Class D offers efficiency, size and cost advantages (but not sound quality), allowing a proliferation of cheap, powerful amplifiers.

Pros & Cons

High power and low cost are desirable amplifier traits and help 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 $, less heat and smaller amplifiers. Class D also simplifies construction and allows fewer and smaller parts, reducing build costs.

There’s no such thing as a free lunch, though, and class D trade-offs revolve around performance or, in this case, sound quality. 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 also often don’t perform as well when driving really low-impedance loads of 2 or even 1 Ohm.

That said, there have been many improvements in class D designs in recent years and some of the latest class D designs perform well on paper, even spectacularly well in a few cases. Note that spectacular on-paper performance does not automatically translate into spectacular sonic performance, however.

For the ultimate sonic performance, class D is not the best choice. That mantle, as always, goes to class A.

This is not an opinion; it is an engineering fact. As with all things technical, there’s a bit to understand, so let’s dig in.

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, class D is sonically inferior. This is why cost-no-object designs never use 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?

Class D offers big cost savings, which means greater margins and therefore bigger profits for manufacturers. It also means greater bang-per-buck in some cases, so it’s not a total loss. One just needs to understand the details.

Use-Cases

Class D is a bit like plastic. Given the choice, most things would be built out of metal and wood rather than plastic, because these are more beautiful, more durable materials. You can make some fancy and cheap parts out of plastic, though. Plastic = lighter, faster, cheaper. Class D = smaller, more powerful, 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 (mis)understanding of the technical aspects of hi-fi engineering generally comes from marketing materials and retailers. The problem again is the conflict of interest and the poor technical literacy of these sources.

Here’s the bullsh*t HiFi+ spewed about the ‘new’ class D amplification in NAD’s plastic C-298 power amplifier, for example, 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, so here’s my translation:

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 now cannot remember. Many turned out to be unreliable due to their 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 really serious gear, 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 almost always find class A in high-end products.

Note that the real high-end does not include NuForce, Bakoon, B&O, NAD, Bel Canto, etc. This stuff is amusing, but most definitely NOT 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 just fantastic.

Be wary of your sources, conflicts of interest and clickbait like “Build the best amplifier in the world for $500”. 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.

Class D has come a long way in recent years, and better designs are coming to market all the time. Class D is a useful topology, but it’s helpful to know and understand why there are better choices when it comes to the best amplifiers.