I’ve been intrigued by class D amplification for many years. But besides some active speakers and a few pro audio models where efficiency and power output takes precedence over fidelity, I’ve yet to hear a true hi-fi class D amplifier. It’s only really been in the last 15 years or so that class D, as a technology, has developed to the point where it is generally accepted within audiophile circles. Not that audiophile acceptance would ever stop me using a product, but the abysmal measurements demonstrated by many first-generation class D products certainly did. Engineers have more recently made considerable efforts to optimise the technology to a point where it can equal, or even surpass, a traditional linear class A or class A/B design.
Perhaps the most notable of these designers in recent times is Bruno Putzeys of Hypex Electronics. Founded in 1996, Hypex supplied plate amplifiers to the pro audio market and later to hi-fi manufacturers, first with a line of plate amps developed for subwoofers and later multi-channel units and active crossovers for studio use. Bruno developped the Universal Class D (UCD) technology during his work at Philips and was hired in 2005 by Hypex to lead its R&D and engineering, after the company agreed a licensing deal with Philips for the UCD technology in 2003.
Unlike the implementations of class D switching typology that came before, UCD modules have a flat frequency response irrespective of load impedance, nearly frequency-independent distortion behaviour and very low radiated and conducted EMI. Putzeys invented the technology working in the Philips lab in Leuven (Belgium) producing an initial 25W module intended for use in a television, and later developing modules with more power and a refined design. Further to his joining Hypex, Putzeys also founded Mola-Mola – a high-end brand for which he designed a complete range of five-figure amplification and source electronics.
Today, Hypex is primarily split into two divisions. Via diyclassd.com, Hypex directly supplies products to the DIY market, while Hypex Electronics also supply products to OEM manufacturers. Many highly respected brands – Marantz and NAD to name just two – are now using Hypex technology in their flagship products, with many more following suit.
The current crop of UCD amplifiers bare little resemblance to the original 2001 circuit. A mathematically exact understanding of self-oscillation allows optimisation of large-signal performance. Improved comparator circuitry, where the PWM signal is created by comparing the input signal to the signal of a high-frequency triangular wave generator, insures that actual behaviour matches the theoretical model as closely as possible. And new gate drive circuitry improves open-loop distortion at moderate signal levels while significantly reducing idle losses. In short, Hypex Class D amplifiers are is ultra-linear, with noise and distortion levels that are next to non-existent and should certainly be inaudible to the human ear.
NCore amps are largely based on the same principles but also implement a control loop adding 20dB of gain across the full audio range without sacrificing stability.
The primary, and really the only advantage of a class D amplifier over and above a good linear design is efficiency and compactness. Class D is an analogue switching typology, and class D amplifiers are often paired with a switching power supply. After all, pairing a class D amplifier with a linear power supply as in a traditional amplifier outweighs much of the class D advantage. Though it has to be said that neither a switching amplifier nor a switching power supply are especially suited to amplifying audio.
A switching amplifier is, as its name implies, one in which the transistors are operated either full on or full off, the pulses modulated by the audio signal. They use either pulse width modulation (PWM) or pulse density modulation (PDM) techniques. Switching transistors use no power when fully off and when fully on present the least resistance and thus the lowest possible power consumption. Class D amplifiers operate at frequencies well beyond the audio band, bringing further challenges in reliability and immunity to noise, both external interference and self-noise.
Before outputting a signal, the switching pulses must be filtered to minimise or eliminate the effects of the unwanted interference and high-frequency noise generated by the switching amplifier. The low pass filtering is passive and can make class D amplifiers load dependant which is a problem, especially given that the load characteristics of a speaker vary with frequency. It is therefore much harder to design a class D amplifier that will behave in a linear, predictable fashion as a well-designed and more traditional class A or A/B amplifier should.
The NC400 is a neat looking module. The circular (88 mm diameter) PCB is mounted to a thick plate of aluminium, acting as a heat spreader to transfer the minimal heat generated by the module to the surrounding enclosure. The signal path is entirely discrete and the board stuffed with high-quality components throughout.
The SMPS600N400 is a more traditional rectangle though is equally well made, with filter capacitance in excess of 4700UF and added circuitry to interface with the NC400 to allow for instantaneous shutdown in case of a fault. When used together the modules feature over-voltage (72V), under-voltage (35V), excessive DC on the output, over-current, over-temperature and short circuit protection. I am aware of some reliability issues with SMPS1200A700 power supplies but not with the SMPS600. My only minor gripe with the design is the metal heatsink which is aligned directly over top of the capacitors and does get hot in operation, especially if you’re running a pair of modules from a single board. Straightening the heatsink, or a plate to couple the heatsink to the enclosure and transfer the heat away from nearby components may improve long-term reliability.
The SMPS600 can handle mains voltages between 100-120 (low line) and 200 – 240 (high line) ±10% at 47-63Hz. It outputs a nominal ±65V DC and can power one or two NC400 modules, though with a performance penalty to be expected if running a pair of modules from a single supply, demonstrated in this Audio Science Review. For reasons of brevity, interested readers can view data sheets for both the NC400 and SMPS at the DIY Class D website.
The NC400 Mono Kit is supplied with all necessary hardware and components. The latest generation is supplied with a pre-soldered XLR chassis cable, pre-crimped power cables and even pre-twisted speaker cable with the ends already stripped. This is a nice touch and means that assembling one of these amplifiers involves only two screwdrivers and a little time and care. A full instruction sheet is provided but it’s quite self-explanatory. Great care should of course be taken when working around the high voltage and current present, and beginners and seasoned pros alike should be sure to double and even triple check their work before powering up the amp.
The full components list is as follows:
- 1 set of case parts including top (denoted by vent holes), bottom (denoted by multiple screw mounting holes), front (a pleasingly thick slab of aluminium) and rear (denoted by connector holes).
- Screws and accessories: Including M3 screws to mount the modules, larger screws and washers for the enclosure, front panel mounting brackets and two front panel grub screws with included Allen key.
- Four chassis feet including screws
- Cables: Including mains connection, chassis ground, switch jumper, module interconnection molex, twisted pair of speaker cable, LED cable, and chassis mount XLR jack with pre-attached cable.
- 1 SMPS600N400 power supply with preinstalled spacers.
- 1 NC400 amplifier module.
- 1 pair (red and black) of speaker terminals.
- One power switch and IEC socket.
- One LED board with screws.
Assembly is straight forward. The rear panel is attached to the base of the enclosure, and the input, output and power connectors fitted. The speaker terminals screw into place at the rear, and the XLR connector is attached by two screws, washers and nuts. The IEC connector and power switch clip into place.
Next, the modules are fitted. There are a lot of holes in the base of the enclosure to accommodate a range of UCD and SMPS modules. The below picture shows the screws used for the configuration of NC400 and SMPS600N400. Four screws hold the power supply and three hold the module, with three of the power supply spacers being plastic and one being metal for grounding.
Next up is the cabling. I routed the input cable beneath the speaker terminals and connected it to the relevant socket on the NC400 module. The plugs are all keyed with clips and will only fit in one direction. The speaker cables are by far the most difficult to connect. The red (positive) connector goes on the terminal closest to the outside edge of the board and then to the upper of the two speaker terminals. Ideally, the cables would be mounted toward the insides of the terminals to allow the pair to be twisted as far as possible. This isn’t possible with thicker cables and requires you opt for something with a thinner conductor which far outweighs the minor benefit that a further couple of twists will offer.
The screw on the top of each terminal is loosened such that the clamp drops sufficiently to accommodate the thick conductor, but not so far as to release the clamp completely as they can be quite frustrating to realign. If you should accidentally release the clamp entirely, flipping the amp over and reinstalling it from beneath makes the task much easier. These terminals are my only gripe with these modules; there are far better PCB-mount screw terminals available. Nevertheless, with the cables mounted to the module they are then installed into the rear of the rear speaker terminals with a small flat-bladed screwdriver.
Next up, power cabling. The large 12-pin cable connects between the NC400 and SMPS600. Again the plug only fits one way but is unnervingly tight. I felt the need to carefully support the connector socket on both module and power supply while pushing this plug home.
The AC wire is connected to the power supply and to the IEC connector, with the neutral line directly to the connector and the live wire to one side of the switch. The included jumper cable connects from the other side of the switch to the live terminal of the IEC connector, and the included ground cable connects to the IEC connector earth and bolts to a designated hole on the chassis beside the IEC connector.
The LED board can then be installed, and its cable connected to the power supply. The power cables should run neatly along one side of the chassis and are secured with the included cable ties. It is important that power cables are kept well away from any signal cabling.
Finally, the top is installed and the two front-panel brackets. Once fitted, the front panel hooks on at the top and drops down, and is secured with the two grub screws at the bottom for which the required Allen key is provided.
In their finished state the, NC400s are smart looking amps. THey’re tiny too, with dimensions more akin to a typical DAC or phono amplifier than a pair of amps able to chuck out RMS figures of 200 watts into an 8 ohm load, doubling down to 400W into 4 ohms and stretching out to 580W into the 2 ohm minimum load. Hypex don’t specify for how long the amplifiers will maintain these figures, though they do quote a continuous figure of 75W into an 8 ohm load when 2 NC400s are driven from a single SMPS600, and 150W when a single NC400 is driven from a dedicated SMPS600 as is the case here. Signal to noise ratio (SNR) extends out to a staggering 125 dB, below that of some of the very best digital components let alone analogue preamplifiers. Frequency response is ruler-flat out to 50kHz. Total harmonic distortion + noise across the audio band (20Hz – 20kHz) is again vanishingly low at just 0.0007% into a 4 ohm load.
The amps claim 93% efficiency with an idle power draw of just 8W each. Input impedance is an unusual 104k ohms and Input sensitivity is again slightly higher than the norm at 2.05 V. Neither of these specifications are likely to affect the amplifier’s use with any commercially available DAC or preamplifier.
There’s no drama on power-up, with a slight click from the power supply and a faint pop from the speakers after a few seconds during which the amp stabilises and comes out of protection. Almost no self-noise is audible in the speakers, with only the slightest hiss barely audible if you press your ear into the tweeter. In use, the amps get warm to the touch though never hot. The temperature may vary slightly between NC400 units due to the way the modules are trimmed in manufacture to minimise distortion. Nevertheless, these amps are certainly cool for their power and size, and certainly run much cooler than any traditional amp of similar power would. If they were mounted in a larger case with larger heatsinks as a linear design would be, they would run stone cold.
Perhaps the only conveniences I would have liked are a trigger input, or at least a front-accessible power switch. The SMPS600 does have a form of triggering but it’s the direct opposite of how a trigger function would normally operate. Applying a voltage to a designated pin will put the SMPS600 and thus the NC400 into standby. When that voltage is removed the SMPS600 powers up, triggering a normal soft start of the NC400. This is the inverse of a typical trigger, which applies a voltage to switch on a connected component, and removing that voltage is the signal for the component to switch back into standby.
An XLR loop output would also be a welcome addition, for which there would just about be sufficient space on the back panel if the terminals were moved slightly. This would allow several NC400 monoblocks to be chained together to bi-amplify the system. One can certainly build the amps into a custom enclosure and add these functions, but it would be nice to see them as standard. A big stack of these little Hypex boxes bi-amping or tri-amping a pair of speakers would I’m sure be as aesthetically pleasing as it would be sonically.
I’m not at all bothered by the lack of a single-ended input. It’s possible to add one; should you wish to do so the wiring is detailed in the NC400 documentation, or RCA to XLR cables are readily available. However running a well-designed differential amplifier with single-ended sources defeats the purpose and will never exploit the full potential of the amplifier. Many preamplifiers, or source components with preamplification, are balanced or pseudo-balanced at the very least and have the required outputs to directly feed these amps. It is recommended to use a good quality screened XLR cable for best results.
There’s little I can say about the sound of these amps. It is well documented that Hypex amps are some of the cleanest, most neutral and uncoloured amplifiers you can buy and I certainly found this to be the case. They’re as close to a straight wire gain as anything I have ever heard, and hence are extraordinarily revealing especially of poor recordings or below average source components. I would generally recommend pairing a better source with a cheaper amp and would stress that this is of vital importance here. Despite their ability to show up just about every flaw in the system or music, however, the Hypex amps are a delight to listen to.
I was surprised at the amount of power on tap. They don’t best a linear design of similar ratings in this area, though they had more than enough ‘oomf’ to drive a pair of reasonably inefficient speakers to uncomfortable levels without showing signs of strain. They pair beautifully with more efficient tannoys, where their output is such that the balanced doesn’t veer towards hardness but there is enough juice for the Tannoys to lap up, bringing a sense of energy. None of this is without finesse or refinement, and the levels of detail you get throughout the audio band are staggering. There is more musical information to tuck into here than I have heard from most amps, even those which make the cost of the Hypex look insignificant.
As I said in the opening paragraph, I’ve been wanting to try a pair of quality class D amps for a long, long time. The Nc400s are a perfect demonstration of what class D technology is capable when properly implemented. They are no-nonsense amplifiers with breakthrough technical performance and stunning sonics, in a package no bigger than a hardback book. Cool running and highly efficient, perfect if you’re eco-conscious, it is tough to find fault with them.
This is one of the few products where I have no gripes, though I would like to see some revisions. I’d like to see proper triggering. It’s highly likely that users of these amps will either keep them tucked away due to their size or have them on show in a rack where their power switches may be difficult to access. I’d also like to see higher power versions with higher current capability for those who need more grunt without the limitations in current and compromised dimensions that come with bridging multiple units. And I’d like to see XLR loop outputs provided as standard to facilitate bi-amping or tri-amping a stack of amps built into these enclosures, as frankly, I think a stack of four or even six of these units would look as beautiful as it would sound.
But despite these minor amendments I would still wholeheartedly recommend Hypex NC400s to anyone looking to own some of the best amplification you can buy at any price. For that is what you get with a little time and effort. At a little over a grand for the pair, these have to be one of the biggest bargains in high-end audio. Highly recommended.
