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Class D FAQ

And here I am, having returned a high end 2,5k Class D amp with "sounding better than most high end AB amps" written in specs because the sound was exactly what everyone had warned me about: aggresive sound and sharp heights. Maybe, just maybe, this was exactly what the music had in it's data and this amp was just too good and all other amps just soften the heights?
 
Which one? There are enough expensive class D amps that use outdated modules rather than modern Ncore or Purifi modules.
Yep. Even other good modules (e.g. B&O ICEPower) are way not as good as Ncore or Eigentakt. Only a very few amps come close to those in terms of distortion performance.

That said, for my money, I still prefer Class AB. Not because it's better or anything, but because repair is generally simpler. A good class AB amp is very easy to repair, a good class D not so much. Something goes wrong on an NCore or Eigentakt or ICEPower module, chances are you're gonna junk the whole board in favor of a new one rather than get into the guts of the module and start probing around with a scope. Something goes wrong on some class AB amp, it's probably through hole and a relatively simple circuit design so component level repair is quick and relatively easy. There are people who can repair class D, but they're few and far between compared to class AB.
 
...because the sound was exactly what everyone had warned me about: aggresive sound and sharp heights. Maybe, just maybe, this was exactly what the music had in it's data and this amp was just too good and all other amps just soften the heights?

Maybe. Or maybe something else. :cool:
 
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/
Is there a class C amplifier?
 
Is there a class C amplifier?
Yep! They're used in non-audio circuits (e.g. radio broadcast) because they conduct less than 50% of the duty cycle. Not a big deal if you use a tuned circuit, but audio can't really... do that.
 
Yep! They're used in non-audio circuits (e.g. radio broadcast) because they conduct less than 50% of the duty cycle. Not a big deal if you use a tuned circuit, but audio can't really... do that.
Didn't know that. Thanks.
 
Which one? There are enough expensive class D amps that use outdated modules rather than modern Ncore or Purifi modules.

Nubert NuPower D. It was kind of a drama, the specs are really really good and there even is a based interview with the developer where he claims how well it's made and even some things that are patentend. They are using an Hypex power supply for sure, I think it's the 3kw one. I could compare 3 amps so far and while this one was really nice and didn't miss a beat in terms of power and dynamics the heights came through really sharp. Not even setting my tweeter to "soft" and reducing the highs in the AVR EQ didn't help. It's the reason why I am very weary of Class D now, at least for my main speakers (nuvero 170).
 
That said, for my money, I still prefer Class AB. Not because it's better or anything, but because repair is generally simpler.
That is certainly true for older designs such as my Quad 606-2, that I had refurbished with just about all components replaced. However, the same would not be possible with a more moderne power amplifier like my son's Yamaha P2500s.
 
Nubert NuPower D. It was kind of a drama, the specs are really really good and there even is a based interview with the developer where he claims how well it's made and even some things that are patentend. They are using an Hypex power supply for sure, I think it's the 3kw one. I could compare 3 amps so far and while this one was really nice and didn't miss a beat in terms of power and dynamics the heights came through really sharp. Not even setting my tweeter to "soft" and reducing the highs in the AVR EQ didn't help. It's the reason why I am very weary of Class D now, at least for my main speakers (nuvero 170).
Problem is most amplifiers specs (and ASR) ranks them by THD+N (SINAD) in 1 kHz, while everyone complaining about class D is because of harsh treble. That's why, IMO, it is very important to check the amplifier's power versus THD+N in 4 ohms especially the 6.67 kHz curve, exactly where sibilance is.

Or just buy NCore or Purifi, which have this perfected. :)
 
Problem is most amplifiers specs (and ASR) ranks them by THD+N (SINAD) in 1 kHz, while everyone complaining about class D is because of harsh treble. That's why, IMO, it is very important to check the amplifier's power versus THD+N in 4 ohms especially the 6.67 kHz curve, exactly where sibilance is.

Or just buy NCore or Purifi, which have this perfected. :)
I can vouch for the Purifi amp module-I have the VTV Eval 1 and it has none of the harsh high frequencies I experienced w/ the recently measured Emotiva PA 1 monobloc amp. Also have a Buckeye amp using the NC252MP module and it sounds almost as good as the Purifi.
 
Is there a class C amplifier?

Yes but it is rarely used for audio and no consumer products AFAIK. Here is a summary from memory so don't hold me to any mistakes:
  • Class A = bias current flows through the output currents all of the time. Most wasted energy and heat, max theoretical efficiency ~50% for a push-pull design (only ~27% for a single-ended design IIRC). Commonly used for low-level circuits like preamps and power amp input and driver stages, rarely for output stages since it is so inefficient. More common in tube amps these days, I think.
  • Class B = bias current flows half the time, so in a push-pull design one device is on and the other is off. Typically one device amplifies the (+) half of the signal and the other the (-) half as it swings around ground (0 V, or a common bias voltage). Can achieve ~67% efficiency in theory. In practice there is crossover distortion around the crossing point as one device is switched off and the other turned on since it does not happen instantaneously. Used for some power amplifiers in the past (do not know about today), with feedback used to reduce crossover (and other) distortion.
  • Class AB = biased in class A for small signals then moves to class B. This lets small signals around the crossing point stay in class A for lower distortion, then as the signal increases and moves out of the small signal region transitions to class B to save power.
  • Class C = bias current flows less than half the waveform cycle. The "missing" energy is usually generated by a resonant circuit (e.g. inductor/capacitor (LC) tank). Common in RF circuits where high power is needed and distortion less an issue, and oscillators which are narrow-band (audio is very wideband, spanning multiple decades) and incorporate a resonant circuit by design.
  • Class D = bias current flows only as output devices switch states, in a form of pulse modulation (pulse width, frequency, or both). Can achieve >90% efficiency. The high switching frequency is provided by a clock source or (for most audio amps) is self-generated by the circuit. The output pulse train is filtered so only the fundamental signal remains. See https://www.audiosciencereview.com/forum/index.php?threads/class-d-amplifiers-101.7355/
  • Class E, F = utilize switching as well but constrain the switching to certain points in the signal cycle (e.g. at voltage or current zero crossings) for higher efficiency since less power is dissipated in the switching transistors. These are used exclusively in RF circuits AFAIK. Class E is used in tuned amplifiers (narrowband, again) and class F is used for generating harmonics of the fundamental so you can say build a high-frequency oscillator output from a lower-frequency circuit.
  • Class G, H = wrap a varying power supply around the core (typically AB) amplifier to improve efficiency. By changing the power supply voltages it uses (wastes) less energy for small signals by applying low supply voltage, then increases the voltage as required as the signal gets larger. Class G uses discrete rails so the power supply switches between two or more (high/low) voltages. Class H uses a tracking supply that varies continuously with the signal level.
There are some more esoteric classes I am not familiar with. I have only designed and worked with the classes above.

HTH - Don

Edit: Found a Wiki page that probably does a better job than I but I didn't read it: https://en.wikipedia.org/wiki/Power_amplifier_classes
 
IMO the difference in SINAD from 94 to 102 of the Hypexes to 104 dB of the Purifi is small next to the average SINAD 80 of the overall amplifier market.
I specifically mentioned THD and not THD+N :). Both have comparable levels of noise, that is dominant at 5W. But if you look at the FFT you will see very different levels of harmonics. The same for THD+N vs power: straight line until the clipping elbow, which means a noise dominated amplifier, for the Purifi, while distortion becomes dominant at some point for the Ncore.
Another difference I forgot to mention is that the Purifi has a 2nd order low pass behavior, while the Ncore has a more classical 1st order behavior.
 
I specifically mentioned THD and not THD+N :). Both have comparable levels of noise, that is dominant at 5W. But if you look at the FFT you will see very different levels of harmonics. The same for THD+N vs power: straight line until the clipping elbow, which means a noise dominated amplifier, for the Purifi, while distortion becomes dominant at some point for the Ncore.
Another difference I forgot to mention is that the Purifi has a 2nd order low pass behavior, while the Ncore has a more classical 1st order behavior.
Agreed, but if the lower harmonics are buried in noise, then there is little to be gained by having them lower, right? That's why THD+N is more realistic imo.
 
Yes but it is rarely used for audio and no consumer products AFAIK. Here is a summary from memory so don't hold me to any mistakes:
  • Class A = bias current flows through the output currents all of the time. Most wasted energy and heat, max theoretical efficiency ~50% for a push-pull design (only ~27% for a single-ended design IIRC). Commonly used for low-level circuits like preamps and power amp input and driver stages, rarely for output stages since it is so inefficient. More common in tube amps these days, I think.
  • Class B = bias current flows half the time, so in a push-pull design one device is on and the other is off. Typically one device amplifies the (+) half of the signal and the other the (-) half as it swings around ground (0 V, or a common bias voltage). Can achieve ~67% efficiency in theory. In practice there is crossover distortion around the crossing point as one device is switched off and the other turned on since it does not happen instantaneously. Used for some power amplifiers in the past (do not know about today), with feedback used to reduce crossover (and other) distortion.
  • Class AB = biased in class A for small signals then moves to class B. This lets small signals around the crossing point stay in class A for lower distortion, then as the signal increases and moves out of the small signal region transitions to class B to save power.
  • Class C = bias current flows less than half the waveform cycle. The "missing" energy is usually generated by a resonant circuit (e.g. inductor/capacitor (LC) tank). Common in RF circuits where high power is needed and distortion less an issue, and oscillators which are narrow-band (audio is very wideband, spanning multiple decades) and incorporate a resonant circuit by design.
  • Class D = bias current flows only as output devices switch states, in a form of pulse modulation (pulse width, frequency, or both). Can achieve >90% efficiency. The high switching frequency is provided by a clock source or (for most audio amps) is self-generated by the circuit. The output pulse train is filtered so only the fundamental signal remains. See https://www.audiosciencereview.com/forum/index.php?threads/class-d-amplifiers-101.7355/
  • Class E, F = utilize switching as well but constrain the switching to certain points in the signal cycle (e.g. at voltage or current zero crossings) for higher efficiency since less power is dissipated in the switching transistors. These are used exclusively in RF circuits AFAIK. Class E is used in tuned amplifiers (narrowband, again) and class F is used for generating harmonics of the fundamental so you can say build a high-frequency oscillator output from a lower-frequency circuit.
  • Class G, H = wrap a varying power supply around the core (typically AB) amplifier to improve efficiency. By changing the power supply voltages it uses (wastes) less energy for small signals by applying low supply voltage, then increases the voltage as required as the signal gets larger. Class G uses discrete rails so the power supply switches between two or more (high/low) voltages. Class H uses a tracking supply that varies continuously with the signal level.
There are some more esoteric classes I am not familiar with. I have only designed and worked with the classes above.

HTH - Don

Edit: Found a Wiki page that probably does a better job than I but I didn't read it: https://en.wikipedia.org/wiki/Power_amplifier_classes
I would not define class by bias current flow (because bias voltage/current sets the class by changing quiescent behavior), but rather by percent of duty cycle the device is conducting.

Class A = 100% of duty cycle, no crossover distortion but low power efficiency and a lot of waste heat.
Class B = 50% of duty cycle, much more efficient but lots of crossover distortion
Class AB = between 50-100% of duty cycle, generally less efficient than B but much lower distortion - there is a good reason this became a standard for audio circuits.
Class C = less than 50% of duty cycle, used for RF where a tuned circuit can take care of distortion
Class D = switching, PWM architecture, acts more like a boost converter than a linear region amplifier.
 
I stumbled upon this list https://docs.google.com/spreadsheets/d/1ZlTOYxmPs938gqHjtDABkWS-MApu7uJjzIGnJ2Elm6Y/edit#gid=0 looking at the signature of Matias and it just made me order two Hypex NC400 Mono. It will be my last try with Class D. If those things don't live up to the hype I'm going to stick to Class AB for eternity. :D
Some manufacturers allow home trials for 15 or 30 days or your money back, so no risk. But the NC400 DIY I think does not apply to that, so good luck. :)
 
Tbh people who intentionally buy Class A or Class A/B amps in 2021 are the anti-maskers of the audio world.

They measure worse (at the same price), they cost more (for the same wattage) and they consume more energy.
I think there are Class A amplifiers that can outperform Class D, but those are extremely expensive.

Benchmark AHB2 is an unique class AB design as far as I know.
 
Some manufacturers allow home trials for 15 or 30 days or your money back, so no risk. But the NC400 DIY I think does not apply to that, so good luck. :)

Even if I would return the NC400, I've seen that the newer kits don't even need soldering anymore. So if I assemble the kit, don't like it and the vendor get's back an almost new preassembled kit to sell again. Kind of a good deal for both sides.

I think there are Class A amplifiers that can outperform Class D, but those are extremely expensive.

Benchmark AHB2 is an unique class AB design as far as I know.
In terms of what?
And yeah, I had my eye on the AHB2 too, but it's quiet expensive and only has 190 watt in stereo.
 
A couple of points to remember: RMS is a way to average a sine wave. The RMS voltage coming out of the wall in most U.S. homes is 120 VAC, but the peak voltage is about 170V. That can be measured across a single wavelength.

Peak versus average power is different--it's how the amplitude (really: area) of the loudest wave (usually a very short period of time including however many waves it includes at the frequencies involved) compares to the average amplitude of all the waveforms in the music.

So, the loudest signals in the music can be measured and reported as peak or RMS values, and the average signals in the music can be measured and reported as peak or RMS values. It seems to me run into trouble when conflating "peak" with "loudest".

When evaluating continuous sine-wave test signals, those values merge.

Heatsinks and continuous ratings and so forth are rated for the average signals continuously and for the loudest signals over some shorter period of time. That's different from the amp's ability to fill out the full waveform of the loudest part of the loudest signal in the short term.

In the amateur radio world, transmitter amplifier power is rated as "peak envelope power", which is defined by the area of the waveform being produced. But they are also rated at that peak power for "intermittent amateur service", meaning that they assume peak power won't be demanded more than some percentage of the time. Amps for morse code running at full power are more demanding than amps for the voice modes--a morse key down is like those test tones, but the morse key is only down about 25-35% of the time in an actual CW conversation. The digital modes can be worse. Voice modes only demand peak output once in a while while the operator is speaking, even if the operator is using limiters to increase average loudness (amateur radio has its loudness wars, too), but presumably the operator is only speaking some fraction of the time, notwithstanding the apparent unwillingness of some operators to listen occasionally. Transmitters for continuous service, such as broadcast stations and (even worse) continuous digital transmission) are built quite differently even at the same rated power. Duty cycle is important. It seems to me that many Class D amps are built for a reasonable duty cycle for actual music, not for continuous full-power output.

I like seeing power plotted against distortion and noise at an important selection of of frequencies--I think that tells me how an amp will perform in the real world.

For me, "harsh" amps are those that distort on the onset of sounds in the treble region, requiring a finite period to stabilize on the correct wave shape. This seems to me what undermines a clean "sssss" attack to avoid a "sh" distortion. But that is arm-waving. I have amps that do that, when driven to their limits (or when malfunctioning, or when playing LP records that have been overplayed or ruined by a past crappy record player). I also have amps that compress output when it approaches the amp's power capability, which dulls the loudest sounds. I like having an abundance of power because it means I never have to worry about some transient peak finding the peak power envelope and going non-linear.

Now, to the topic: My current amps are Class AB amps of good design, reputation, and measurement (by the standards of a decade or two ago). They are rated to produce less than 0.09% THD at 125 watts into 8 ohms from 20-20K, with a dynamic headroom of 1.2 dB. The dynamic headroom describes how much louder the loudest signals can be without running out or the ability to fill out the waveform. An amp of noticeably greater power would need to be in the 250-300-watt range (into 8 ohms) RMS continuous, it seems to me.

But my old B&K Class AB amps have a signal/noise ratio of well over a hundred dB, which means that they are effectively transparent for me. But I do notice a very low hiss from them when they are on but no music is being played--I have to be within a foot or two of the tweeters to hear it. A Hypex NC502 amp would certainly be cleaner in terms of measured distortion, but the only noticeable improvements from my perspective, and in my ears, would be 1.) the greater power (maybe), and 2.) the absence of that quiescent hiss. I would do it for fun, and for efficiency and space, but it will not otherwise change my life. That's why I haven't done it yet.

Rick "finding it ironic that a few people who praise 20-watt SET amps complain about the way Class D amps are rated" Denney
 
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