in a fabric overlay, and white gloves were provided for handling. The dm10 is also supplied with a Shunyata Diamondback power cord, the first instance I can recall where a manufacturer has supplied an aftermarket AC cord as part of the package.—Paul Bolin