• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). Come here to have fun, be ready to be teased and not take online life too seriously. We now measure and review equipment for free! Click here for details.

Class D FAQ

Matias

Major Contributor
Forum Donor
Joined
Jan 1, 2019
Messages
2,519
Likes
4,054
Location
São Paulo, Brazil
#1
This topic is dedicated to Frequently Asked Questions about class D amplifiers. If you have been directed here the reason is that it is easier to concentrate the frequent answers here instead of repeating them in several topics.


Q: Class D power ratings cannot be true. How can these small and light "toy amplifiers" generate all these watts??

A: The key word is efficiency. That means less power is wasted as heat, and as a consequence, the manufacturers don't have to attach heavy heatsinks to the amplifier and build a large chassis. But these watts and amperes are all true, measured with Audio Precision gear.


Q: Yeah, but the continuous power is a fraction of the rated power. They simply cannot play 20 kHz full scale sine waves continuously for long! Look at the datasheet, they even admit it! Their FTC power rating is a fraction of what they advertise. This is a fraud!

A: Music signal is dynamic, its RMS it about -12dB from full scale. Also it has much more energy in the bass area, 20 kHz is usually -40dB. In practice, playing loud music in difficult speakers, class D amplifiers run cool and for long periods. More info here.


Q: This high frequency switching noise must be audible! If not directly, it will mirror artifacts into the hearing range. Will I get headaches listening to class D amplifiers?

A: The switching noise is typically around 400-500 kHz. This is way above what your tweeters can reproduce, and even if they could, it is way higher than what humans (and dogs and bats) can hear, so no headaches. There are no high frequency artifacts showing up on measurements, just check any IMD FFT graphs.


Q: But will this high frequency switching noise burn the tweeters of my precious speakers?

A: No, it will not damage your tweeters. The switching noise is very low in level, from the amplifier idle up to full power. More info here.


Q: But class D amplifiers have phase shift in high frequencies. Does the treble really come later than the rest of the music? I don't want to distort my music.

A: No, class D amplifiers do not have phase shift. They have a constant time delay that, when plotted against the diminishing wave period, looks like a curve. But in fact it is a constant delay for all frequencies. Music arrives at you ears all together. More info here.


Q: I read that class D amplifiers are only suitable for the bass range, and mids and treble are better served with a class A/AB amplifier. Is it true that the highs sound "metallic" with class D amplifiers?

A: It may have been true some decades ago because old designs had more distortion on the mids and highs compared to the bass, specially when demanded with more power. But modern high end designs have similar low levels of distortion in all frequencies and in all loads, so that they are fine for bass, mids and treble. In fact, the high frequency IMD from Purifi Eigentakt is state of the art compared to any topology, see here.


Q: I read about GaN transistors and how they are the future. With this technology we can shift the switching frequency way up to megahertz and solve all the issues, right? I will never have to worry about output filters and high frequency distortion again!

A: While the technology is interesting and promising, we have yet to see an amplifier that has significantly better measurements than what we have with silicon today. More info here.


Q: Class D amplifier manufacturers are not true "manufacturers". All these "assemblers" do is mounting boards into boxes, and connect cables and connectors. This is not true designing like a traditional amplifier manufacturer.

A: While this is partially true for some complete amplifier modules, other modules need more parts to work (buffer boards and opamps, auxiliary power supplies, etc). Some manufacturers also add more features (auto-on, variable gain, subwoofer out with low pass) or even integrate other functions (DAC, streaming, DSP, room correction, etc). As for the "romantic" view of designing and building amplifiers in house as it has always been done, see here.


Q: I am never buying a class D amplifier! Ever! I hate them! They will have to take my class A/class AB/tubes from my dead hands!

A: That's fine, no one is forcing you to buy a class D amplifier. Just please stop spreading FUD. :)
 
Last edited:
OP
Matias

Matias

Major Contributor
Forum Donor
Joined
Jan 1, 2019
Messages
2,519
Likes
4,054
Location
São Paulo, Brazil
Thread Starter #7
It's not my fault that Class D is topping the SINAD chart on this forum.
Actually the best measuring amplifier known is the Benchmark AHB2, which is class AB with switching power supply. But the majority of amplifiers in the blue portion of the ASR chart are class D.
 
Last edited:

pjug

Major Contributor
Forum Donor
Joined
Feb 2, 2019
Messages
1,140
Likes
935
#8
Actually the best measuring amplifier known is the Benchmark AHB2, which is class AB with switching power supply. But the majority of amplifiers in the blue chart are class D.
Also consider that it is lot easier to send a Class D amplifier to Amir so there are a lot of good Class AB amplifiers that have not been measured here.
 
OP
Matias

Matias

Major Contributor
Forum Donor
Joined
Jan 1, 2019
Messages
2,519
Likes
4,054
Location
São Paulo, Brazil
Thread Starter #10

pjug

Major Contributor
Forum Donor
Joined
Feb 2, 2019
Messages
1,140
Likes
935
#11
For 30$ per Watt (@ 8 ohm) it honestly better.

I consider it an outlier to be honest.
Consider it however you'd like. And go on a Quixotic quest to save the world with Class D if you'd like. I live in a house built in the 1800s so I'm not going to be able to fix my carbon footprint with Class D.
 

abdo123

Major Contributor
Forum Donor
Joined
Nov 15, 2020
Messages
3,158
Likes
2,739
Location
Brussels, Belgium
#12
Consider it however you'd like. And go on a Quixotic quest to save the world with Class D if you'd like. I live in a house built in the 1800s so I'm not going to be able to fix my carbon footprint with Class D.
I'm not asking anyone to throw away their perfectly working Class A or Class A/B amp and go buy a Class D amp.

But if you're not buying Class D you're effectively a science denier, people here shun 'cables make a difference' people so hard, and want the best and most efficient equipment at all time. But when it comes to class D they become really stingy about it (Thus the thread).
 

somebodyelse

Major Contributor
Joined
Dec 5, 2018
Messages
2,166
Likes
1,593
#14
Q: Class D power ratings cannot be true. How could these small and light "toy amplifiers" generate all these watts??

A: The key word is efficiency. That means, less power is wasted as heat, and as a consequence, the manufacturers don't have attach heavy heatsinks to the amplifier and build a large chassis. But these watts and amperes are all true, measured with Audio Precision gear.
This may or may not be true, depending on which "toy amplifiers" you're talking about. It's most likely true at the Purifi/Hypex/ICEPower/Crown end of the market, but at the bottom end they're usually quoting datasheet power of the chip they're using at 10% distortion into 4R, which only a subset will manage to achieve let alone maintain. If you look at what they're actually capable of they can still be good value, but the headline claims just aren't comparable to what we'd think of as a normal power rating. Compare the measured output of the Aiyima A07 with the headline 300Wx2 in their Amazon listing, and @pma's measurements at the heatsink with different supply voltages.

Another aspect that deserves a mention is that, depending on the amp topology, the high end frequency response may be load dependent as the load affects the response of the output filter. Hypex, Purifi and some others don't suffer from this, but many of the low end class D amps do. It's not necessarily a problem, but something you should be aware of, and look into, if you're going to buy one.
 

pma

Major Contributor
Forum Donor
Joined
Feb 23, 2019
Messages
1,918
Likes
3,158
Location
Prague

pjug

Major Contributor
Forum Donor
Joined
Feb 2, 2019
Messages
1,140
Likes
935
#16
I'm not asking anyone to throw away their perfectly working Class A or Class A/B amp and go buy a Class D amp.

But if you're not buying Class D you're effectively a science denier, people here shun 'cables make a difference' people so hard, and want the best and most efficient equipment at all time. But when it comes to class D they become really stingy about it (Thus the thread).
You are making all kinds of assumptions about people that probably are not true. There is a lot to consider about buying an amp. Have you thought about the generally much shorter warranty on Class D amps?
 

abdo123

Major Contributor
Forum Donor
Joined
Nov 15, 2020
Messages
3,158
Likes
2,739
Location
Brussels, Belgium
#17
You are making all kinds of assumptions about people that probably are not true.
Then the person who wrote this thread is making these same assumptions.

Have you thought about the generally much shorter warranty on Class D amps?
please elaborate, do you think Class D has a shorter lifespan?

or you think Class D is generally made by start-ups that cannot afford to be held liable for their devices?

The first one would require proof.

the second one is irrelevant.
 

DonH56

Major Contributor
Technical Expert
Forum Donor
Joined
Mar 15, 2016
Messages
5,485
Likes
9,605
Location
Monument, CO
#18
Science denier here. :)

Plenty of class AB amps have inaudible distortion and noise levels, and using switching (class G) or tracking (class H) power rails with an A/AB amp allows them to approach the efficiency of a class D amp. It should also be noted that class D is most efficient at maximum power; at low power levels it is much lower, though still very good. They each have their pros and cons.

One other misconception that keeps popping up is that the "D" is for "digital". No, it was just next in line in the invention of amplifier classes, it is not a "digital" amplifier.

For a very high-level handwaving look at how a class D amplifier works you can look at this: https://www.audiosciencereview.com/forum/index.php?threads/class-d-amplifiers-101.7355/
 

pjug

Major Contributor
Forum Donor
Joined
Feb 2, 2019
Messages
1,140
Likes
935
#19
Then the person who wrote this thread is making these same assumptions.



please elaborate, do you think Class D has a shorter lifespan?

or you think Class D is generally made by start-ups that cannot afford to be held liable for their devices?

The first one would require proof.

the second one is irrelevant.
I would just say I have more confidence in Class AB and linear power supplies. I have never had to throw out a Class AB amp. Class D just don't have much of a lifetime track record yet.
 
Top Bottom