Class D offers advantages in terms of high efficiency, small size and low 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 $. Class D also simplifies construction and allows the use of fewer and smaller parts, reducing build costs. This is great for those wanting smaller, cheaper amplifiers.
There’s no such thing as a free lunch though and class D trade-offs include higher distortion and noise, and unwanted RF byproducts. Class D amplifiers also often don’t perform as well as conventional designs into tough low-impedance loads either and for ultimate sonic performance, class D is not the right choice. As with all things technical, there’s a bit to understand, so let’s dig.
Conflicts of Interest
Many will try to convince you that class D is inherently sonically superior to class A or class AB, but there is nothing from a technical perspective that would make class D superior and various technical reasons why it is inferior. It doesn’t sound better either, but there are many reasons why some hearing for the first time might jump to that incorrect conclusion, which I’ll cover below.
Class D is cheaper and more efficient, so it suits the fancy one-box solutions dressed up as high-end gear (Devaliet etc) and it works very well for less critical home cinema, AVR and subwoofer duties for example. Engineers and experienced audiophiles know this of course but manufacturers and retailers need to sell new equipment, so we have a conflict of interest problem.
Think about it. Retailers advising people about new hi-fi equipment NEED to sell that equipment you can’t expect anything other than a sales push. Imagine taking your old car to the dealership and asking the salesperson if they think you should keep it or get one of the new ones they sell. Imagine asking them about a brand they don’t sell. Do you honestly think the advice you’d receive would be worth anything? My point here is to be very critical about where you source information.
But Mike, if class D is no good, why do manufacturers use it?
Bemused enquirer
This is a better question. I’m not saying class D is “no good”. Class D is great in certain use cases and, perhaps more significantly, offers big savings on production costs, which means greater margins and therefore bigger profits for manufacturers. Sadly, the mainstream hi-fi media and retail industry receive kickbacks, junkets and deals and they’re hardly going to tell people this because it jeopardises their ability to get the gear and therefore make money.
Plastic
Class D is like plastic. Given the choice, most designers would build things out of metal and wood rather than plastic. These are more beautiful and durable 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 if you need lighter, faster and cheaper!
Likewise, given the choice, 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 $100,000 and 200kg each, and nobody would buy them! Plastic dashboards crack and fade too, and you won’t find too much plastic on a tank or in an aeroplane wing. You’ll see where this is going.
I remember chatting with a retailer about speakers. He told me that a famous loudspeaker manufacturer’s drivers now used injection moulded plastic baskets and frames and that this was better than metal because the marketing materials said so. I explained some engineering and pointed out that this manufacturer’s most expensive speakers still used drivers with cast metal chassis.
Is plastic ever the best choice? If you need to save money and weight, sure it is. Is class D ever the best choice? Again, if you need to save money and weight, or if low cost and high power are the overriding considerations then yes, class D is the best choice.
Use-Cases
Class D amps are great for subwoofers and home cinema amplifiers where high power density and low cost are critical. Who doesn’t want a small, affordable 1000W subwoofer?! Manufacturers like NuForce and B&O have produced class D amplifiers for the hi-fi market, whilst B&O’s ICEpower class D modules find use in concert, club and live venue environments where high power, efficiency and ruggedness are more important than absolute sound quality.
All-in-one amplifier/DAC/streamer things, sound bars, subs, PA amplifiers and AV receivers use class D amplifiers for one set of reasons: low cost/small size/high power/high efficiency/high margins. In these roles, class D performs perfectly.
Power
Powerful amplifiers sound impressive, especially to hi-fi enthusiasts who’ve previously only owned low-powered amplifiers. It’s even more impressive when they are affordable and tap into the ‘upgrader’ market of people moving up from basic gear. People taking that next step are impressed by the punch and drama that only powerful amplifiers can create and many of these folks are the ones raving about class D.
Bull$hit
Here’s what HiFi+ reckons about the ‘new’ class D amplification in NAD’s plastic C-298 power amplifier, 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 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
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.
True high-end gear doesn’t need to be small, lightweight, efficient or affordable, it just needs to sound and perform THE BEST. Therefore, one should always look to these real high-end use cases to see what “the best” looks like. Note: real high-end does not include NuForce, Bakoon, B&O, NAD, or Bel Canto.
At the real high end, there are no advantages to using class D and many disadvantages. For this reason, you won’t find class D in high-end products, and there’s your answer.
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” despite their extraordinarily high cost. Good on Stereophile for being brave enough to call it. I’m sure ML was not pleased.
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. People trying to save money or receiving a kickback you don’t know about usually aren’t the best sources of impartial advice and rarely have the listening experience with serious gear to offer useful opinions on it. Be wary of clickbait like “Build the best amplifier in the world for $500”. You don’t need me to explain why.
For the record, please know that I want a $500 class D amplifier to be better than a $50,000 class A amplifier like everyone else does, but it isn’t, it won’t ever be and it’s foolish to imagine that it could be. Come on people.
Class D is not bad. A class D amplifier may turn out to be the best amplifier you’ve heard and if so, great. Just be sure you listen to a range of products before forming an opinion and maintain a very sceptical eye, and ear!
