July 2003

World Premiere!
Halcro dm10 Pre-Amplifier And dm68 Power Amplifier
Review by Alvin Gold
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  Over a long career I have had the pleasure or otherwise of testing and sometimes owning a variety of amplifiers, some poor, many better, and a few – very few - that could be described as great in the best sense of the term. The first and also the most recent Krells some into this most select category, so a couple of decades ago did the Audio Note Ongaku (editor's note: as agreed upon the Ongaku's excellence by editor Steven R. Rochlin who personally had one in his humble abode for quite a few years). The Ongaku is an unfeasibly expensive low power single-ended "Class A" zero feedback tube amplifier. Then there were the Naims: iconoclastic, lean and mean, but somehow right when playing music though a wider variety of systems than they are commonly given credit for. And of course there were others.

But with my first sight of the dm58, the smaller of the two Australian made Halcro power amplifiers, I knew beyond question that amplifier design had reached a new zenith. But of what? Musical achievement, Aerodynamics? Technology? Or absurdity perhaps? What exactly is this amplifier that looks more like a wind tunnel model for a hypersonic re-entry vehicle – not even a proper airplane please note. Is it for real? Or is Halcro just taking the – excuse the word, but there's no really satisfying Angle-Saxon alternative – piss?

Doubleplusgood

The dm68 is the more costly of two Halcro power amplifiers – the other is the dm58. This is not a single-ended amplifier, remember, and it is solid-state, not valve powered. The matching pre-amplifier is only a little less absurd – for one thing few pre-amplifiers are as heavy as this 22.5kg leviathan – but we'll come back to this later. All these questions, and more like them, real and implicit, will, or in some cases will not be answered in what follows. A selective reading of the maker's specifications for the power amplifier shows that it weighs 125 lbs. and measures 31 x 16 x 16 (HxWxD in inches). The same as the junior dm58 in fact, but power output is up, all the way from 200 Watts to 225 Watts, which is a fraction of a decibel in real money. But power really isn't the issue here. The differences mainly concern the inputs (see later) and in the distortion residual, which are significantly lower on the senior model. Halcro also claim that sound quality is better, but this is not something that can be tested in the context of this review.

The introduction to the instruction manual makes the central pitch for the amplifier. 'Halcro designs and manufactures the only ‘Super Fidelity' (sic) amplifiers in the world... (which) produce better than 99.9998% purity of all tones across the entire audio range. This’ (it concludes with almost British understatement) 'is as close to perfection as you can reasonably expect.' So Halcro is pinning everything on distortion, and the claim is that the amplifier nowhere introduces more than 0.0002% THD, corresponding to <120dB up to 20Hz, or <1,000 parts per billion at 400 Watts/4 Ohms, which is full power, or -134dB at 1kHz (200 parts per billion). Intermodulation is held to similar levels. Other high end amplifiers are typically around the 200,000 parts per billion mark. But these figures are not achieved through the usual means, swamping the amplifier feedback, which would work with steady state signals, but which would impact on transient performance. In fact there is no global feedback, only modest levels of local feedback; each stage has been designed from the ground up such that distortion is defined not by the active devices, with or without feedback, but by the ability of the resistors to follow Ohm’s Law. Not that distortion is the be-all and end-all. The designer, Bruce Candy, has been an audiophile and music lover since childhood, and his aim was to produce an amplifier 'which does not color the sound with its own electronic characteristics'.

The dm68 is a monoblock, designed to be placed as close to the loudspeakers as possible. Given that this monoblock is as large as many loudspeakers in their own right, this could lend a cluttered appearance to the business end of some rooms. But there is no gain saying the superb build and imposing presence.

Sweet Dreams (Are Made Of This)

Like almost everything else about Halcro, the in and outputs hold their own surprises. There are two pairs of bi-wirable loudspeaker terminals per monoblock designed for spade terminals, with a rubberized caps which do away with the traditional wrench to achieve a good mechanical grip. There are four switch selectable inputs. The standard unbalanced input uses phono connectors, and has a 100k Ohm input impedance, enough to ensure that even components with a high output impedance will not see any frequency response modification. A balanced input is available using an XLR connector.

The real surprise comes with inputs three and four, one a so-called minimal path voltage input, and the other a current mode input. Both use RCA phonos, with an impedance of 660 Ohms and 50 Ohms respectively. Balanced mode has all the usual advantages over unbalanced: great signal/noise, and immunity from external AC fields and any residual hum loops (which of course should not be there in the first place), due to common mode rejection. Essentially any interference is picked up equally on both signals lines, and is cancelled out at the receive end of the cable. The direct input offers no such immunity, but it bypasses the first buffer stage and some other components early in the amplifier, and therefore should give a purer sound at the expense of gain. Coincidentally, the operating bandwidth is wider through this input. The intrinsic response of the amplifier is roughly DC to 1MHz, but low pass filtering restricts the output to around 70kHz (-3dB) through the normal inputs. Though the direct input, the bandpass filtering is relaxed to around 190Hz to 100kHz, and simultaneously the high pass filter that blocks frequencies below about 3Hz is removed. Finally, the current mode input is designed for connection to a high impedance current mode source outputs like the dm10, is essentially immune to cable differences. The only other user features are the main power switch near the mains input on the back, and the standby switch which is under the top horizontal section of the unit. Bicolour LEDs on the front and back of the unit indicate power and standby status.

Reliability and protection are predictably taken extremely seriously. All capacitors for example are rated for 105 degree C operation or higher instead of the usual 85 degrees. All ICs are 'at least' industrial grade, suitable for use from -40 degrees C to +85 degrees C or better, and highly linear resistors and capacitors are employed in the audio signal path. 4- and 6-layer PCBs are specified throughout, and the various circuit blocks are physically separated (where beneficial) and rigorously screened or housed in their own Faraday cages to avoid inductive pickup of stray EMF fields. EMI filtering is used between the amplifier circuits and the power supply. It is this separation into functional, mutually separated blocks that defines the external appearance of the Halcro power amps. The finish is satin anodized aluminum, and it looks incredible – in both senses.

Internal construction is mirrored by external appearance. The switch mode power supply section and the output circuits are housed in the box section in the base. The low level circuits are in the upper box section, and the output MOSFETs and associated heat sinks are in the left hand vertical member, the right hand section being essentially empty, though it does contribute to the amplifier's cooling.

Given the elaborate design and sheer physical scale of this amplifier, the 225 Watt power rating may seem rather modest. Halcro points out that there is at least one documented case in which an amplifier whose output doubles for every halving in load impedance caused its cables to catch fire. The dm68 is limited to 15 amps, but as the company points out, all tube amps are inherently current limited, and real loudspeakers are generally limited to 4 Ohms or above, and if they are not ‘one must wonder why’. There are large transient reserves, and in practice there was never the slightest hint that the amplifier could not deliver everything it was asked to, and more. The output devices – MOSFETs – are protected by a circuit that calculates their power dissipation and allows them only to operate within their design envelope, while other circuits monitor DC offsets, excessive current flows in the power supply as well as the output stage, main transients, input overloads and amplifier inter-stage protection. Over voltage protection is also provided for the small signal stages. Gradual limiting is introduced if the amplifier becomes too warm, and it almost goes without saying that the Halcro is short circuit proof. So I won’t.

The Universal part of Universal Power Factor Correction allows the Halcro to operate without any user switching or configuration with inputs ranging from 85V to 270V RMS and 45Hz to 65Hz. Power Factor Correction in effect relates to the switch mode power supply, which is designed to address the poor mains waveform that is available at most mains sockets which have the top and bottom of the waveform flattened, with typical THD level around the 90% level. The Halcro supply avoids this problem, allowing the power supply to behave as an ideal resistor. Switch mode power supplies are only rarely used high fidelity amplifiers (Linn and Chord are exceptions), and in general they have not had a good press. But this one at least has about as much to do with the supply in your personal computer as a peanut does to an elephant. It always operates around 115kHz, and is electrically efficient, wasting little heat and demanding only passive cooling. The supply stage is heavily shielded, and is flitted on its in and output side with filters to reduce RF interference and mains spikes.

We Two Are One

The dm10 is the senior of two pre-amplifiers which have just been launched almost simultaneously. The test sample is one of the first, perhaps the first fully production ready dm10 out of the factory, and it looks like a scaled down replica of the pre-amplifier. But it still weighs 48 lbs. and measure 17.64 x 9.44 x 15.74 (WxHxD in inches), figures that would do credit to a decently powerful high end power amplifier. The dm10 differs from the dm8 by including a phono stage, headphone amplifier and mono switch on the front panel.

The design brief is very similar to that of the power amplifiers, including superior reliability and "the lowest distortion of any pre-amplifier in the world" (our italics). Specific claims are that all circuits have distortion levels (THD, IM etc) that are 'unmeasurable' and have exceptionally low noise. Component quality is of the highest order (Vishay resistors and FKYI capacitors are given as examples). The circuits are designed to be largely immune to interference from EMF, with first order filters and common mode chokes on some in and outputs, with more stringently designed filters for the power supply and control microprocessors, of which there are several, placed in close proximity to the circuits they control. Again a switch mode power supply has been specified, with the switching frequency restricted to above 200kHz. Additionally, the power supply rails are said to be exceptional well double regulated, with switch mode and linear servo loop regulation.

The input stage is followed by a high impedance FET buffer which feeds an inverting amplifier, than the volume control, which in turn drives a high input impedance amplifier input. A continuously variable capacitor is also connected between the final amplifier and ground which helps set the loading conditions for the final amplifier stage, and also forms part of the MM phono loading. Halcro designates this arrangement as an ‘active load’ which is designed to lower the noise floor at high frequencies. There is a 3rd order high pass Bessel filter operating on the phono input below a few Hertz to suppress rumble or other low frequency noise. An RIAA filter follows, claimed accurate to within 0.25dB at all frequencies.

The pre-amplifier includes single ended and balanced inputs, and also a current mode and bridged current mode output. Single ended outputs are available for tape and headphones. Switching is performed using mechanical (not solid state) relays, a side effect of which is an initially disconcerting but innocuous ticking noise when selecting inputs. The volume control uses a 'unique magnetic incremental indexing mechanism' which has a similar effect. All knobs and buttons are covered in a soft rubber-like material.

A large, high resolution backlit LCD display show the full operating stats, with prominence given to the most important data, such as the input selected, volume level and balance. Phase (switchable) stereo/mono (phono only) and similar information is shown in smaller type. The remote control is made from solid extruded aluminum, with most of the controls in the arc naturally described by the thumb when it is being held – if you happen to be right handed. The switch mode power supply, display, panel electronics and the audio circuits are contained in their own internal screened enclosures, with minimal cabling. The control scheme is simple and rational.

Even more effort has been expended around the back. Selectable features here include adjustable resistance and capacitance, MC/MM switching and a three way gain setting for the phono input, earth local remote switching to avoid ground lops and a control to select single ended, balanced and current mode outputs. Doubling up of the unbalanced current outputs allows the dm10 to drive bridged power amplifiers, which is an option for the Halcro power amps. The balanced outputs can also be used for this purpose, but custom cables are required.

The dm10 will handle systems of almost arbitrary complexity, with nine pairs of input sockets capable of handling six source components – phono MM and MC (balanced and single ended!), a current source input, three balanced mode inputs and three single ended inputs. All bar the phono inputs are assignable to any numbered input. Separate volume control settings can be defined for the main output and headphones, and each input retains its own volume setting, which neatly resolves the problem posed by sources with differing voltage outputs – typical CD players and tuners for example.

The two Halcro components did not arrive by the usual route. The UK importer did not have samples; in fact he received his first pre-amplifier the very day I relinquished mine. The test samples came from Focal/JMlab in France, recently acquired for their own use, to use for exhibitions and the like, and presumably also to help with further product development in the future. They were supplied to me along with a pair of Focal. JMlab's new flagship Grande Utopia Beryllium, which was the ostensible reason for the test. Their thinking was to ensure that reviewers had ancillary equipment appropriate for driving loudspeakers with the Grande Utopia Be's resolving ability, and in more general terms in keeping with their sound quality, but the review was agreed with the Australian manufacturer and the UK importer.

I should also add a word of thanks to the US importer, Philip O' Hanlon of the US importer On a Higher Note, whose advice proved invaluable. This is not the kind of product that any reviewer can get to know fully in any reasonable time period. Not this reviewer anyway. It was at one of his dems at the last CES (Las Vegas January 2003) that I had a chance to audition the Halcro power amps with Wilson loudspeakers, and where I heard the first pre-amplifier to leave the factory – pre-production I believe.

Take Me To Your Heart

Although most of the Halcro listening, which extended over a period of a little over a month, was with the Grande Utopia Be (once settled in place, this is not a loudspeaker that is readily moved!), I did gather as much competitive machinery as I could. Included in the roster was a dCS Verdi SACD/CD transport, Purcell upsampler and Elgar Plus pre-amplifier, a Mark Levinson 390S used as a CD player and as a CD/Pre, a Musical Fidelity 1kW power monoblocks, a Krell KPS25 CD/pre and Krell FM400 power amplifier. Cables were from Synergistic Research, Nordost and Transparent. Last but definitely not least, a Roksan TMS2 turntable, Artemiz arm and Shiraz MC cartridge were used, driving the dm10 phono stage direct rather than Roksan’s own stepup.

The Synergistic Research mains connected loudspeaker and interconnects, which have something called Active Shielding technology, didn't last long in the system. They generated a sound that could only be described as messy and muddled. This are cables with mains polarized screens and blue indicator LEDs. All very exotic, but in this system at least they just didn’t deliver. There were plenty of dynamics, but the system simply wouldn’t focus. Nordost Valhalla was a much better proposition in most of the combinations tested. It was a little flatter dynamically, but resolving ability was on another planet, and the soundstage and musical information was organized in a way that was palpably musical. Nordost Valhalla single ended and balanced interconnects were used for all line level connections, and worked beautifully. Transparent loudspeaker cables, MusicWave Plus, which is one of the low end offerings from this stable, also worked very well, delivering a good compromise with resolving ability below Nordost levels, but firm, dynamic voicing. It is the kind of sound that makes you forget there is cable in the system.

Of the competing pre and power amplifiers only the Krell gave the Halcro dm68 a serious run for its money, and then only in very specific areas. Just one in fact: the bass, where the Krell simply runs rings around almost anything you can name. I should add that we are talking about the current Class A cx series power amplifiers here, and that from experience the 400cx is consistent with the rest of the series, the 750 in particular. The clearest demonstration of the Krells; ability was first identified with a peculiarly difficult track called "Big in Japan" from Tom Waits Mule Variations album with its extraordinary driving rhythms and the trademark Waits percussive attack. What the Krell bought to the party was the extraordinary timing, and the way the various parts of the song fitted together. It even sounded better in passages that were deliberately distorted. At least in my experience, and in combination with this loudspeaker, the Krell is the more convincing; and delivers an extraordinarily balanced performance, especially when used with Krell's own KPS25 (but without the CAST current mode interface on this occasion).

There were hints of the same generic shortcoming with a range of other discs too, which on consideration appears to be a system problem or characteristic. The Grande Utopia Be is a remarkable loudspeaker, but it doesn’t breathe as freely in my 10 x 4 meter room as it does in large spaces (I have heard it with Levinson electronics in Focal's own quite well designed demonstration room). In my room, the bass tends to spread and loose it’s focus to a degree, and the Krell's extraordinary discipline helps cut through this in a way that is unusual, and perhaps even unique. The Halcro is less forgiving. It would have worked better in the bass in Focal's own listening room, I suspect, but again this is improvable here.

What the Halcro does however is something very special indeed, something that I have never heard to the same extent from any other amplifier. The Krell may have timing and control down to a fine art, but the Halcro components, individually and together, offer definition such as you haven’t heard from an amplifier before, solid state or tubed. The level of detail extracted from recordings was extraordinary, aided and abetted by the Grande Utopia Be's equally exceptional beryllium tweeter it should be said. It seems pointless to expect this kind of resolving ability to be a factor unless the rest of the system is at least working in the same direction, but this was a system that thrives on what the Halcro does best, and one that benefits from the holographic image the Halcro presents of the source.

Needless to say some of the time was spent listening to SACD’s (the recent Benjamin Zander/Philharmonia Mahler 6 on Telarc for example. Enough to determine that the system works beautifully with worthwhile high resolution discs. But some of the most impressive experiences were with compact discs like Takemitsu Quotation of Dreams (Oliver Knussen/London Sinfonietta) on DGG, an exquisitely colourful and subtly refined recording, but only a compact disc. The most remarkable factor with the Takemitsu was the way that the Halcro was able to cast light into its deepest recesses, uncovering layers of jewel-like detail that had previously lain obscured. I think I played this disc about half a dozen times right through during the Halcro/JMlab's sojourn, which is some going for an already familiar disc.

This was not the only disc to be reinvented by the Halcro in this way, and unsurprisingly the extra light cast into the deep recesses of known recordings was not always an unalloyed benefit. The Halcro combination has the rather alarming property of always telling the truth, and the truth is not always palatable. Poor, threadbare recordings are not improved, and the effort of applying such a powerful audio microscope to material that doesn’t warrant such close up attention is a sound no more musical than through any other half decent amplifier. The real point here however is that when the recording is up to it, the Halcro does much more than extract detail, which is something you can do simply by turning the treble up with almost any amplifier. The Halcro however is not tonally bright. It is intrinsically detailed, but it is tonally near neutral, it is simply very expressive and very clean. There is no exaggeration of high frequency detail, or for that matter vinyl roar or other low frequency artifacts off vinyl, an the result is reproduction quality that is open and detailed but clean, and out of the box. The Halcro combo goes about its business holistically, and with perfect balance.

All right then, near perfect balance. The mid and treble are near neutral, while the bass is extraordinarily deep and disciplined, but some may find it dry at times. The opportunity didn’t arise to try the Halcro with a full range electrostatic like the Quad ELS (which however would not have been able to handle the power) or the (now sadly defunct) MartinLogan CLSIIz, but the mid and treble should be a perfect match to both. How the bass behaves is anyone's guess, but the clean, disciplined quality of the amplification will probably allow them to be driven harder than many other combinations.

Dynamically too the Halcro combination does a superb job. If you like your music pre-digested in the manner of many modern commercial recordings, you’d better look elsewhere. There’s no apparent cap on the dynamic range, and perhaps more pertinently, it is practically immaterial where you set the volume level. It is possible to 'talk over' a Halcro system even when it is playing quite loud, always a good sign, but when the volume is turned down to a whisper, fine detail remains audible and in proportion to the overall sound in a way that eludes other amplifiers. Similarly, it is not only virtually impossible to clip the Halcro power amp (in fact I was completely unable to do so, but those with very insensitive loudspeakers and large rooms many have better luck), the working dynamic range does not shrink perceptibly as the volume increase. This dynamic consistency, even more than the dynamic range itself, is truly the mark of a fine amplifier.

The other obvious property of the combination is that it sounds extremely clean. At first this is less noticeable than the opposite, which is that after the Halcro, everything else sounds vague grubby and inarticulate, slightly grey and even at times dirty. The fresh, open quality of the Halcro pair (music permitting) is what gives the music its tremendously liveliness and transparency, and the ability to resolve information, sometimes down to and into the noise floor is simply breathtaking.

So much for the combination. What about the individual components? The first thing to notice is that the pairing, which has not been reviewed in print elsewhere so far, is extremely consistent. Everything that the power amp does, can be heard in the pre-amplifier and vice versa, so that the effect of adding the pre-amplifier to the complete system (which is what happened, as the pre-amplifier arrived a week after the other components) is to reinforce the sense of cleanness, openness and transparency, the expressiveness and detail - all the things that the dm68 brings to the party. As far as could be judged, the MC phono stepup when driven by the Roksan combination is as free from an aural fingerprint and as dynamically wide ranging as the line inputs.

Naturally, some time was spent evaluating the various ways of connecting the two Halcro components together. The balanced mode input was a clear improvement over single ended, and the current mode input failed to show any real advantage over the latter, but perhaps would have done had I not been using cables of the calibre of Nordost Valhalla. But the direct input was the clear winner on all counts. There was plenty of gain in hand though this input, at least in the combination and in the room tested, and the aural results were clear: an unmistakably more immediate, cleaner and more transparent sound. Detail was even better resolved, and the sound had a more tactile feel.

No Fear, No Hate, No Pain (No Broken Hearts)

Which brings us almost to the bottom line. No excuses here about cost, weight and size. These things go with the territory at the audiophile bleeding edge. If you want practicality, go buy a sack of flour and learn to bake bread. This is hard core audiophile amplification which in most respects does nothing less than define a new state of the art for music reproduction in the home. I can't wait to hear what Halcro does next.

Specifications

dm10

Type: Component stereo pre-amplifier with MM & MC phono inputs

Inputs: balanced (XLR) and single ended (RCA): three single ended line inputs, assignable and three balanced line inputs, assignable plus one current mode single ended line input

Output: Line: tape monitor, Main: single ended x 2, balanced x 1, current mode single ended x1, direct single ended x1

Functions: stereo/mono (phono inputs only), mute, absolute phase switch, balance, volume, phono gain, resistance, capacitance. 

Supplied with full function remote control

Line Input Impedance: 10k Ohms single ended, 10k + 10k Ohms balanced

Output Impedance: 170 Ohms single ended, 170+ 170 Ohms balanced

Phono Stage: Moving magnet 10k Ohms – 60 Ohms + 60p–350pF
Moving coil 220 Ohms+4.7nF. 50 Ohms for current mode

Distortion: unmeasurable, below noise floor. 

At Full Output: <250 parts per billion (-132dB) for balanced, single edged & current modes.

Volume Control: +20dB - -60dB in 0.5dB steps. -60dB - +60dB in 1dB steps

Dimensions: 17.64 x 15.74 x 9.44 (WxDxH in inches)

Weight: 48 lbs.

Price: $15,990



dm68

Inputs: single ended, balanced, current mode single ended, direct single ended

Terminals: Bi-wire spade for loudspeaker

Power Output: 225 Watts 8 Ohms, 400 Watts, 4 Ohms

Distortion At Full Output: <-120dB THD up to 20kHz, 400 Watts, 4 Ohms. SMPTE IM intermodulation products each <-120dB.

Inputs: single ended 100k Ohms, balanced 100k Ohms + 100k Ohms, current mode 60 Ohms, direct 450 Ohms 

Dimensions: 16 x 31 x 16 (WxDxH in inches)

Weight: 125 lbs.

Price: $37,990

Company Information

Halcro
118 Hayward Avenue
Torrensville
South Australia
5031

Voice: +61 8 8238 0807
Fax: +61 8 8238 0852
E-mail: admin@halcro.com
Website: www.halcro.com

USA Distributor
On a Higher Note, LLC
26081 Via Estelita
San Juan Capistrano, CA 92675

Voice: (949) 488-3004
Fax: (949) 488-3284
E-mail: pohanlon@cox.net