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When did class D start to not suck?
- Thread starter ameerm
- Start date
Class D didn't 'suck' when it was indistinguishable from class AB, G, H or whatever design in a blind test using different speakers and within its power bandwidth and with distortion levels below audibility although I am admittedly too lazy to find all the references.
The whole point of G and H is they reduce the losses of an AB output stage by reducing the voltage accross the transistors that are operating in the linear region.
This is not necessary in a Class D output stage - since Class D outputs don’t operate with the transitors in the linear mode.
So again - I’m struggling to understand your post.
What a train wreck... Following is my opinion/memory...
There were some really great class-D amplifiers decades ago, as others have noted, such as the Sony unit @restorer-john showed us. And some pretty lousy ones of the same vintage (Infinity's I think? The one that self-destructed with capacitive loads...) In the late 1980's through 1990's IIRC the number of class-D products grew, but were rarely accepted by "audiophiles" and were found more in car and industrial/PA/sound reinforcement applications. Certainly Crown and others developed some great designs for pro audio use. I am not sure when self-oscillating designs were first developed or by whom but Hypex and ICEPower became big players. I think (not sure) they managed to commercialize the technology with a high degree of success, due largely to closing the feedback around the output filter and thus providing lower output impedance and distortion over the full audio (plus) bandwidth. That was early 2000's best I recall, probably when class D started to make inroads into the mainstream audiophile market.
We are still running a Crown K2 at church. I do not think it is class D but am not 100% sure. I think they called it a "current amplifier" or something like that. My impression is it is a low-bias G or H design but I really don't know, not sure I ever did. Life was focused on other things then though I did help set up the system.
As for amplifier classes, the Wiki article is decent, and below is the post I dredge up whenever this comes up. Note some have argued my use of terms such as "bias current" in describing the classes; when I was designing them that is how I thought about them and, well, old dogs... The classes are named in the order they were developed, A being first and H the last, at least for now. The D is not for "digital". Crown(?) defined a class-I topology that appears to be sort of an interleaved class-D scheme but I do not think it has been officially recognized by any standards body. Interestingly enough I used a similar scheme for an RF amp long before I knew about the "class-I" audio version; I did it as a science experiment trying to get high efficiency RF amplification without needing device bandwidth beyond what was available at the time (early 1990's).
Amplifier Classes
Here is a summary from memory so don't hold me to any mistakes:
Class A = bias current flows through the output devices 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% SE, ~78% push-pull 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
There were some really great class-D amplifiers decades ago, as others have noted, such as the Sony unit @restorer-john showed us. And some pretty lousy ones of the same vintage (Infinity's I think? The one that self-destructed with capacitive loads...) In the late 1980's through 1990's IIRC the number of class-D products grew, but were rarely accepted by "audiophiles" and were found more in car and industrial/PA/sound reinforcement applications. Certainly Crown and others developed some great designs for pro audio use. I am not sure when self-oscillating designs were first developed or by whom but Hypex and ICEPower became big players. I think (not sure) they managed to commercialize the technology with a high degree of success, due largely to closing the feedback around the output filter and thus providing lower output impedance and distortion over the full audio (plus) bandwidth. That was early 2000's best I recall, probably when class D started to make inroads into the mainstream audiophile market.
We are still running a Crown K2 at church. I do not think it is class D but am not 100% sure. I think they called it a "current amplifier" or something like that. My impression is it is a low-bias G or H design but I really don't know, not sure I ever did. Life was focused on other things then though I did help set up the system.
As for amplifier classes, the Wiki article is decent, and below is the post I dredge up whenever this comes up. Note some have argued my use of terms such as "bias current" in describing the classes; when I was designing them that is how I thought about them and, well, old dogs... The classes are named in the order they were developed, A being first and H the last, at least for now. The D is not for "digital". Crown(?) defined a class-I topology that appears to be sort of an interleaved class-D scheme but I do not think it has been officially recognized by any standards body. Interestingly enough I used a similar scheme for an RF amp long before I knew about the "class-I" audio version; I did it as a science experiment trying to get high efficiency RF amplification without needing device bandwidth beyond what was available at the time (early 1990's).
Amplifier Classes
Here is a summary from memory so don't hold me to any mistakes:
Class A = bias current flows through the output devices 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% SE, ~78% push-pull 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
Last edited:
ZolaIII
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If it uses class G-H power structure we can consider it class G-H which you will recognise by presence of charge pump in design block diagram. It can have more rails and separate amplification stages that are only used when needed.
There are and class D-G amplifiers like mentioned Hipex modules.
Another example with description:
DG amplifier uses PWM to produce a rail-to-rail digital output signal with a variable duty cycle. In this respect, a Class DG amp is the same as a Class D amp. The Class DG amp, however, also uses a multilevel output stage to sense the magnitude of the output signal. It then switches the supply rails, as needed, to supply the required signal power more efficiently. A Class DG amp, such as the MAX98308, uses the same dual-power concept as a switching Class D topology for even higher efficiency.
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I’ve not seen a controlled same/different listening test between UCD and nCore…I suspect there is a reason for that!
@Putter had the real answer - and that would have first been possible when class D amps with flat FR into real speakers came on the market. In audio, when evolutions are marketed on abstract distortion reduction, often that’s a smokescreen for lack of audible differences. The new thing can still be “better” and there are other important ampilfier performance aspects that can improved as well (notably self-noise, energy efficiency at idle and standby).
ZolaIII
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Hire you go a classy design with class H embedded amplifier.
It was really tough to get those transistors switching fast. I'm sure they did the best anyone could, but modern transistors are at least 10x faster. That means better efficiency and you can switch faster, which makes the notorious output filter a bit easier.
@abdo123 doesn’t have that, does he?
Besides, it’s a DAC with a headphone amp, it says it right there at the top!
ZolaIII
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I know just having fun. It's still a class leading design and not just for the DAC and AMP design and it's efficiency but also for claimed EMI rejection rate which is area in which class D isn't very good.
That's why portable devices (working in harsh conditions) based upon it lead SINAD charts for such and get to the point of state of art desktop contraparts (in great designs of course).
Your comparing a 30 mW max headphone amp (which is not a lot even for a headphone), to 180+W class D amps?
At so little power?
Better get this one then:
Topping G5 Review (Portable DAC & HP Amp)
This is a review and detailed measurements of the Topping G5 portable, battery operated DAC, headphone amplifier with bluetooth. It was sent to me by the company and costs US $299. The G5 comes in a super stout metal enclosure. It is on the heavy side for a portable product (2 to 4 times...
www.audiosciencereview.com
ZolaIII
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@voodooless no I said class leading which can't mean anything other than similar same class designs.
Shrink the G5 by order of magnitude (10x) and fead it with dirty USB power source and you get Luxury & Precision W2.
Shrink the G5 by order of magnitude (10x) and fead it with dirty USB power source and you get Luxury & Precision W2.
The question in this tread is a bit loaded, audiophiles may have accepted classs-D recently , but we may have been listening to them a lot when going to concerts and other live events for decades but unknowingly of course .
imho the audibility of any electronic component is overated to compared to speakers and acoustics .
The older classs-D designs did not have the filter in the feedback loop and gave unpredictable results with speakers , they may peak or dip in the treble do to the output impedance at high frequencies or be just fine , it's case by case .
But in PA and concert halls you use EQ anyway , so these things could be mitigated for the particular use case
but unknowingly of course .imho the audibility of any electronic component is overated to compared to speakers and acoustics .
The older classs-D designs did not have the filter in the feedback loop and gave unpredictable results with speakers , they may peak or dip in the treble do to the output impedance at high frequencies or be just fine , it's case by case .
But in PA and concert halls you use EQ anyway , so these things could be mitigated for the particular use case
ZolaIII
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Example which @restorer-john shown is also a D-G design (so no surprises there).
Now it’s class D-G? Earlier it was class H? Make up your mind… how it it a class DG design?
D*mn-Good?
ZolaIII
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Why wouldn't you read complete post instead?
Tip; do it slowly and couple of times if you need.
Yes, agreed. That's when you could get decent MOSFETs for a reasonable price.
Why do I need the rest? You claim Hypex amps are class DG, so I ask why you think that is.. If I understood that wrong, enlighten me.
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