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Since its inception in 1990, Helix has enjoyed considerable growth worldwide. An audacious yet modest company, its expansion has not only encompasseed acumen but also stable size, so that today’s Helix is a vastly more mature and experienced company to that of thenadays. This veritable master of electronics now offers everything from speakers and subwoofers through to processors and amplifiers — even combining the latter for certain models. In short you don’t find ‘bad’ Helix products, period.

Its specialty, though, is amplification. Though looking at the H 400X you could be forgiven for initially dismissing it as just another simple power amplifier. Sure it’s got a flashy appearance, but isn’t it just one among the myriad of other amplifiers on the market? What could possibly make the Helix H 400X worth owning over the competition? 

Well friends, take a seat — this about to get interesting, because there’s a whole swag of reasons… equating to the world of difference between a simple amplifier and a good one. 

Of course the eye-watering performance specifications are the immediate stand-out for this unit. But there’s also some very clever technology implemented here, especially where power delivery is concerned. 

Switching things up
More on that later though; let us first talk switching, as this was one of the more pressing concerns Helix faced when designing the H 400X. 

It was originally envisaged as an amplifier operable primarily within the Class-A realm — which is conducive to the warmest and purest sound. But sadly it’s also somewhat inefficient, and Helix no doubt didn’t want this unit drawing an ongoing flow of amperes even at idle. Put simply, the design needed to offer a warm sound without causing an in-car brownout. 

Helix has realised this prerequisite by introducing a certain level of Class A-B amplification, to ensure that the amplifier didn’t draw exorbitant levels of current during operation. 

Expanding upon this a little… we know analogue amplifiers are classed according to how much current is flowing during each wave cycle. We measure this in degrees, with 360º equating to current flowing during the entire cycle. The majority of Class-A amplifiers use the same output transistor to reproduce both the top and bottom halves of the audio waveform, and because it’s reproducing both, said transistor stays switched on permanently with full current flowing through it. Even when devoid of audio signal it still runs a 360º cycle continually flowing direct current, and is thus conducive to serious inefficiencies both thermally and electrically. 

Class-B switching, on the other hand, employs twin transistor groups with one reproducing the top or positive half of the waveform, while the other produces the bottom negative half — basically 180º apiece. This is more efficient because there’s no idle current flowing through the output transistors when there is no audio signal waveform. While this sounds a far better arrangement, all transistors require a small amount of current to turn on, and so this causes a blip in the sound waveform to appear as the transistors switch roles. So not as pure for sound quality, despite offering vastly superior efficiency to Class A. 

Hence the Class-AB design was born, whereby two groups of transistors are employed akin to Class B, but because transistors require power to switch on, the Class-AB design leaves a little bias current constantly running through them. This ensures they remain switched on, which in turn eradicates that blip in the waveform. So the overall ideology is that Class-AB is more efficient than Class A, yet enjoys a similarly warm-sounding output.

Theory in practise
Departing the theory and getting back to the practise, then, the H 400X is a four-channel design with each channel outputting 70 watts continuously when presented with a four-ohm load. Loading it down to two ohms will equate to upping the power output to 140 watts per channel, while bridging the channels will return you 250 watts at four ohms. If you load it down to two ohms bridged, you’ll double this figure… however if you find yourself needing to load it down like that, can I suggest you get yourself an SPXL1000 monoblock instead? 

These power ratings are fairly impressive, if not altogether staggering; but wait, there’s more. (Just grip something firm whilst I run you through the ancillary specifications, because they’re incredible.) First up the power comes wide a total harmonic distortion of 0.009%. Yes that is three decimal places. Hell, if that doesn’t ensure seriously clean and detailed sound, nothing will. Next up we deal with undesirable cone movement and to that end the damping factor is over 300, remembering that we humans cannot detect much difference over 50. Reciprocal accuracy is maintained thanks to an impressive slew rate of 7V/µs and channels are kept separated with very little cross-bleed thanks to a separation figure of 85dB. Operational noise is kept to a bare minimum thanks to a signal-to-noise ratio of over 100dB, and the overall operational frequency range is a sensible 20Hz to 20kHz, thus fulfilling its human hearing range without expending needless resources trying to reproduce frequencies only whales or canines will appreciate.

As if the sound wasn’t impressive enough, just wait until you happen upon the presentation of the unit. Akin to many Helix products the H 400X is stunningly presented, and utilises a delectable blend of aluminium and Perspex to great effect. The majority of the case is extruded aluminium, anodised black. It retains relatively small dimensions, with a footprint of 200mm x 336mm, and height of just 32mm, meaning that finding real estate for it is straightforward. 

The top of the unit is covered by a clear smoked Perspex cover, and why not? When you have an internal topology like this, the last thing you want to do is hide it. This cover not only allows you to see what’s inside but also doubles as protection for both the internals and connections it overhangs at each end. At the north end of the amplifier are gold-plated power, earth and RCA input terminals, in addition to the channel routing switches. At the other end are the four gold-plated speaker output terminals and half the audio controls, the other half being accessible through the top plate itself. The switch locations on the top govern the crossover configuration between high, low or through-pass, while the four potentiometers on the end control the crossover frequency, which is adjustable between 15Hz and 4kHz. Crossover slopes are second order with a Q logarithm of 0.7. The sensitivity controls are also located upon the top plate, up the other end adjacent to the RCA inputs.

With the internals on display for all to see, laid out like a Canberra street directory, do let’s delve into them further. The H 400X is a masterclass in intelligent amplifier design, chock full of high-quality goodies, an electrical odyssey starting with a bank of six capacitors to stiffen and smoothen the incoming voltage before it’s fed into twin toroid-core transformers for step-up to a final voltage of ±27V. 

Power is stored ready for usage in twin 40V/3900µf power capacitors, before being shunted through to two lines of bipolar output transistors clamped along the heat sink for maximum thermal dissipation.

Installation is hard to mess up, as is setting the gains if you’re handy with an oscilloscope. I ended up with the gains a tad above zero, raised ever so slightly to match my processor’s  output voltage.Not surprisingly it passed the zero-noise test with flying colours, and in  actuality you can set it well past three-quarters on the gain potentiometers before any  appreciable level of hiss presents itself. 

That’s dealing in sine waves though. Where the unit really shines is when you start playing actual music through it. If you’ve owned anything Helix previously, you’ll already be accustomed to its signature sound. But if this is your first foray into the brand, then rest assured you’re in for a real treat. It’s not so much the volume that’s staggering but the sheer tonal control and astute accuracy. You hear far more detail in your music than that provided by almost any other brand of amplifier. No matter the genre, be it the dulcet tones of a pipe organ through to the shrill screech of a Charvel guitar, the H 400X is scarily truthful and exact to the point of lifelikeness. Linearity-wise the output is also extremely smooth, as it concentrates more upon the business of providing you with an honest listening experience rather than hammering you blindly into submission completely oblivious to anything relating to quality...

Conclusion
The H 400X is one of those amplifiers where we feel genuinely despondent as our audition  reaches its conclusion and we prepare to remove it again, so much does it bring to the musical experience. (Permanent owners will not face this “reviewers’ curse”.)

Meanwhile for those out there who believe there’s little difference between amplifiers, I challenge you to swap out your existing unit for these. You may find yourself revisiting that opinion. ...

Helix H 400X incar amplifier 
Cost: $1099
+ Clean detailed sound
+ And plenty of it
- Hard to criticise
 
Type: Class AB four-channel amplifier
Power: Continuous power rating: 4 x 75W/140W continuous at 4/2 ohm
Features: High, low or through-pass crossover
 
Contact: Dynamic Audio