Class D advantages are high efficiency, smaller size and lower cost, allowing a proliferation of cheap, powerful amplifiers.

This topic is so poorly understood, however, that I’ve written this FAQ just to address the marketing spin and general misunderstandings around it! For those who need the quick answer:

Class D is not new and is a cost-cutting solution, not a sound quality-enhancing one. Class D is like plastic, or front wheel drive – useful, cheap to implement and not inherently bad, but just not the best solution, where cost is no object.

This is a technical observation, not disputed by those who understand the engineering.

Pros & Cons

High power and low cost are desirable amplifier traits and explain the popularity of class D amplifiers in the low-cost, high-value sectors of the market. Class D delivers high efficiency compared to class AB amplifiers, meaning more Watts per dollar, less heat and lighter, smaller amplifiers. This reduces manufacturing, inventory and shipping costs, whilst retaining retail pricing, meaning everyone makes a little more money.

There’s no such thing as a free lunch, though, and class D trade-offs revolve around certain aspects of performance. Class D amps generally have higher distortion, noise and unwanted RF byproducts caused by the high-frequency switching inherent in these designs. Class D amplifiers often don’t perform as well when driving low-impedance loads of 2 or even 1 Ohm. Also, because of their budget design intent, class D amplifiers often don’t last anywhere near as long as conventional class AB designs.

There have been many improvements in class D designs in recent years. Some of the issues associated with class D can be designed around, and some of the latest class D designs perform well on paper, spectacularly well in a few cases. For the ultimate sonic performance, class D is not the best choice. That mantle always goes to class A. This is not an opinion; it is a simple engineering truth.

Conflicts of Interest

Some will try to convince you that class D is inherently sonically superior to class A or class AB, but from a technical perspective, this is simply incorrect and easily observable in testing. This is why cost-no-object designs are not class D. Class D is cheaper and more efficient, though, so it suits one-box solutions – Bose, B & O, Devaliet, etc. It works well for less critical home cinema, AVR and subwoofer duties.

Engineers and experienced audiophiles understand this, but manufacturers and retailers need to sell new equipment, creating a conflict of interest ever-present in audio. Imagine taking your old car to the dealership and asking the salesperson if they think you should keep it or get one of the fancy new ones they sell. Do you think you can receive impartial, technically informed advice from people whose livelihoods depend on selling these new cars?

But Mike, if class D is no good, why do manufacturers use it?

Simple: class D offers cost savings, which means greater margins and bigger profits. It also means greater bang-per-buck in lower-cost market segments.

Use-Cases

As I mentioned, class D is a bit like plastic and front wheel drive. Given the choice, most objects would be built out of metal or wood rather than plastic, because these are more beautiful, more durable materials. Likewise, cars would be rear or all-wheel drive rather than front wheel drive. You can make fancy, cheap parts out of plastic, though, and front wheel drive cars are cheaper to build. Class D = smaller, cheaper.

Class A offers the best performance and is the best option in amplifiers where cost is not a factor. Cost is always a factor, though. You can’t build 500-Watt class A PA amplifiers because they would cost $100,000, weigh 200kg each, and nobody would buy them.

Class D is perfect where high power, small size and low cost are needed, like PA gear, subwoofers, home cinema amplifiers and receivers and low to mid-priced power amplifiers. All-in-one amplifier/DAC/streamer things, sound bars, subs, PA amplifiers and AV receivers use class D amplifiers for one set of reasons: low cost/small size/high power/high efficiency/high margins.

Manufacturers like NuForce, B&O and Hypex have produced class D amplifiers and amplifier modules for the hi-fi market. B&O’s ICEpower class D modules find use in concert, club and live venue environments where high power, efficiency and ruggedness are more important than absolute sound quality. In these roles, class D performs well.

Bull$hit

Consumer and retailer (mis)understanding of the technical aspects of hi-fi engineering is historic, unchanging and just something we must deal with. Conflicts of interest and poor technical literacy are why this will never go away.

For example, here’s the bullsh*t HiFi+ spewed about the ‘new’ class D amplification in NAD’s plastic C-298 power amplifier, with nonsense highlighted:

NAD has moved away from the old-fashioned and very power-hungry linear power supplies and class AB output stages that waste nearly half of the energy consumed, producing heat rather than sound. Instead, the company has developed even better performing circuits based on switch mode (sic) power supplies and class D output stages. Once thought to be inferior to traditional topologies, NAD’s advanced work in this area has created some of the best-performing amplifiers regardless of basic design principle. These new designs are very linear over a wide bandwidth and provide consistent performance into all speaker loads, providing a dramatic advance over previous models.

HiFi+ ‘staff’

HiFi+ needs to do better. Here’s my translation, with a healthy dose of reality added back in:

NAD has abandoned proven, high-performance but more expensive to manufacture linear power supplies and class AB output stages. These are less efficient because they deliberately use some energy to improve sonic performance. Instead, the company has recycled existing, poorer-performing but cheaper to make circuits based on switching power supplies and class D output stages. Known to be inferior but currently trending in hi-fi land, NAD has used old designs from the ’50s and ’60s to save money on transformers, parts, metal and therefore production, weight and shipping costs. These designs provide no real advancement over previous A and AB models except increased margins and profitability, which helps NAD.

Liquid Mike

The High-End

Class D was the flavour of the month in the naughties when people like Srajan Ebaen at 6 Moons endlessly pumped brands people had ever heard of and cannot remember. Much of this turned out to be the typical unreliable junk we see at the boutique end of hi-fi manufacturing due to the use of cheap, off-the-shelf modules and low-cost build and manufacturing.

Class D has come a long way since then, but you only need to look at serious equipment (actually serious rather than marketed as serious), and things become clear. True high-end gear doesn’t need to be small, lightweight, efficient or affordable; it just needs to be superbly engineered and sound and perform THE BEST. At the real high end, there are no advantages to class D and many advantages to the gold standard, class A. For this reason, you always find class A in high-end products.

Note that genuine high-end does not include NuForce, Bakoon, B&O, NAD, Bel Canto, etc. This stuff is amusing, yes, but NOT remotely high-end. The Mark Levinson No 53 class D monoblocks are a rare exception, and Stereophile bravely described them as “disappointing and flat-sounding” despite their extraordinarily high cost.

Bottom Line

But Mike, lots of hi-fi gear is class D, and I’ve read that it’s fantastic.

Be a sceptical reader and carefully consider your sources. Remember that the magazines and online publications where you see advertising are heavily compromised and effectively cannot be trusted to reliably report on such things – the HiFi+ bullshit above is a classic example of this.

I don’t know anyone who doesn’t want a $500 class D amplifier to be better than a $35,000 class A amplifier like my Accuphase A-75. The problem is, this is like wanting a Hyundai to be a Ferrari, a Timex to be a Rolex, or a Fosters to be a fancy IPA. Want it, but know that it’s also just silly.

Class D has come a long way, and better implementations are coming to market all the time. Many of the disadvantages of class D can be designed around, but it’s far easier to start with something that doesn’t have those disadvantages. Class D is a useful topology, but there are better choices when it comes to the ultimate performance.