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.
Q1: Class D power ratings cannot be true. How can these small and light "toy amplifiers" generate all these watts??
A1: 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.
Q2: 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!
A2: 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 and here.
Q3: 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?
A3: 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.
Q4: But will this high frequency switching noise burn the tweeters of my precious speakers?
A4: 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.
Q5: 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.
A5: 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.
Q6: 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?
A6: 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 and here.
Q7: Yeah, but when we use a higher bandwidth measurement like 45 kHz or 90 kHz we can see the THD of high frequencies "ballooning" with high distortion. These manufacturers are lying by using using lower bandwidth 20 kHz to hide them! What about the measurements standards from last century?
A7: Remember that this is THD, meaning the harmonics of 10 kHz are 20 kHz, 30 kHz, etc, and the harmonics of 15 kHz are 30 kHz, 45 kHz, etc. They are all inaudible. This is intentional by design: maximize feedback loop to reduce distortions where it is audible below 20 kHz, and let it go where it is inaudible. See here. The 20 kHz bandwidth measurements just represent better what we actually hear.
Q8: 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!
A8: 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 and here.
Q9: 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.
A9: 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.
Q10: 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!
A10: That's fine, no one is forcing you to buy a class D amplifier. Just please stop spreading FUD.
Q1: Class D power ratings cannot be true. How can these small and light "toy amplifiers" generate all these watts??
A1: 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.
Q2: 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!
A2: 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 and here.
Q3: 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?
A3: 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.
Q4: But will this high frequency switching noise burn the tweeters of my precious speakers?
A4: 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.
Q5: 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.
A5: 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.
Q6: 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?
A6: 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 and here.
Q7: Yeah, but when we use a higher bandwidth measurement like 45 kHz or 90 kHz we can see the THD of high frequencies "ballooning" with high distortion. These manufacturers are lying by using using lower bandwidth 20 kHz to hide them! What about the measurements standards from last century?
A7: Remember that this is THD, meaning the harmonics of 10 kHz are 20 kHz, 30 kHz, etc, and the harmonics of 15 kHz are 30 kHz, 45 kHz, etc. They are all inaudible. This is intentional by design: maximize feedback loop to reduce distortions where it is audible below 20 kHz, and let it go where it is inaudible. See here. The 20 kHz bandwidth measurements just represent better what we actually hear.
Q8: 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!
A8: 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 and here.
Q9: 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.
A9: 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.
Q10: 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!
A10: That's fine, no one is forcing you to buy a class D amplifier. Just please stop spreading FUD.
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