Halcro dm38 Stereo Power Amplifier 
Home Theater Power Amplifiers Stereo Amplifiers
Written by Brian Kahn   
Friday, 01 August 2008

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.

I was lucky enough to have several months to audition this amplifier in both a traditional, two-channel stereo system and a multi-channel theater system.  In the stereo system, I used my favorite CD player, the Classe CDP-202, and the excellent Conrad Johnson CT-5 tubed preamplifier upstream of the Halcro dm38.  The primary speakers were my MartinLogan Summits, but I also used the Acoustic Zen Adagios.  I began with Transparent and Cardas cables and was easily able to discern the differences between them with these highly resolving electronics.

I later moved the dm38 into a new room and integrated the stereo system into my home theater system via the Conrad Johnson’s theater pass-through feature.  At this time, I changed the cabling to Kimber cables, as the theater system primarily utilized Kimber cables.  The cables used in the two-channel portion of the system were from the Kimber Select line.  I note here that all the stereo components were plugged into my venerable Equi=Tech, with the exception of the dm38, which was plugged directly into the wall mains outlet with the Kimber Palladian cord.

Music and Movies
I began with by checking out Halcro’s claims of a power supply that could handle anything thrown its way.  One of the DVDs I have been listening to lately is Godsmack’s Changes (Coming Home Studios), specifically the track “Battalla de los Tambores.” While this is far from the traditional audiophile review material, it provided the dm38 an excellent opportunity to demonstrate its dynamic capabilities.  This track features two drummers battling it out in an enthusiastic and powerful piece, reminiscent of multiple drum solos pieced together to show off the drummers’ skills and impact.  This track makes for an exhilarating and exciting demo on a system with good dynamic range and extension.  The detail between the various drums was easy to discern, both spatially and tonally.  The dm38 retained an iron grip over the speakers at all times, keeping this explosive track under control.

The Halcro dm38 had no problems driving either pair of speakers as loud as I dared listen, without any sign of stress or compression.  I noted with interest that the Halcro MC70, with the same power rating, could not drive the MartinLogans to the same level as the dm38. I attribute this to the massive power supply of the dm38, which is shared by only two channels.

Sticking with bass heavy popular music rather than audiophilia, I played Crystal Method’s Vegas (Outpost Records) and went straight to one of my favorite tracks, “Busy Child.”  The deep and powerful synthesizer track on this piece has long been a favorite for testing a system’s bass speed and control.  The dm38 maintained complete control of the speakers at these lower octaves without any loss of detail.

As I listened to Dire Straits’ Brothers in Arms (Warner Brothers), the dm38 simply got out of the way of letting me enjoy the music.  I found myself listening to the entire album, not concentrating on the system, but simply absorbing the tracks.  This happened with many albums during my time with the dm38.  Mark Knopfler’s voice was reproduced with such detail and balance that I could easily close my eyes and believe I was at a live performance.  The guitars and drums were all well-balanced among each other with a good sense of space in a large soundstage.  I noted that the electric guitars were particularly piercing (in a realistic manner) when the dm38 was coupled to the Acoustic Zen Adagios, which feature a ribbon tweeter.  My experience with ribbon tweeters is that the slightest bit of distortion upstream of the speaker causes them to be quite harsh.  The Adagios’ tweeters remained smooth, clean and extended throughout my listening session with the dm38.

I then moved on to Jeff Buckley’s rendition of “Hallelujah” on Live at Sin-e (Columbia Records). The track begins with just Buckley’s voice above the acoustic guitar.  Both were portrayed with detail and solidity that lent a sense of realism.  What struck me even more listening to this track was the sense that the guitar and vocals were coming from a real point in space that was solidly anchored between and slightly behind the speakers.

Although the dm and MC series of amplifiers are quite different in design, they are said to share sonic similarities.  In order to ascertain this for myself, I found my notes from my MC50 review.  I again listened to two pieces from that review. One was Shawn Mullins’ Soul’s Core (Sony).  The MC50 had done a good job with rendering the vocal details in the song “Anchored in You.”  In comparison, the dm38 rendered detail on a whole new level.  I would not have known I was missing anything had I listened only to the MC50, but once I’d heard the dm38, I realized that there was much more information that was previously unrevealed.  The second piece I went back to was Elvis Presley’s Elvis is Back (DCC), specifically listening to Elvis’ version of “Fever.”  My notes focused on the sense of space.  My listening impressions through the dm38 closely mirrored my notes, yet I have the feeling that dm38 again rose above, even if not as obviously as before.

Lastly, I went to audiophile recordings, as I suspect that Halcro, especially the reference line, has a large audiophile following.  Jennifer Warnes’ Famous Blue Raincoat (BMG/Classic) remains a favorite selection of Leonard Cohen songs.  The track “Bird on a Wire” features Warnes’ husky vocals solidly positioned in the center, the triangle was to the left where it belonged, the drums were a good distance back next to the horns with a few other instruments filling out the stage.  The soundstage was solid extending beyond the outer edges of my speakers and away past my front wall.  Despite the size of the sound stage it was continuous, without gaps but with each instrument solidly placed.  The Halcro did this consistently throughout the album. For example, the vocals and each of the instruments on the title cut were not only well-anchored in their respective places on the soundstage, but they also had the proper sense of space around them.  The string section was coherent and lush, yet I was still able to pick out individual instruments and notes.  The tenor saxophone just sounded right.  It had the proper tone and body, without any of the artificial harshness of recorded horns.

The Downside
The dm38 is very difficult to fault sonically. It really doesn’t do anything wrong.  It is just slightly on the cool side of neutral.  As with the MC50, I was not enamored of the speaker connectors, although the connectors on the dm38 are completely different than those of the MC series.  The dm38 only has a single set of posts per channel, making bi-wiring a bigger chore.  Also, as described above, the binding posts accept only spade connectors.  Lastly, while I found the unusual industrial design to be a welcome change, some will not like it and will be upset to see that it will not fit neatly into the shelves of their a/v units.  Personally, if I owned a piece of equipment this stunning, I would not want it stuck in the middle of a shelf unit, but rather front and center where everyone could see it.

I have reviewed and/or auditioned all of the competition, and the Halcro dm38 is one of the single finest stereo amplifiers available at any price. It is extremely fast and detailed, without introducing a sense of analytical harshness.  Its revealing nature will necessarily mean that it should be paired with clean, high-quality electronics, as lesser-quality gear will be ruthlessly revealed.  If I was pressed to identify any sonic signature, it would be that the dm38, like the MC series, is slightly cool.  If it makes any errors on veering towards thinness or lushness, it is the former.  As with the MC series, lovers of tubes, especially triode amplifiers, will find the Halcro sterile.

Overall, the dm38 is one of the most detailed and revealing stereo components I have ever heard at any price in any system.  It is expensive, but at this level of investment in your system, you get to choose the very best of the very best, so in the end, it comes down to what sonic flavor you prefer most.  If that flavor is one characterized by speed, detail and neutrality, the dm38 is unquestionably your choice.
Manufacturer Halcro
Model dm38 Stereo Power Amplifier
Reviewer Brian Kahn

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