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Halcro dm38 Stereo Power Amplifier Print E-mail
Friday, 01 August 2008
Article Index
Halcro dm38 Stereo Power Amplifier
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Halcro amplifiers have built themselves quite an impressive reputation in the last few years.  I remember first hearing Halcro at a 2003 CES DTS exhibit. The exhibit had the gorgeous huge H-shaped monoblocks placed next to each speaker.  Shortly thereafter, several other audio magazines reviewed the various available Halcro amplifiers with much praise; one even called a Halcro amplifier “the best amplifier ever.”  Recently, I have had the opportunity to review home theater components from Halcro’s lower-priced and more attainable Logic series.  As one can imagine, I was quite excited when Halcro offered me a chance to review their DM38, a stereo amplifier from their Reference series.  I was anxious to see if Halcro could transfer the sonic qualities from their reference monoblocks to the DM38, which at $20,000 is the least expensive amplifier in their Reference line, and whether the Reference line was worth the premium over the Logic line.

The industrial design of the dm series of amplifiers is nothing short of stunning.  Many of my guests did not recognize it as a piece of stereo equipment.  Its unique looks and exceptional fit and finish would easily allow one to confuse the amplifier for a piece of modern art or fine furniture.  The best way to describe the unique design is that of a large H, albeit one with two crossbars.  The amplifier is 31 inches high, 16 inches deep and wide.  The 120-pound amplifier is skinned in a brush-finished aluminum, with attractive wooden feet.  The uprights of the H are oval in shape when viewed from above, reminiscent of an aerofoil.  The uprights are slotted on the outside surfaces to form large heat sinks.  The amplifier’s housing not only provides a unique look, but it actually contributes to the audio performance, in that its power supply is in its own box to isolate it from the amplifier stage.

The lower of the two crossbars contains the switch mode power supply and the upper has the audio connections and signal amplification circuits.  The upper section is further internally divided into three separately-shielded compartments for input circuits, power stage and output connections, which include a large inductor acting as an RF trap.  The shielding for the individual compartments is extensive, using a 16mm-thick aluminum plate between the input and output stages, in addition to numerous strategically placed smaller plates.
The IEC power plug is on the underside of the lower crossbar and the switch is a discreet rubber-housed push button on the underside of the upper cross-bar.  There dm38 has three inputs: balanced, unbalanced voltage mode and an unusual unbalanced current mode.  Selection is via a rotary switch.  The speaker binding posts are covered with large, easy to grasp knobs.  The current mode works with the Halcro amplifiers to achieve an even lower noise floor.  This is done with a very low input impedance of 60 ohms (as compared to a normal voltage input of 100,000 ohms), the interference is literally swamped by the low impedance letting the signal through.

The dm38 is a large, powerful stereo amplifier, with 350 watts per channel into a four-ohm load.  Unlike the previously-reviewed MC series amplifiers, the dm series is a traditional analog Class AB design.  Total harmonic distortion is less than 3000 parts per billion, which is very, very low.  The noise rating is provided in the format of less than 5nV/sqrt at 1 kHz in the more commonly used voltage mode (this is one-sixth the noise of the MC series).  Halcro claims numerous built-in protections.  Luckily, I never had the chance to test them.  The built-in protections include a guard against short circuits, excessive temperature or current, D.C. offset, etc.  The power supply features its own built-in protection against transients, over-voltage, incorrect frequency, etc.  Halcro notes that the noisiest portion of any amplifier is the output stage and that the dm38’s output stage is less noisy than the entirety of most other amplifiers.  The use of high-speed, complimentary FETs, coupled with distortion-canceling circuits, ensures a fast, accurate and quiet output section.

The audio amplification circuits of the dm38 utilize extremely high-speed components, limited negative feedback and active correction circuits.  Much care is spent paying attention to details, such as careful layout and shielding. The dm38 uses a combination of floating power supplies to eliminate the distortion caused by the non-linear nature of the semiconductors.  The PCB layout and design maximizes the power supply and ground distribution, while isolating the signal from external influences.  The PCB in the power stage is six layers; in the input stage, it employs four layers to achieve this objective.  Each stage has been optimized to obtain speed and minimize distortion.

Halcro is especially proud of their Power Factor Corrected power supply design. The Halcro website has a white paper, which explains the system in detail. Still, I will try to explain it here briefly, in simple layman terms.  The power factor corrected, voltage from wall is in phase with the current from where it is drawn.  With most amplifiers, the voltage and current are out of phase.  Keeping the voltage and current in phase causes less problems with other devices by introducing very little noise or instability back into the electrical system.  The main’s AC voltage is rectified to DC and then switched back to an AC signal at a much higher frequency to accomplish the power factor correction.  The AC signal is then rectified to DC, then AC and DC once again to generate the required voltage.  The resulting power supply is a very stable and quiet voltage source isolated from the AC mains; with a large reserve of energy, you can actually unplug the dm38 and it will still run for a short time. A further benefit in today’s world of rising energy costs is that the switch mode power supply is very efficient.


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