I’ll not beat about the bush. Until recently, I’d not heard any full-range Class D amplification that I felt to be on a level with what’s possible from well-executed Class AB or Class A amplification. While the Class D that I have heard did seem to give a good account of itself when asked to handle a limited bandwidth, it consistently came up more or less short when given the full-range task. I’ve heard very convincing subwoofers powered by Class D, for example the Fathoms by JL Audio, but also absolutely the worst ever full-range monoblock power amplifiers of my audio career. These – I will not name them to avoid blushes all round – used much-lauded Class D modules by a famous OEM developer and, despite their five digit price ticket, sounded dynamically flat, tonally bleached and did very weird stuff with timing.
But last year Danish loudspeaker brand Dali sent me a pair of Oberon 7C active speakers, powered by Class D modules. Much to my surprise I liked them a lot, finding very little to pick at. Might it be that with the advantage of being able to design and therefore control the entire chain from input to speaker cone Dali had been able to deliver something quite unusual, or was Class D finally developing some respectable high-end credentials?
As design engineers like to remind us, Class D is highly efficient. It turns into heat only around 10% of the electricity it uses, whereas even the most efficient Class AB amplifiers waste some 50% as heat, and Class A a ball-park 80%.
This is not only of relevance to those concerned about the size of their electricity bill, but it has implications for the cost of manufacture too. Class A and -AB are, on the surface, less expensive to make than Class D, but when the necessary supporting elements are factored in, such as the cost of the required heatsinking and a chassis large enough to support such a radiating structure, Class D is the cheaper technology.
For example, because it wastes less power as heat, a typical 200 Watt Class D amplifier will require no heatsink, and can therefore be housed in a smaller and less expensive chassis than an equivalent Class A or -AB amplifier. For the amplifier vendor this translates into an approximate 30% saving. As Skip Taylor of Texas Class D module developer Elegant Audio Solutions likes to point out, this is one reason why over the last decade Class D has gradually become the dominant amplifier technology in portable, mobile and automotive applications.
In an audiophile context though, as someone smarter than me once observed, Class D is a technical response to a question that nobody ever asked. It hasn’t been in the works because it promised better sound quality than Class A or -AB designs, but because it allows the space and power consumption savings for non-audiophile applications, and cost reductions that may (or may not) be passed on to the buyer at the point of sale.
That Class D might now be said to be knocking on the door of Class A and -AB sonic performance is only because of the technical curiosity, some might say bloody-mindedness, of pioneers such as Putzeys, Lyngdorf and Taylor – I mention just three engineers, but there are significant others – who have been on a mission to ‘make’ Class D viable for audiophile applications.
Until recently they have had their hands tied to a significant degree by the speed and accuracy with which contemporary silicon gain devices are able to switch states. This is a complex area and I am going to skate over it. If readers want to acquire a full understanding of what’s going on when semi-conductors are used in an amplification chain that is pulse width modulated as in Class D, there are plenty of resources on the internet at various dive depths.
The key point is that until recently the go-to transistor used in Class D was the MOSFET. While MOSFETs from various manufacturers differ to a degree in their technical specs, MOSFETs can be made to switch states only so quickly. This generic speed limit, along with other characteristics, results in unwanted noise smack in the audio band. To mitigate the noise Putzeys and others apply almost Biblical levels of negative feedback in their designs. The claim is that it has no sonic consequences, but based purely on listening tests, I am not so sure, and neither is our Editor Jason Kennedy with whom I discussed the matter in preparation for writing this review.
Reader; if you have a current generation Class D amplifier and you are happy with it, then blessings be upon you. Really. But Jason and I – many others too – hear very clearly the sonic signature of much Class D. He calls it a hollowing out of the sound while I would call it a lack of dynamic expression; instruments and voices just don’t have the air-compressing energy that they have in real life and which Class A and -AB amplifiers (there are exceptions) can generally provide a reasonable facsimile of.
Musical veracity thief
Some in audio design circles who have similar aural sensitivity think this sonic deficit results from the feedback, and they are welcoming the gallium nitride field effect transistor (GaNFET) as an advance that may at last enable Class D to match Classes A and -AB in the sonic quality stakes. Simply – I warned you I was going to skate – GaNFET performance parameters are superior to those of MOSFETS; much faster switching and with lower levels of distortion and noise which in turn means the amount of negative feedback required is reduced. If feedback is the thief of musical veracity then the potential wins from GaNFETs are obvious.
Skip Taylor’s Elegant Audio Solutions has been one of the first to put into production amplifier modules built around GanFETs, and his company’s commercial off the shelf GaNAMP1002 module and HP800 switch-mode power supply are to be found in the newly-launched Peachtree Audio GaN400 stereo power amplifier. Peachtree produces a complementary pre-amplifier, called the preDAC, which incorporates an ESS delta-sigma DAC chip, a phono stage and headphone amplifier.
As a package for buyers wanting an affordable and competent audio entertainment hub, the preDAC is competitive. But from a review perspective, what really interested me was the GaN400 amplifier and how it compared with my reference Class AB amplifier.
Class D amplification requires the deployment of serious engineering chops, and there are consequently only a few companies currently with the resources required to produce OEM modules. It was therefore almost inevitable that the amplifier boards and switch-mode power supply found in this amplifier would be specified by brands other than Peachtree, and so they are. However, I particularly like the look of Peachtree’s implementation of the EAS modules in the GaN400. It is thoughtful. We get an auto-sensing voltage selection, a neat input board, tidy hook-up wiring that aims to minimise the potential for pollution of the audio signal path by the power supply, as well as liberal use of ferrite cable clamps to mitigate electro-magnetic interference. As far as I am aware the Peachtree remains unmeasured on an independent test bench, but by eye alone it’s a more reassuring deployment of the EAS boards than some others.
Externally, Peachtree’s GaN400 amplifier and the preDAC have necessarily different face plates but are otherwise identical in dimensions and finish, the review samples an immaculate piano finish ‘mocha’ veneer. Black is also available.
Sound quality
Back-to-back with my reference Bryston 4B Cubed Class AB stereo power amplifier, the GaN400 gave a good account of itself, sounding rather un-Class D-like. Fed the output of a Mola Mola Tambaqui DAC and a PS Audio Phono stage via first the Peachtree preDAC, and then for most of the time an Icon4 Pro passive line stage, the GaN400 turned out to give a surprising and pretty even measure of the four key musical qualities.
Ignore suggestions you may have read elsewhere that the GaN400 sounds like a Class A tube amplifier – that’s just fanciful thinking. In fact it’s a commendably neutral, uncoloured and subjectively electrically quiet gain device with an output of 400 Watts per channel. Where it differs from much of the MOSFET Class D amplification field is in its solid musical believability, giving a level of dynamic expression that comes close to the best that Class AB can do.
I was ready to dismiss the GaN400, but right out of the box it gave an overall performance that grabbed attention, and in a good way. Over extended listening, to mostly electronically-derived recordings, the GaN400 proved to be a fairly close match to the Bryston 4B Cubed, only really losing out in the lower reaches to the Bryston’s bass slam, but in terms of apparent bandwidth and linearity proving very competent.
On acoustic material, and particularly recordings where there had been minimal intervention at the production desk, the Bryston’s relative refinement pulled it clear by a significant margin though. Against the 4B Cubed, the GaN400 fell a little short in portraying vocal weight, and the air-moving power of strings for example. It also had a drier, slightly more grainy presentation, and a flatter, less dense portrayal of the performance space. But these criticisms are relative; we need to remind ourselves that the Bryston is more than twice the price of the Peachtree.
Regular Class D amplifiers generally offer a lot of power for the money. The GaN400 does too, but adds to it a level of sonic sophistication that left this Class D sceptic thoughtful and marginally disorientated. Driven by the accompanying Peachtree pre-amp the GaN400 gave a better, more rounded and believable musical performance than I associate with even the best of MOSFET Class D amplification.
When the Icon4 Pro passive line stage was used in place of the preDAC the GaN400 revealed itself to be capable of better still performance, with generous servings of the immediacy, dynamic expression, timing acuity and spatial detail that we expect as audiophiles. I have no hesitation in observing that thus paired, the GaN400 is truly competitive with Class AB stereo power amplifiers at a similar price point or higher and is therefore well-deserving of consideration. Audiophile Class D may be truly coming of age.