Class-D offers the key advantages of high efficiency and low cost, allowing powerful amplifiers to be produced cheaply.
High power and low cost are desirable characteristics and help explain the popularity of class-D amplifiers in the value sector of the market. Class-D delivers very high efficiency compared to class-AB amplifiers and more Watts per $. It also simplifies construction and allows the use of fewer, physically smaller parts, reducing build costs.
There’s no such thing as a free lunch though and class-D tradeoffs include higher distortion, noise and unwanted RF byproducts. Better sonic performance is not a class-D advantage, but, as with all things technical, there’s a bit to understand, so let’s dig.
Frame of Reference
Don’t let anyone try to convince you that class-D is sonically inherently superior to class-A or class-AB, because it isn’t. It IS more efficient though, so it suits some of the fancy one-box solutions you see these days dressed up as high-end gear. Engineers and experienced audiophiles know this, but manufacturers and retailers need to sell new equipment of course.
Think about the conflict of interest. The retailers potentially advising you about new hi-fi equipment NEED to sell you the equipment. It always makes me laugh when people don’t see this. I mean, imagine taking your old car to the dealership and asking if the salesperson thinks you should keep it or get a new one. Better still, imagine asking them for a model they don’t sell! 😂
Class-D offers big production cost savings which means greater margins and therefore profits. Sadly, the mainstream hi-fi media and retail industry consists of folks who receive kickbacks, promos, deals and who generally aren’t well-placed to advise on topics such as this.
But Mike, if class-D is no good, why do manufacturers use it?
Bemused enquirer
This is a much better question. I’m not for one second saying class-D is “no good”. Class-D is VERY good in certain use cases, I’m merely clarifying what those are and why.
Plastic
You can think of class-D as being a little like plastic. Given the choice, most designers would build things out of metal and wood rather than plastic. These are more durable and more beautiful materials, but also more expensive. Plastic knives and forks anyone..? You can make some really fancy and very affordable parts out of plastic though, like car and motorcycle parts for example. Plastic = lighter, faster, cheaper. This is good, by some measures.
I remember chatting with a retailer about speakers. He was telling me how a famous loudspeaker manufacturer’s drivers now used injection moulded plastic baskets and frames. He tried to tell me that this was better than metal, because the marketing materials implied so. I explained some engineering aspects and pointed out that this manufacturers most expensive drivers still used die-cast metal baskets.
Are plastic or class-D ever the best choices? That depends on the use case. If low cost and high power are the over-riding considerations then yes, class-D and plastic are the best options. If the highest performance, appearance and longevity are the goals, then they will not be.
So now you know why there is so much plastic and class-D amplifiers in modern cars and hi-fi gear and it isn’t because plastic or class-D are inherently superior. All-in-one amplifer/DAC/streamer things, sound bars and AV receivers use class-D amplifiers for one set of reasons: low cost/high margins/high bang-per-buck.
Power
Powerful amplifiers sound impressive, especially to power-deprived hi-fi enthusiasts who’ve previously owned low-powered amplifiers. If they are also affordable, it’s even more impressive and taps into the ‘upgrader’ market moving up from basic gear. Class-D often appeals here because people taking that next step are impressed by the drama powerful amplifiers can create.
Class-D amps are great for subwoofers and home cinema amplifiers too, where high power density and low cost are important. Who doesn’t want an affordable, small, 1000W subwoofer?! Power IS desirable and manufacturers like NuForce and B&O have produced class-D amplifiers for the hi-fi market, whilst 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.
The High-End
True high-end gear doesn’t need to be small, lightweight, efficient or affordable though, it just needs to sound and perform THE BEST. Therefore, one should always look to these end-game use cases to see what the best really looks like. At the real high end, there are no advantages to using class-D, so you basically don’t find it and there’s your answer. NuForce? B&O? NAD? Bel Canto..? This is not high-end gear, fancy though it may look.
Class-D was the flavour of the month in the naughties, but many designs turned out to be unreliable due to their use of SMD components, cheap, off-the-shelf modules and low-cost build and manufacturing. Class-D definitely has a place, mostly in the high-value, low cost sector of the market.
By every quantitative and qualitative sonic measure, class-D is inferior to class-A and class-AB, when other factors are equal. Therefore, where cost is not a factor, there is almost no class-D, except the Mark Levinson No 53 class-D monoblocks for example, and Stereophile described them as disappointing and flat-sounding.
But Mike, lots of hi-fi gear is class-D and I’ve read that it’s just fantastic. A guy on YouTube says they are the best amplifiers in the world!
Bemused enquirer
Reality
Keep in mind that most of what you read in the mainstream media including YouTube is paid for and technically ill-informed. Makers trying to save money often aren’t the best sources of impartial advice, so be wary of clickbait like “Build the best amplifier in the world for $500”.
Who doesn’t want a $500 class-D amplifier to be better than a $50,000 class-A amplifier?! I certainly do, but it isn’t. A class-D amplifier may turn out to be the best amplifier you’ve heard and that’s great, but just be sure you listen to a range of products before forming on opinion. Wanting class-D to be the best performer overall is like wanting to be taller. Instead, try to understand why Accuphase, Bryston, Gryphon etc use class-A or AB designs for their best equipment.