Class D offers advantages in efficiency, size and cost, allowing small, powerful amplifiers to be produced cheaply.

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 very 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 traditionally include higher distortion, noise and unwanted RF byproducts. Class D amplifiers also often don’t perform as well driving tough low-impedance loads (really low-impedance loads, not 4 Ohms which is the new 8 Ohms!). That said, there have been many improvements in these designs in recent years and this is a good thing.

For the ultimate sonic performance, class D is rarely the right choice, unless high-power is the overriding consideration. As with all things technical, there’s a bit to understand though, so let’s dig in.

Conflicts of Interest

Many will try to convince you that class D is inherently sonically superior to class A or class AB, but first, you need to look at who is doing that and why. Note: I have nothing to sell you and a strong interest in educating people, as you might expect of an ex-science teacher.

There is nothing from a technical perspective that makes class D superior and various technical reasons why it is inferior. There are however many things that make class D designs powerful and cheap to manufacture. There are many reasons why folks hearing a powerful class D amp for the first time might jump to an incorrect conclusion though and we will get to that.

Class D is cheaper and more efficient, so it suits one-box solutions – Bose, B & O, Devaliet etc – and it works very well for less critical home cinema, AVR and subwoofer duties. Engineers and experienced audiophiles understand this of course but manufacturers and retailers need to sell new equipment. This creates a very real conflict of interest that is 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. Imagine asking about a brand they don’t sell. Do you honestly think you’d receive impartial, technically informed advice?

My point here is to be very critical about where you source information. Sadly, the mainstream hi-fi media and retail industry receives kickbacks and incentives to sell new gear, not promote older, better gear. They’re hardly going to tell people this because it jeopardises their ability to get the gear and therefore make money — retail 101.

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

Bemused enquirer

This is a better question. It’s not that class D is “no good”. Class D is conceptually very interesting and works very well for certain use cases. Perhaps more significantly, class D offers big cost savings, which means greater margins and therefore bigger profits for manufacturers. It’s fair to say though that there is no inherent sonic advantage in class D, and many inherent sonic advantages in class A.

Plastic

Class D is a lot like plastic. Given the choice, most designers would build things out of metal and wood rather than plastic, because these are more beautiful and durable materials. You can make some fancy and affordable parts out of plastic though, car and motorcycle parts for example. Plastic = lighter, faster, cheaper. Class D = smaller, cooler, cheaper.

Class A is the best option where cost is not a factor, but cost is always a factor. You can’t build 500 Watt class A PA amplifiers because they would cost $100,000 and 200kg each, and nobody would buy them because nobody in the nightclub gives a sh*t about the better sound that you can’t hear through the garbage PA speakers!

I remember chatting with a retailer about speakers. He told me that a famous loudspeaker manufacturer’s drivers now used injection moulded plastic baskets and frames and that this was better than metal because the marketing materials said so. I explained some engineering and pointed out that this manufacturer’s most expensive speakers still used drivers with cast metal chassis.

Is class D ever the best choice? If a manufacturer wants to save money and weight, or if low cost, decent performance and high power are the overriding considerations then yes, class D is undoubtedly the best choice.

Use-Cases

Class D amps are great for subwoofers and home cinema amplifiers where high power density and low cost are critical. Who doesn’t want a small, affordable 1000W subwoofer?! Class D also works well for low to mid-priced high-power amplifiers.

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.

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. In these roles, class D performs perfectly. Powerful amplifiers sound impressive, especially to hi-fi enthusiasts who’ve previously only owned low-powered amplifiers.

Power is even more impressive when the equipment is affordable and taps into the ‘upgrader’ market of people moving up from basic gear. People taking that next step are impressed by the punch and drama that only powerful amplifiers can create and many of these folks are very often the ones raving about class D.

Bull$hit

The problem with the way class D gear is marketed is one of a lack of scientific challenge to the claims made, and a general lack of critical engagement with the narrative. Here’s what HiFi+ reckons about the ‘new’ class D amplification in NAD’s plastic C-298 power amplifier, for example, with nonsense underlined:

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’

Here’s the de-marketingBSed translation. HiFi+ needs to do better:

NAD has abandoned the tried and tested, more expensive and better performing linear power supplies and Class AB output stages that use some of the energy consumed 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 to traditional topologies, NAD has used designs that have been around since the 1950s and saved a ton of money on transformers, metal and therefore production and shipping costs, helping boost an ailing manufacturer. These designs are not as linear over a wide bandwidth as Class AB designs (but we can’t say that). They provide consistent performance into all speaker loads as any good amplifier does (oops), providing no advance over previous models, (again oops).

Liquid Mike

Improvements and The High-End

Class D was the flavour of the month in the naughties when people like Srajan Ebaen at 6 Moons pumped brands nobody had ever heard of and people lapped it up. Many products turned out to be unreliable due to their use of SMD components, cheap, off-the-shelf modules and low-cost build and manufacturing. Try to find out how many NuForce amplifiers are still working…

Class D has come a long way since then and now has more of a place in the low to mid-price sector of the market. It’s still just a tool though, a way to achieve high power, low cost and a small form factor with an acceptable suite of compromises. You only have to look at really serious, expensive gear to see what amplifier topologies are used there and things become clear.

Measured alone, some of the newer Hypex class D modules do very well. We must always consider that we don’t listen to measurements though. Ultimately, it comes down to how equipment performs sonically, and how well it lasts after the warranty expires. In terms of value though, there is no denying the price-performance ratio of some of the newer class D modules.

True high-end gear doesn’t need to be small, lightweight, efficient or affordable, it just needs to sound and perform THE BEST and be incredibly well-made. Therefore, one should always look to these real high-end use cases to see what “the best” looks like. The real high-end does not include NuForce, Bakoon, B&O, NAD, or Bel Canto, etc, etc.

At the high end, there are no advantages to using class D and many advantages to using the gold standard, class A. For this reason, you generally won’t find class D in high-end products, almost without exception. The Mark Levinson No 53 class D monoblocks are an exception and Stereophile described them as “disappointing and flat-sounding” despite their extraordinarily high cost. Good on Stereophile for being brave enough to call it.

But Mike, lots of hi-fi gear is class D and I’ve read that it’s just fantastic. Guys on Audiogon reckon its great and a guy on YouTube says they are the best amplifiers in the world!

Bemused enquirer

Reality

Much of what you read is nonsense, you have to know this and if you don’t, I’m telling you it is. People trying to save money through DIY often aren’t the best sources of impartial advice and rarely have the listening experience with serious gear to offer useful opinions on it.

Industry kickbacks are a problem across the board, top to bottom, and the low-cost high-value sector is not immune from them, so keep this in mind. Be wary of clickbait like “Build the best amplifier in the world for $500”. For the record, I want a $500 class D amplifier to be better than a $50,000 class A amplifier like everyone else does. Show me one that is and I will be proudly telling everyone about it, that I can promise.

Class D is not bad, it’s very useful, like plastic, but there are simply better choices when it comes to serious high-end gear. A class D amplifier may be the best amplifier you’ve heard though, and if so, great. Just be sure you listen to a range of class A/AB/D products before finalising your opinion.